HOST DEFENSE FACTORS

BACTERIAL FACTORS

Bacterial pathogens have evolved various virulence factors and mechanisms that enable them to overcome host defenses, including adherence factors, enterotoxin and cytotoxin elaboration, mucosal invasion, and a variety of others.

Adherence

The ability of bacteria to adhere to host mucosal cells is a critical virulence factor in enterotoxin-producing and invasive bacteria as well as in enteroadherent E. coli (EAEC) and enteropathogenic E. coli (EPEC). Bacterial adherence to host mucosal cells may be the predominant virulence factor, as in the case of EPEC; one of two important factors (adherence plus toxin elaboration), as in the case of enterotoxigenic organisms; or only one of several factors required for expression of full pathogenicity, as seen with invasive organisms. Bacteria that cause disease by adhesion alone do not elaborate any of the traditional enterotoxins, but rather they adhere tightly to the mucosa of both the small and large intestine.²⁵ The classic EPEC and the EAEC²⁶ are typical of this group. Other organisms, including enterotoxigenic E. coli (ETEC) and the invasive Salmonella and Shigella species, also must adhere to the intestinal surface to be fully pathogenic.

Studies on the mechanism by which EPEC cause diarrhea show that they attach to the intestinal mucosa in a characteristic manner, producing ultrastructural changes known as attaching-effacing lesions²⁷ that lead to elongation and destruction of microvilli.^25,28 This pattern of bacterial binding to enterocytes also has been referred to as attaching and effacing adherence²⁷ and the particular morphologic alteration as pedestal formation (Fig. 107-1).²⁵

Figure 107-1. Electron micrograph of enteropathogenic Escherichia coli (EPEC) adherent to the jejunal mucosa and demonstrating the attaching and effacing lesion, also called pedestal formation. Tightly adherent E. coli are obliterating the brush border.

The laboratory counterpart of mucosal colonization is adherence in tissue culture to various cell lines such as Hep-2 and HeLa. A characteristic form of localized adherence is observed only with classic EPEC serotypes. These events occur in the following three phases²⁹:

1. Nonintimate attachment of EPEC to intestinal epithelial cells: Attachment is mediated by a bundle-forming pilus associated with a large plasmid common to EPEC isolates

2. A signal transduction event that leads to cytoskeletal changes in the enterocyte via activation of protein kinase and the release of intracellular calcium

3. Intimate attachment of the bacterium to the host cell membrane: Attachment is mediated by an outer membrane protein called intimin, which is encoded by the eaeA gene cluster on the EPEC chromosome³⁰

The presence of a plasmid in EPEC serves to increase intimin production; this process is needed for localized adherence to occur.³¹ EPEC strains with localized adherence produce acute diarrhea when these strains are administered to normal volunteers.³² The role of the eaeA gene as a virulence factor in human EPEC infection has been confirmed in volunteer challenge studies.³³

Enterotoxigenic organisms also require expression of bacterial adherence for proliferation of the organisms and colonization, as well as for full expression of toxicity.³⁴ ETEC adhere to the surface of the small bowel epithelium without penetrating the epithelial layer and do so by mechanisms different from those used by EPEC. The most important mechanism by which enterotoxigenic bacteria adhere to the intestinal mucosa is related to specific protein antigens on the surface of the bacterial cell known as pili or fimbriae, also referred to as adherence antigens or colonization factor antigens.³⁵ These pili bind to specific receptor sites on the surface of the intestinal cell via specific ligand-receptor interactions.

The antigenic structure of the adherence pili determines the host specificity of the ETEC strains. For example, those bearing a K88 antigen are pathogenic for piglets, whereas others bearing K99 antigen cause disease in calves and lambs. ETEC adhesion antigens for humans include type 1,3,P and BFP pili.

Evidence that these colonization factors (e.g., pili and lectins) are important to the pathogenesis of E. coli diarrheal disease in animals is derived from the observations by Moon³⁶ that loss or gain of fimbriae by genetic manipulation results in the loss or gain of the ability to adhere to and colonize the intestine. Adherence not only permits colonization but also can facilitate the delivery of enterotoxin to the epithelium and might even enhance the ability of the organism to elaborate enterotoxin.^34,36

Enterotoxin Production

Enterotoxins are polypeptides, secreted by bacteria, that alter intestinal salt and water transport without affecting mucosal morphology.^5,37,38 Many organisms elaborate enterotoxins (e.g., V. cholerae, Shigella, ETEC, and Staphylococcus aureus), and several enterotoxins may be elaborated by a single organism. Although most enterotoxins affect the small intestine, the colon also may be a target organ.

Several enterotoxin-producing organisms cause food poisoning and induce disease without requiring intestinal colonization, such as S. aureus and C. perfringens; their toxins are ingested preformed in food, thus accounting for their characteristic brief incubation period.

Whether the enterotoxin is ingested preformed or is first expressed within the intestinal lumen, the toxin-enterocyte or toxin-colonocyte interaction begins with the binding of the enterotoxin to a specific mucosal receptor. The toxin-receptor interaction increases the concentration of an intracellular mediator, resulting in alteration of salt and water flux. Thus far, three intracellular mediator systems have been shown to be involved in the pathogenesis of enterotoxigenic diarrhea: adenylate cyclase and cyclic adenosine monophosphate (cAMP), the guanylate cyclase and cyclic guanosine monophosphate (cGMP) systems, and intracellular calcium.^39,40 Alterations in these mediator systems have similar effects on transport processes to decrease the coupled influx of sodium and chloride and to stimulate the active secretion of chloride from the cell into the intestinal lumen. Other intracellular mediator systems involved in the pathogenesis of bacterial diarrhea include protein kinase C and arachidonic acid metabolites, among others.

Cholera toxin (molecular weight ≈84,000 kDa), which stimulates adenylate cyclase, is a prototypical enterotoxin (Fig. 107-2 and Chapter 99), the toxin-enterocyte interaction of which is well understood. Cholera toxin is composed of an A subunit surrounded by five B subunits that bind the toxin to a ganglioside (GM₁) receptor on the brush border membrane of the villus epithelial cell. The A subunit, which consists of two parts (A₁ and A₂) linked by a disulfide bond, slowly penetrates the brush border membrane and is cleaved into its two component peptides. Reduction of this bond releases the active A₁ peptide that traverses the cell to the basolateral membrane, where it stimulates the ribosylation of G_s, the stimulatory subunit of a heterotrimeric G protein. This action results in the irreversible activation of G_s and an increase in cytosolic cAMP. This cAMP in turn activates cAMP-dependent kinases that inhibit NaCl-coupled transport and stimulate chloride secretion.

Figure 107-2. Subunit structure of cholera toxin (choleragen) (see text). mw, molecular weight.

(From Fishman PH. Action of cholera toxin: Events on the cell surface. In Field M, Fordtran JS, Schultz SG, editors. Secretory Diarrhea. Bethesda, Md: American Physiological Society; 1980. p 86.)

In addition to cholera toxin, other important enterotoxins are those elaborated by E. coli.^41,42 Two classes of E. coli enterotoxins are known: heat-labile (LT) and heat-stable (ST) toxins. LT, which exists in two forms (LT-1 and LT-2),⁴² is a large molecular-weight protein that causes diarrheal disease similar to, but less severe, than, cholera. The subunit structure and mechanism of action of LT-1 and LT-2 also are similar to those of cholera toxin; although cholera toxin and LT bind to a glycolipid receptor, specifically GM_1, an additional glycoprotein receptor might exist for LT.⁴² Heat-stable enterotoxins (ST_as) also may be elaborated by E. coli and bind to brush border receptors on enterocytes and colonocytes, a receptor guanylate cyclase, to increase intracellular levels of cGMP. ST_b is an unrelated enterotoxin elaborated by some E. coli pathogenic for pigs. Other organisms that elaborate highly homologous heat-stable enterotoxins include Yersinia enterocolitica, Citrobacter, and non-O1 vibrios.⁴²

EVALUATION OF THE PATIENT

The initial step in the diagnostic evaluation of a patient with acute diarrhea should be a thorough history and physical examination, the goals of which are to identify patients who may be at risk of severe illness or susceptible to complications and those who will benefit from specific therapy. Most patients simply need rehydration therapy. Consideration of the patient’s general health, severity and duration of illness, and the setting in which the illness was acquired should enable the clinician to determine who needs further evaluation (Fig. 107-3).

Figure 107-3. Algorithm for the diagnosis and treatment of infectious diarrhea. AIDS, acquired immunodeficiency syndrome; C. difficile, Clostridium difficile; E. coli, Escherichia coli; EHEC, enterohemorrhagic E. coli; EIEC, enteroinvasive E. coli; EPEC, enteropathogenic E. coli; ETEC, enterotoxigenic E. coli; HIV, human immunodeficiency virus; PMN, polymorphonuclear neutrophil.

Patients who are debilitated, malnourished, or immunocompromised and those who have severe comorbid illnesses are at increased risk for complications of diarrhea and infection. They can require hospitalization and early diagnostic tests. Other patients who also require a more-aggressive approach include those with systemic signs and evidence of an inflammatory diarrhea, illness lasting more than three to four days, a history or physical examination suggesting a disease process that will benefit from specific therapy, and infection with certain specific organisms (Table 107-1).⁶

Table 107-1 Pathogens Indicating Need for Antimicrobial Therapy in Patients with Infectious Diarrhea

Noninflammatory Diarrhea

Patients with noninflammatory diarrhea usually present with large-volume watery stools without blood or pus, or they present with severe abdominal pain. These patients generally have few systemic signs or symptoms, and fever usually is absent. Abdominal cramping, nausea, and vomiting can occur. The most likely causes are viruses (rotavirus, norovirus), enterotoxigenic E. coli, V. cholerae, staphylococcal and clostridial food poisoning, and Giardia and Cryptosporidium infections. Noninflammatory diarrheas generally do not require extensive evaluation. Pathogens causing noninflammatory diarrhea usually infect the small intestine and merely adhere to the mucosal surface without invading the epithelium or causing acute inflammation. Most of these organisms (e.g., cholera, rotavirus) elaborate enterotoxins that stimulate intestinal secretion, occasionally resulting in profound dehydration.

Inflammatory Diarrhea

Patients with inflammatory diarrhea usually present with numerous small-volume stools that may be mucoid, grossly bloody, or both. Such patients may appear toxic and usually are febrile. Abdominal cramping may be severe. Because of the small stool volumes, these patients are less likely to be dehydrated than those with noninflammatory diarrhea. Physical findings might point to a specific diagnosis (Table 107-3).

Table 107-3 Clinical Finding(s) That Suggest the Causative Organisms for Some Inflammatory Diarrheas

From Park SI, Giannella RA. Approach to the adult patient with acute diarrhea. Gastroenterol Clin North Am 1993; 22:483.

Organisms causing inflammatory diarrheas usually affect the colon and either invade or elaborate cytotoxins, resulting in an acute inflammatory reaction with disruption of the epithelial barrier, mucus, red blood cells, and white blood cells in the stool (Table 107-2). Microbes causing this syndrome include Shigella, Campylobacter, EHEC, C. difficile, Salmonella, Yersinia, and Entamoeba histolytica. Fecal leukocytes (or positive stool lactoferrin test) indicate an acute inflammatory process, and sheets of polymorphonuclear leukocytes (PMNs) usually indicate colitis. The acute inflammatory diarrheal syndrome also can have an noninfectious etiology, for example, ulcerative colitis, Crohn’s disease, radiation or ischemic colitis, and diverticulitis. Table 107-4 lists the organisms that may be associated with the presence of fecal leukocytes.⁴⁷

Table 107-4 Fecal Leukocytes in Intestinal Infections

DAEC, diffusely adhering Escherichia coli; EAEC, enteroaggregative E. coli; EHEC, enterohemorrhagic E. coli; EIEC, enteroinvasive E. coli; EPEC, enteropathogenic E. coli; ETEC, enterotoxigenic E. coli.

Proctitis Syndrome

Proctitis syndrome is characterized by frequent painful bowel movements containing blood, pus, and mucus. The sensation of tenesmus, often with rectal pain, usually is prominent. Infectious causes include Shigella, herpes simplex virus type 2, and Neisseria gonorrhoeae, Treponema pallidum (syphilis), and Chlamydia (lymphogranuloma venereum). The importance of this syndrome is that many of the causes have specific treatments. Noninfectious causes include idiopathic ulcerative proctitis, Crohn’s proctitis, radiation proctitis, and solitary rectal ulcer syndrome.

Fecal Leukocytes

A particularly useful technique to focus the differential diagnosis is microscopic examination of the stool for PMNs (see Table 107-4). Invasive pathogens that primarily affect the colon, such as Shigella and Campylobacter, produce a “sea of polys,” as well as red blood cells. The toxigenic organisms, viruses, and food-poisoning bacteria cause a watery stool that harbors few formed elements. Stool tests for lactoferrin or calprotectin (proteins made by PMNs) in fecal specimens are available and provide a rapid and sensitive alternative to microscopy for identifying PMNs and, by inference, inflammatory diarrhea.⁴⁸

Salmonella, Yersinia, Vibrio parahaemolyticus, and C. difficile diarrhea have unpredictable and variable numbers of fecal leukocytes, depending on the degree of colonic involvement; most cases show only occasional PMNs. An acute exacerbation of ulcerative or Crohn’s colitis can produce a stool with many PMNs, thereby resembling bacillary dysentery. Although fecal microscopic examination is neither infallible nor even helpful in all cases, it is inexpensive and yields immediate information that can guide antibiotic therapy. Although the fecal lactoferrin and calprotectin tests are slightly more expensive than microscopy for fecal leukocytes, they have several advantages, including ease, rapidity, and the absence of a requirement for a fresh stool specimen.

Stool Cultures

Stool cultures are ordered too frequently. In most microbiology laboratories, routine stool cultures are processed for Shigella, Salmonella, and Campylobacter. Other enteric pathogens such as Yersinia, Vibrio, and E. coli O157:H7 are not sought routinely. Therefore, if clinical suspicion is high, the microbiology department needs to be notified to search for these pathogens. Because of sporadic shedding of pathogens (nontyphoidal Salmonella spp., Salmonella typhi) and because most episodes of acute diarrhea are caused by viruses, undetectable pathogens, or noninfectious causes, stool cultures are not usually positive. At Massachusetts General Hospital, the isolation rate of bacterial pathogens from 2000 fecal cultures in 1980 was 2.4%.⁴⁹ In patients with severe diarrhea requiring hospitalization, the bacterial isolation rate from feces is somewhat higher, ranging from 27% to 43%,^50,51 and up to 58% in a study that used more-advanced techniques.⁵² Even in patients hospitalized for dysentery, the rate of positivity for microbiologic diagnosis is only 40% to 60%. In community patients with severe acute gastroenteritis (more than four fluid stools per day, lasting at least three days and with at least one associated symptom), the yield of a stool culture and ova and parasite examination increased to 88%.⁵³ In outbreaks of gastroenteritis in the United States, only one half of the cases have a confirmed etiology, of which two thirds are bacterial in origin. These figures suggest that many cases of acute diarrhea are caused by unidentified pathogens.

When parasitic or protozoal infection is suspected, stool examination for cysts, trophozoites, larvae, or eggs should be performed. When either Giardia or cryptosporidia is suspected, a stool enzyme-linked immunosorbent assay (ELISA) for antigens of these organisms should be requested. ELISA also is done for rotaviruses or noroviruses, but this test rarely is required in adults. It is not appropriate to request stool cultures and stool examination for ova and parasites for every patient with diarrhea. These tests should be performed only when the clinical suspicion is high that parasitic infection is the cause of diarrhea and the results of the stool tests will change management and influence outcome.

Endoscopy

Endoscopy may be useful to obtain aspirates and biopsies of the small intestine to detect Giardia, cryptosporidia, microsporidia, Isospora belli, or Mycobacterium avium-intracellulare. Flexible sigmoidoscopy can be useful in evaluating patients with proctitis, tenesmus, or sexually transmitted diseases or in identifying the pseudomembranes of C. difficile. In HIV-infected patients, colonoscopy with multiple biopsies is used to detect cytomegalovirus ulcers, which may be confirmed by the presence of inclusion bodies on biopsy.

An algorithm for the diagnosis of acute diarrhea is presented to help determine which patients should be treated symptomatically and which require further diagnostic studies and treatment (see Fig. 107-3). Approximately 90% of cases of acute diarrhea fall into the “No” category.

VIBRIO CHOLERAE

Cholera is a severe diarrheal disorder that can cause dehydration and death within three to four hours of onset. Stool output can exceed 1 L/hr, with daily fecal outputs of 15 to 20 L if parenteral fluid replacement keeps up with losses. The acutely ill patient typically has marked signs of dehydration, poor skin turgor, “washerwoman’s” hands, absent pulses, reduced renal function, and hypovolemic shock.

Cholera is the prototypical toxigenic diarrhea. More has been learned about pathophysiology—and normal intestinal function—from cholera than from any other intestinal disease. Treatment programs have been devised, including an oral rehydration regimen used to treat all major diarrheal illnesses; the cholera enterotoxin has been purified; the immunology and epidemiology of cholera have been clarified; the secretory mechanisms and second messenger systems involved have been elucidated; and anticholera vaccines have been developed.

OTHER VIBRIO SPECIES

In addition to the cholera vibrios, at least nine other vibrios have important pathogenic significance.^61,73,74 These strains represent a diverse group of organisms that are morphologically and biochemically identical to V. cholerae but that do not agglutinate with the O group antiserum of the three cholera serotypes.^74,75 The non-O1 cholera vibrios produce several toxins and cause a wider range of infection than do the cholera vibrios, including watery diarrhea, dysentery, wound infections, ear infections, and septicemia.^61,74

The non-O1 cholera vibrios can be isolated from salty coastal waters of the United States, most commonly in the summer and fall when the temperature rises. Mollusks, particularly oysters, have a reported contamination rate of 10% to 15% and are the major source of non-O1 Vibrio disease; clams, mussels, and crabs also have been implicated.

Strains within the same species can produce different enterotoxins, cytotoxins, and hemolysins. The diversity of toxin production is matched by the diversity of clinical symptoms: diarrhea ranges from watery dehydrating diarrhea to frank dysentery; some strains penetrate the intestinal mucosa and produce bacteremia; others have been incriminated in wound infections after exposure to ocean water or handling raw seafood.⁷⁶

In East Asia, non-O1 cholera vibrios have been associated mainly with severe dehydrating diarrhea. In Peru, serogroups O10 and O12 were isolated from patients with liquid diarrhea associated with mild to moderate dehydration.⁷⁷ In the United States, reported cases of disease caused by non-O1 cholera vibrios include wound and ear infections, septicemia, and infections of the lung and biliary tract.⁷⁴ The most common antecedent history is consumption of raw oysters within the previous 72 hours. In outbreaks, there is a high attack rate, with incubation periods that range from as short as six to 12 hours to as long as three days. A one-week course of diarrheal illness is common. Because the gastrointestinal disease is self-limited and relatively benign in the United States, antibiotics are not recommended; however, septicemia, wound infections, and deep organ infections should be treated with appropriate antibiotics.

The incidence of Vibrio intestinal infections was studied among participants at an antimicrobial conference in New Orleans, many of whom had consumed raw oysters. Of 479 persons surveyed, 11% had a positive stool culture for vibrios, mainly V. parahaemolyticus, and approximately one third of those with a positive culture had diarrhea. Samples of local seafood, especially oysters, were found to harbor five different species of vibrios.⁷⁸ In the Chesapeake Bay area, the annual incidence of Vibrio infections related to consuming seafood is estimated to be 1.6 per 100,000 persons.⁷⁹

AEROMONAS SPECIES

Aeromonas species are ubiquitous environmental organisms found principally in fresh and brackish water, especially in the summer months. These Gram-negative organisms often are mistaken for coliforms in the laboratory and, as a result, reported incidence rates are falsely low. Aeromonas species are divided into two groups: psychrophilic (Greek: psychros, cold) aeromonads, which grow optimally at temperatures ranging from 22°C to 28°C, and mesophilic aeromonads, which grow best between 35°C and 37°C.⁴³ Psychrophilic strains usually are isolated from environmental water sources and fish; Aeromonas salmonicida is the most common strain in this group. Based on their phenotypic features, the mesophilic aeromonads are grouped into three complexes: Aeromonas hydrophila, Aeromonas caviae, and Aeromonas veronii. All three of these Aeromonas species have been associated with human infection.^86,87 Aeromonas strains produce an array of toxins, including heat-labile enterotoxin, hemolysin, and cytotoxin.⁸⁸

PLESIOMONAS SHIGELLOIDES

Plesiomonas shigelloides also is a member of the family Vibrionaceae but is isolated less often than Aeromonas in the United States.^86,91,93 Most cases have been associated with consumption of raw oysters or recent travel to Mexico or Asia.⁹⁴ Diarrhea ranges from mild and watery to severe colitis with visible blood. Abdominal pain often is prominent. Antibiotic sensitivity is similar to that of Aeromonas, but little information is available on the efficacy of treatment.

ESCHERICHIA COLI

E. coli are major components of the normal intestinal microflora in humans and animals. Although most strains are relatively harmless in the bowel, others possess virulence factors that are related to diarrheal disease. At least six types of E. coli intestinal pathogens have been recognized (Table 107-7). Their virulence factors include toxin production, adherence to epithelial cells, and invasiveness, each encoded by specific genetic elements (plasmids or chromosomal genes) that determine pathogenicity.

Enterohemorrhagic Escherichia coli

Acute hemorrhagic colitis, which first was recognized in two separate outbreaks in Michigan and Ohio in 1982,¹⁰⁶ has been associated mainly with a specific serotype of E. coli, O157:H7. This organism is estimated to be responsible for 0.6% to 2.4% of all cases of diarrhea and 15% to 36% of cases of hemorrhagic colitis in Canada, the United Kingdom, and the United States.^107,108 The spectrum of disease associated with E. coli O157:H7 includes bloody diarrhea, which is seen in up to 95% of patients, nonbloody diarrhea, hemolytic-uremic syndrome (HUS), and thrombotic thrombocytopenic purpura (TTP). Currently, the class of EHEC includes more than 100 different serotypes.¹⁰⁸

Epidemiology

EHEC has become the most commonly isolated pathogen from the stools of patients with bloody diarrhea in the United States.¹⁰⁹ EHEC disease is most common in northern climates such as in Massachusetts, Minnesota, and the Pacific Northwest, but it occurs throughout the United States. It also is well known in Canada, Great Britain, and throughout Europe. Infections occur sporadically or in large outbreaks. The leading vehicle of infection is hamburger meat, although outbreaks have been associated with precooked meat patties, roast beef, salami, fresh-pressed apple cider, lettuce, alfalfa sprouts, and unpasteurized milk.^29,110–113 Water-borne outbreaks also have been associated with contaminated swimming pools and other recreational water bodies, well water, and municipal water systems.²⁹ Person-to-person transmission probably has played a role in outbreaks in daycare centers, and nursing homes.^107,114,115 Infection rates vary seasonally, and peak incidence is from June to September.

EHEC strains are found in the fecal flora of a wide variety of animals, including cattle, sheep, pigs, goats, chickens, dogs, and cats. Many of these strains are of serotypes other than E. coli O157:H7. The most important reservoir of infection is cattle, hence, transmission via hamburger meat.

Virulence Factors

EHEC strains possess at least two virulence factors: an adherence mechanism causing attachment-effacement lesions similar to those seen with EPEC (see earlier) and two Shiga-like toxins (STX cytotoxins I and II).^107,108,116 The toxins, which are identified either in stool samples or from culture of the organism itself, cause characteristic lesions in tissue culture lines such as Vero cells and HeLa cells. Some EHEC strains produce only STX I or II, whereas others produce both toxins. Most strains of E. coli O157:H7 possess the eaeA gene, which is associated with intimate attachment to the intestinal mucosa, as in EPEC (see earlier). They also produce enterohemolysin and are capable of using both heme and hemoglobin, a property that may enhance their virulence.¹¹⁷ E. coli O157:H7 toxins can result in colitis that resembles ischemic colitis because they can cause endothelial damage, platelet aggregation, and microvascular fibrin thrombi.^{107,108,118,118a}

Clinical Features

After an incubation period of one to 14 days (mean, three to four days), watery nonbloody diarrhea begins. It is associated with severe abdominal cramping and often progresses to visibly bloody stools. Other symptoms include nausea, vomiting, low-grade fever, and chills. The development of frankly bloody diarrhea often results in admission to the hospital.

Examination of the colon by endoscopy demonstrates a segmental colitis (i.e., friable inflamed mucosa with patchy erythema, edema, and superficial ulcerations). The process usually is most evident in the right colon (Fig. 107-4), but virtually any part of the colon may be affected, just as with idiopathic ischemic colitis. Plain films of the abdomen might show subepithelial edema and hemorrhage (thumbprinting), usually in the ascending and transverse colon. Leukocytosis with a shift to the left usually is present, but anemia is uncommon unless infection is complicated by the development of HUS or TTP.¹⁰⁷ Microscopic examination of the stool reveals red and white blood cells in low to moderate amounts. The median duration of diarrhea is three to eight days, with longer durations in children and persons with bloody diarrhea.¹⁰⁷

Figure 107-4. Colonoscopic appearance of the sigmoid colon in a patient with enterohemorrhagic Escherichia coli (EHEC) infection. The mucosa is edematous and violaceous, with diffuse subepithelial hemorrhage.

A striking association has been noted between intestinal infection with EHEC and HUS. In Minnesota, the incidence of HUS increased progressively during the 1980s to a current rate of 2.0 cases per 100,000 child-years. E. coli O157:H7 was isolated in 46% of children presenting with HUS. Risk factors for HUS include age younger than five years, attendance at a large daycare center, presence of bloody diarrhea, and a high white blood cell count.¹¹⁰ A study from the British Isles showed that 95% of the cases of HUS had a prodromal diarrheal illness. The disease was seen most commonly in the summer. Most EHEC strains were O157:H7 or O157:H− (H not able to be typed) and approximately 30% of the isolates belonged to nine other serogroups of E. coli.^119,120 HUS is characterized by acute renal failure, microangiopathic hemolytic anemia, and thrombocytopenia.

Diagnosis

Several laboratory methods are used to diagnose EHEC infections. Because most isolates of E. coli O157:H7 do not ferment d-sorbitol, screening for this pathogen usually is done with sorbitol-MacConkey agar (SMAC). Sorbitol-negative colonies can then be serotyped with commercially available O157:H7 antisera. Such colonies should be sent to a reference laboratory for confirmation.

The chances of obtaining a positive culture in stool depend on the time between the onset of symptoms and collection of the stool. Within two days of onset, virtually all stool specimens from EHEC-infected patients are positive for EHEC, whereas after seven days only one third are positive.¹²¹ In contrast, other studies have found that the median duration of excretion of EHEC is 17 to 29 days, with some patients shedding the bacterium for as long as 124 days.^29,122,123

Testing for Shiga toxin–producing E. coli (STEC) has been widely used but requires special facilities. Newer tests, including DNA probes, polymerase chain reaction (PCR), and enzyme immunoassays, can detect STEC I and II directly in stool specimens but are not in wide use.

One report detailed use of peroxidase-labeled antibody directed against whole E. coli O157:H7, and subsequent immunohistochemical staining to identify the organisms on archival paraffin block tissue sections from patients with hemorrhagic colitis and ischemic colitis.^118a Positive staining was found in three of 11 cases of ischemic colitis, suggesting an etiologic role for E. coli O157:H7 in some cases of presumably idiopathic ischemic colitis.

Treatment

The desire to treat EHEC infections is understandable because of the presence of bloody diarrhea and concern that complications such as HUS will develop. Several reports, however, have raised concern that the risk of HUS is increased by antimicrobial therapy. In a murine model, certain antibiotics, notably ciprofloxacin, caused enhanced STX production by E. coli O157:H7 in vitro via the induction of bacteriophage encoded genes; this occurrence was associated with an increased death rate in antibiotic-treated mice.¹²⁴

Antimicrobial therapy in humans does not appear to provide much benefit and might even be harmful. A randomized, controlled trial of TMP-SMX in children with E. coli O157:H7 enteritis found no effect of therapy on the duration of symptoms, pathogen excretion, or incidence of HUS.¹²⁵ One prospective cohort study identified 71 children with acute E. coli O157:H7 gastroenteritis, of whom only 9 had been treated with antibiotics. However, 5 of the 10 children in whom HUS developed had received either TMP-SMX or a cephalosporin.¹²⁶ In this study, antibiotic therapy was associated with a significantly increased risk of HUS, but this conclusion has been challenged by others.^127,128

Because antibiotic use has not been shown to decrease morbidity resulting from EHEC and might increase the risk of HUS, routine use of antibiotics is not recommended in the treatment of gastroenteritis if E. coli O157:H7 is the known or suspected cause. In cases of confirmed E. coli O157:H7 infection, patients should be followed closely for manifestations of HUS.

Thorough cooking of ground beef is an important preventive measure.

SHIGELLA SPECIES

Shigella organisms cause bacillary dysentery, a disease that has been described since early recorded history. The inhabitants of Athens in the second year of the Peloponnesian War were ravaged by dysentery. In the American Civil War, more than 1,700,000 soldiers suffered from dysentery, with 44,500 deaths. World War I also produced a high incidence of dysentery: 371,000 total casualties in France and up to 486,000 casualties in East Africa. Although dysentery is a disease that becomes more prevalent in wartime, there is a constant endemic incidence in tropical countries and in temperate zones.

Pathogenicity

All strains of Shigella cause dysentery, a term that refers to a diarrheal stool that contains an inflammatory exudate composed of PMNs, mucus, and blood. The exudative character of the stool is a point to be emphasized: This is not mere watery diarrhea but rather a loose bowel movement that contains pus. The inflammatory exudate is related to the main pathologic event: invasion of the colonic epithelium.

Humans are the only natural host for the dysentery organism. Experimental infections can be produced in monkeys and guinea pigs, and disease is made worse by starving them, feeding them antibiotics (thus altering the colonic microflora), or administering opium to reduce intestinal motility.

The major site of attack of Shigella is the colon, although scattered ulcerations can be seen in the terminal ileum as well. Invasion by Shigella is associated with a constellation of virulence factors that are related to various stages of invasion and lead eventually to death of the intestinal epithelial cell, focal ulcers, and inflammation of the lamina propria. These virulence factors are encoded by both chromosomal and plasmid genes, all of which are needed for the full expression of virulence. All virulent Shigella, as well as EIEC, contain large 120- to 140-megadalton plasmids, which are related to outer membrane proteins. Various loci encode for an invasion plasmid antigen (ipa); invasion factors (inv); and a series of vir proteins that are involved in cell regulation mechanisms within the cell.¹⁴¹ Having penetrated the mucosal surface, the organisms multiply within the epithelial cells and extend the infected area by cell-to-cell transfer of bacilli. Shigellae rarely penetrate beyond the intestinal mucosa and generally do not invade the bloodstream; however, bacteremia can occur in malnourished children and immunocompromised hosts.

The initial lesions are confined to the epithelial layer, and the local inflammatory response is severe, consisting of PMNs and macrophages. There is edema, formation of microabscesses, loss of goblet cells, degeneration of normal cellular architecture, and ulceration of mucosa. These events give rise to the characteristic clinical picture of bloody, mucopurulent diarrhea. As the disease progresses, the lamina propria is involved extensively with the inflammatory response. Crypt abscess formation is a nonspecific but prominent feature (Fig. 107-5).

Figure 107-5. Shigellosis. A, Colonoscopic view of this rectum shows luminal narrowing and mucosal inflammation similar to that seen in ulcerative colitis. B, Histopathologic features include a severe inflammatory infiltrate of polymorphonuclear neutrophils and macrophages in the mucosa. Notice that the glands are straight, without architectural distortion or branching, because the process is acute.

(A and B from Wilcox CM. Atlas of Clinical Gastrointestinal Endoscopy. Philadelphia: WB Saunders; 1995.)

Fluid and Electrolyte Therapy

Most patients with dysentery can be managed with oral rehydration. High-volume diarrhea is seen occasionally with shigellosis and can result in severe dehydration and hypovolemia. Intravenous fluid replacement is indicated in this situation and also when severe vomiting prevents oral replacement. Fluid losses can be replaced within a few hours by intravenous solutions, and oral replacement should be encouraged as soon as possible (see Table 107-6). Antidiarrheal remedies generally are unhelpful, and some believe they might even aggravate bacillary dysentery. Kaolin and pectate and other water-binding agents do not diminish stool volume or frequency.

Antimicrobial Agents

The major determinant in the decision to use antibiotics is the severity of the illness. In practice, patients with moderate or severe cases of dysentery should receive antibiotic therapy. Mild cases often pass as self-limited events, without a visit to a physician. If such cases are seen in the clinic or in the physician’s office, antibiotic therapy might not be required in light of the relatively benign course of infection. A reappraisal should be made when the culture report returns positive for Shigella. In many cases, diarrhea has already ceased. Patients with diarrhea for longer than one week should receive antibiotics.

Ampicillin was previously the preferred antibiotic, with TMP-SMX as an alternative choice; however, many strains, if not most, in the United States and abroad now are resistant to these antibiotics. The quinolone antibiotics, such as ciprofloxacin, ofloxacin, and norfloxacin, are highly active in vitro against Shigella and are the drugs of choice. Single-dose therapy with 1 g of ciprofloxacin is as effective as two doses or a five-day standard regimen in patients with Shigella infection; however, single-dose therapy proved less effective than multiple-dose regimens for patients with S. dysenteriae 1.¹⁵¹ Problems with using quinolones in the treatment of Shigella include the high cost of the drugs and concern about cartilage damage in young children. Nalidixic acid is an alternative therapeutic agent that has produced good results, although resistance develops rapidly with widespread use of this drug.¹⁵²

Because there is now increasing evidence of the skeletal safety of quinolones in children,¹⁵³ these drugs are being studied increasingly in pediatric populations. With single-dose pefloxacin therapy of infected children during an outbreak of multidrug-resistant S. dysenteriae 1 in Burundi, 91% of treated children became symptom free by day five; the remainder were substantially improved.¹⁵⁴ None of the children experienced any joint problems during the four-week period of follow-up. Similarly, a double-blind trial of pivmecillinam compared with ciprofloxacin suspension for childhood shigellosis found that ciprofloxacin resulted in clinical responses in 80% of children, with no associated arthropathy.¹⁵⁵

Although early animal and human volunteer studies indicated that the use of antimotility agents in the treatment of invasive diarrhea might lead to prolonged fever and pathogen carriage, a recent study has challenged this dictum. Treatment of dysenteric patients with a combination of the synthetic antidiarrheal agent loperamide and ciprofloxacin resulted in a significantly shortened duration of diarrhea and decreased number of stools when compared with ciprofloxacin alone.¹⁵⁶ The use of loperamide did not lead to prolonged fever or excretion of the pathogenic bacilli.

Antibiotics for shigellosis must be absorbed from the bowel to reach organisms within the intestinal wall and lamina propria, and the only effective delivery system is the bloodstream.¹⁵⁷ Nonabsorbable drugs, such as neomycin, kanamycin, paromomycin, colistin, and polymyxin, are clinically ineffective, despite in vitro sensitivity. Intravenous cefamandole also has proved disappointing. Curiously, amoxicillin, which is well absorbed and achieves higher serum levels than ampicillin, is not effective therapy for shigellosis.¹⁵⁸

Chronic carriers of Shigella are rare. Postinfection carriage generally lasts less than three to four weeks and rarely exceeds three to four months. In circumstances in which eradication of the carrier state is deemed necessary, TMP-SMX or a fluoroquinolone should be used, guided by antibiotic sensitivity results. Such treatment eliminates the carrier state in approximately 90% of patients.

Mild diarrhea and cramps can continue for days to weeks after treatment of bacillary dysentery, even when the organism is no longer present and the acute episode seems to have passed. These symptoms are not necessarily a cause for alarm, because the bowel might have sustained severe mucosal injury that requires time for repair. Approximately 10% of patients with shigellosis, however, may be left with these symptoms chronically, a condition called postinfection irritable bowel syndrome.¹⁵⁹

Shigellosis is highly contagious. Spread within a family is common. Secondary cases can occur in hospitals among other patients, nurses, and physicians. Careful hand washing and stool precautions are important to prevent dissemination of this disease.

Microbiology

Salmonella species are a group of Gram-negative bacilli, most of which are motile and produce acid and gas from glucose, mannitol, and sorbitol (except S. typhi and rare strains that produce only acid); they are active producers of hydrogen sulfide; and they are closely related to each other by somatic (O) and flagellar (H) antigens. These organisms are primarily intestinal pathogens, although some can be found in the bloodstream and internal organs of invertebrates; they are often isolated in sewage, river and sea water, and certain foods. Most Salmonella species have a wide range of hosts.

The typing scheme for Salmonella species is based on antigenic structure, but in recent years, the name of the strain has been derived from the city in which it was first isolated (e.g., Montevideo, Heidelberg, Dublin, Newport). Most salmonellae are flagellated; using the proper growth conditions, the H (flagellar) and O (somatic) antigens can be tested for separately. Some strains, notably typhoid bacilli, have an additional somatic antigen associated with virulence (Vi). The Vi antigen prevents agglutination with O antigen. A positive correlation exists between virulence in mice and the amount of Vi antigen in a specific strain; however, this correlation does not carry over completely to humans. More than 70 anti-Vi phage types have been identified.

For convenience in the laboratory, a series of serogroups, the Kauffmann-White serotypes, was developed based on shared antigens among the most common Salmonella types. Ninety percent of Salmonella species pathogenic for humans falls into groups A to E, which contain 40 serotypes. The application of newer molecular methods to the taxonomy of Salmonella has revealed that all serotypes of Salmonella belong to one species that includes seven subspecies, which can be differentiated with biochemical tests. To avoid confusion with previous nomenclature, the new species Salmonella enterica has been proposed.¹⁶⁰ Using this approach, the typhoid bacillus would be named S. enterica subspecies enterica serotype typhi. Because this lengthy name is cumbersome, however, simpler acceptable versions are S. typhi or S. enterica serotype typhi.

Epidemiology

Salmonella is one of the great food-borne infections.² The major route of passage is by the five Fs: flies, food, fingers, feces, and fomites. The disease can cause large outbreaks, which often are associated with common-source routes of spread. A typical setting is an institutional supper or barbecue. Community outbreaks can persist for several months. For example, Riverside, California, experienced an epidemic involving 16,000 persons that raged for months and was related to a contaminated municipal water supply.

Although approximately 45,000 cases of salmonellosis are reported annually, these numbers reflect vast under-reporting, and it is estimated that 1.4 million cases of Salmonella food poisoning occur each year.² The two most common serotypes in the United States are Salmonella enteritidis and Salmonella typhimurium.

Attack rates of Salmonella show a strong relationship to age. Children younger than one year have the highest attack rate, a susceptibility that may be related to immunologic immaturity. There also are high attack rates and increased mortality in elderly persons.

Nonhuman reservoirs play a crucial role in the transmission of the disease. In 500 outbreaks investigated over a 10-year period, almost 50% were related to animals or animal products. Poultry, meats, eggs, and dairy products were involved most often when a causative product was identified (Fig. 107-6).

Figure 107-6. Source of infection in 500 human salmonellosis outbreaks between 1966 and 1975. *Includes more than 50 sources that individually caused less than 3% of outbreaks.

(Redrawn from the Centers for Disease Control. Salmonella Surveillance, Annual Summary, 1976. Washington, DC: U.S. Department of Health, Education and Welfare, Public Health Service; 1977.)

Salmonellae have a tendency to colonize domestic animals. Poultry has the highest incidence of Salmonella carriage, particularly hens, chickens, and ducks. Vertical transmission via the transovarian route can occur in chickens, so even normal-appearing eggs can be contaminated with Salmonella. Pigs and cattle also may be heavily contaminated. Many of these animals can cohabit peacefully with salmonellae and usually are asymptomatic. Other animals known to harbor Salmonella include buffalo, sheep, dogs, cats, rats, mice, guinea pigs, hamsters, seals, donkeys, turkeys, doves, pigeons, parrots, sparrows, lizards, whales, tortoises, house flies, ticks, lice, fleas, and cockroaches.

Commercially prepared foods may be contaminated with salmonellae: 40% of turkeys examined in California, 50% of chickens in Massachusetts, and 20% of commercial egg whites have been shown in surveys to harbor these organisms. Large national and international outbreaks have been traced to commercially prepared chocolate balls, precooked roast beef, smoked whitefish, frozen eggs, ice cream, raw-milk cheese, alfalfa sprouts, cantaloupe, peanut butter, and powdered milk. Other commercial products not directly related to foods, such as carmine dye or brewer’s yeast, also can be contaminated. Infected pets, especially turtles and lizards, have been implicated in the transmission of salmonellosis.

Pathogenic Mechanisms

Salmonellae attack the ileum and, to a lesser extent, the colon. They cause mild mucosal ulcerations, rapidly make their way through the epithelial surface to the lamina propria and then to the lymphatics and bloodstream, and then rapidly spread to other organs hematogenously. Histologic sections show edematous, shortened crypts and PMNs in the lamina propria.

A series of pathogenic factors, each controlled by plasmids or chromosomal loci, are required for a Salmonella strain to be fully pathogenic. Specific plasmids encode for bacterial spread from Peyer’s patches to other sites in the body^161,162 or the ability to survive within macrophages after phagocytosis.¹⁶³ The outer membrane lipopolysaccharide and the Vi antigen are additional virulence factors. Another virulence factor imparts the ability of salmonellae to elicit transepithelial signaling to neutrophils,¹⁶⁴ which contribute to cell damage and secretion. Finally, Salmonella strains produce enterotoxins that can play a role in diarrhea.^161,165,166

The infectivity of a specific strain is related to its serotype and the inoculum size. For example, 10⁵ Salmonella newport produce illness in some volunteers, whereas 10⁹ Salmonella pullorum are unable to do so. The latter strain is poorly adapted to humans, as suggested by its rarity in clinical infections; it is well adapted to chickens, from which it is often isolated. A dose-response curve has been determined for certain strains of Salmonella; an approximately 50% infection rate is seen with 10⁷ organisms, whereas the infectivity rate rises to 90% with 10⁹ organisms.

In experimental animals, the number of bacteria required to produce infections can be reduced by pretreating the animals with antibiotics. Antibiotic exposure in humans also increases susceptibility to Salmonella infection.¹⁶⁷ In addition, reduced or absent gastric acid is known to increase susceptibility to infection, because acid in the stomach kills many of the challenge organisms.^140,141

Predisposing Conditions

A number of conditions increase the risk of salmonellosis (Table 107-9). The relationship between sickle cell anemia and Salmonella osteomyelitis is well known. Indeed, several forms of hemolytic anemia predispose to this infection, including malaria, bartonellosis, and louse-borne relapsing fever. The presumed mechanism of increased susceptibility is blockage of the reticuloendothelial system by macrophages that have ingested breakdown products of red blood cells, thereby reducing their ability to phagocytose salmonellae.¹⁶⁸ Patients with sickle cell anemia also have a decreased capacity to opsonize salmonellae because of defective activation of the alternative complement pathway.¹⁶⁹

Table 107-9 Conditions That Predispose to Salmonella Infection

AIDS, acquired immunodeficiency syndrome.

Neoplastic disease is associated with an increased risk of salmonellosis. Leukemia, lymphoma, and disseminated malignancy predispose patients to bloodstream invasion by this organism.¹⁷⁰ Use of glucocorticoids, chemotherapy, or radiation therapy also is associated with Salmonella sepsis. In AIDS patients, persistent Salmonella bacteremia, only temporarily yielding to antibiotic therapy, is related to the profound suppression of cell-mediated immunity.¹⁷¹ Gastric surgery appears to be an important predisposing condition in the development of Salmonella infection because destruction of the gastric acid barrier enhances the host’s susceptibility to infection.⁷ Schistosomiasis also is associated with invasive salmonellosis.¹⁷² Ulcerative colitis also can predispose to Salmonella infection and the carrier state.¹⁷³

Clinical Features

Although many serotypes of Salmonella are restricted to animals, some strains are less fastidious and can cause serious human infection. S. typhimurium causes a spectrum of disease ranging from gastroenteritis to bacteremia; S. newport causes septicemia, and S. typhi, S. paratyphi, Salmonella schottmülleri, and Salmonella hirschfeldii (the last three known formerly as S. paratyphi A, B, and C, respectively) cause enteric (typhoid) fever.

Five clinical syndromes are seen with Salmonella (Table 107-10), including gastroenteritis, noted in 75% of Salmonella infections; bacteremia, with or without gastrointestinal involvement, seen in approximately 10% of cases; typhoidal or enteric fever, seen with all typhoid and paratyphi strains and in approximately 8% of other Salmonella infections; localized infections (e.g., bones, joints, meninges, and blood vessels), seen in approximately 5%; and a carrier state in asymptomatic people (the organism usually is harbored in the gallbladder).¹⁷⁴

Table 107-10 Relative Frequencies of the Clinical Syndromes of Salmonella Infection

AIDS, acquired immunodeficiency syndrome.

The most common syndrome caused by Salmonella is gastroenteritis. The incubation period is usually six to 48 hours but can last as long as seven to 12 days. Initial symptoms are nausea and vomiting, followed by abdominal cramps and diarrhea. The diarrhea usually lasts three or four days and is accompanied by fever in approximately 50% of persons. Diarrhea can vary from a few loose stools to dysentery with grossly bloody and mucopurulent feces to a cholera-like syndrome, in patients who are hypochlorhydric or achlorhydric.^11,175 Persistent fever or specific findings on physical examination suggest bacteremia or focal infection. Salmonella bacteremia is similar to sepsis caused by other Gram-negative bacteria. Recurrent Salmonella bacteremia is seen in patients with AIDS.¹⁷¹

Once the organism invades the bloodstream, almost any organ can become involved, (e.g., meningitis, arteritis, endocarditis, osteomyelitis, wound infections, septic arthritis, and focal abscesses).¹⁷⁴

Patients can become chronic carriers (defined as persistence for longer than one year) of nontyphoidal Salmonella. The overall carrier rate is between 2 : 1000 and 6 : 1000 infected persons. Children, especially neonates, and persons older than 60 years of age become carriers more commonly than do others. Also, structural abnormalities in the biliary tract (e.g., cholelithiasis) or the urinary tract (e.g., nephrolithiasis) predispose to and perpetuate the carrier state.¹⁷⁶

Salmonella Colitis

Involvement of the colon in the course of Salmonella gastroenteritis probably is common. Although most patients with Salmonella present with mild diarrhea and watery bowel movements, colonic involvement can dominate the clinical picture; toxic megacolon and perforation due to Salmonella can occur.¹⁷⁷ Patients with Salmonella colitis typically have diarrhea for 10 to 15 days before the diagnosis is established. In contrast, patients with the usual form of gastroenteritis are symptomatic for five days or less. In the colonic form, diarrhea is more persistent, even though the organism might have disappeared from the feces on clinical presentation. Bowel movements are grossly bloody in approximately one half of the patients with Salmonella colitis. Sigmoidoscopic findings include hyperemia, granularity, friability, and ulcerations. Rectal biopsy specimens reveal mucosal ulcerations, hemorrhage, crypt abscesses, straight glands, and a PMN infiltration that involves the lamina propria. Barium enema films confirm these findings and usually show pancolitis.

In the acute period, there is no reliable method to distinguish idiopathic ulcerative colitis from Salmonella colitis, except by a positive stool culture. Any patient with an acute onset of colitis, no past history of colitis, and a duration of symptoms of three weeks or less should be considered to have infectious colitis; Salmonella, as well as EHEC, Shigella, Campylobacter, and C. difficile, are important diagnostic considerations.

The course of Salmonella colitis varies and can be as short as one week or as long as two to three months. The average duration of illness is three weeks. Complications include toxic megacolon, bleeding, and overwhelming sepsis.

It is important to recognize Salmonella colitis, so that inappropriate therapy is not administered. Glucocorticoids can exacerbate Salmonella colitis and result in silent perforation and septicemia. Finally, patients can be reassured of the self-limited course of Salmonella colitis as opposed to the chronic relapsing course of idiopathic ulcerative colitis.

Treatment

Although many antibiotics have been used to treat nontyphoidal Salmonella gastroenteritis, all have failed to alter the rate of clinical recovery. In fact, antibiotic therapy increases the frequency and duration of intestinal carriage of these organisms.¹⁷⁸ A review of 12 randomized trials found no differences in the duration of illness, diarrhea, or fever between patients treated with antibiotics and those treated with placebo.¹⁷⁹ Relapses were more common in those treated with antimicrobial agents, as were adverse drug reactions. Thus antimicrobial therapy should not be used in most cases of Salmonella gastroenteritis.

Despite this general rule, antibiotics should be used when Salmonella gastroenteritis complicates certain conditions (Table 107-11), such as lymphoproliferative disorders, malignant disease, immunosuppressed states (AIDS and congenital or acquired disorders), organ transplantation, known or suspected abnormalities of the cardiovascular system (e.g., prosthetic heart valves, vascular grafts, aneurysms, and rheumatic or congenital valvular heart disease), foreign bodies implanted in the skeletal system, hemolytic anemia, extreme ages of life, and pregnancy. In addition, patients with Salmonella gastroenteritis should be treated with antibiotics when they exhibit findings of severe sepsis including high fever, rigors, hypotension, decreased kidney function, and systemic toxicity.

Table 107-11 Indications for Antibiotic Therapy in Salmonella Gastroenteritis

AIDS, acquired immunodeficiency syndrome.

If a decision is made to initiate therapy, the choice of drug may be problematic because of high levels of antibiotic resistance to ampicillin or TMP-SMX; currently, a fluoroquinolone is the drug of first choice. For patients with strains sensitive to ampicillin or TMP-SMX, these agents can be used (Table 107-12). The quinolones, particularly ciprofloxacin, have shown good results in patients with enteric fever¹⁸⁰ and in chronic carriers.¹⁸¹ Ciprofloxacin therapy in patients with uncomplicated Salmonella gastroenteritis, however, has led to a high relapse rate that is associated with more-prolonged fecal excretion of salmonellae than seen in placebo-treated control subjects.¹⁷⁸ As might be expected, resistance to ciprofloxacin has been observed during therapy.¹⁸²

Table 107-12 Antibiotic Therapy for Bacterial Enteropathogens in Adults

Microbiology

S. typhi is biochemically similar to other Salmonella species and is distinguished primarily by its specific antigens. As a rule, this organism produces little or no gas from carbohydrates, elaborates only small amounts of hydrogen sulfide, and bears the Vi antigen on its surface. These markers should alert the laboratory to the possibility of this pathogen; confirmation of S. typhi is accomplished by serotyping.

Pathogenic Mechanisms

The pathologic events of typhoid fever are initiated in the intestinal tract after oral ingestion of typhoid bacilli.¹⁶ The organism penetrates the small intestinal mucosa and makes its way rapidly to the lymphatics, the mesenteric nodes, and the bloodstream. There is a paucity of local inflammatory findings, which explains the lack of intestinal symptoms at this early stage. This sequence of events is in marked contrast to that of other forms of salmonellosis and shigellosis, in which intestinal findings are prominent at the onset.

After the initial bacteremia, the organism is sequestered in macrophages and monocytic cells of the reticuloendothelial system. It undergoes multiplication and re-emerges several days later in recurrent waves of bacteremia, an event that initiates the symptomatic phase of infection. Now in great numbers, the organism is spread throughout the host and infects many organ sites. The intestinal tract may be seeded by direct bacteremic spread to Peyer’s patches in the terminal ileum or via drainage of contaminated bile from the gallbladder, which often harbors large numbers of organisms.

Hyperplasia of the reticuloendothelial system, including lymph nodes, liver, and spleen, is characteristic of typhoid fever. The liver contains discrete micronodular areas of necrosis surrounded by macrophages and lymphocytes. Inflammation of the gallbladder is common and can lead to acute cholecystitis. Patients with preexisting gallbladder disease have a tendency to become carriers, because the bacillus becomes intimately associated with the existing chronic infection and may be incorporated within gallstones. Lymphoid follicles in the intestine, such as Peyer’s patches, become hyperplastic and infiltrated with macrophages, lymphocytes, and red blood cells. Subsequently, a follicle may ulcerate, penetrate through the submucosa to the intestinal lumen, and discharge large numbers of typhoid bacilli. As the bowel wall progressively is involved, it becomes paper-thin (most commonly in the terminal ileum), and is susceptible to perforation into the peritoneal cavity. Erosion into blood vessels produces severe intestinal hemorrhage.

Epidemiology

Improvements in environmental sanitation have reduced the incidence of typhoid fever in industrialized nations. Approximately 400 to 500 cases occur each year in the United States, chiefly in young people. Large-scale epidemics of typhoid occur on a regular basis in developing countries and usually are traced to contaminated food that is imported from an endemic area or to contaminated water supplies.¹⁸⁵

Because S. typhi cohabits exclusively with humans, the appearance of a case could indicate the presence of a carrier. An investigation by public health authorities should be instituted to determine the source and the presence of other cases. As they are discovered, chronic carriers are registered with the health authorities, and the particular microorganism is phage-typed so that it can be traced in the event of an outbreak. Registered carriers represent only some of the potential reservoir, however, and do not take into account imported cases of typhoid, which represent more than 70% of the acute infections in the United States.¹⁸⁶

Clinical Features

In its classic form and without treatment, typhoid fever lasts about four weeks and evolves in a manner consistent with the pathologic events. The illness is described traditionally as a series of one-week stages, although this pattern may be altered in mild cases and by antibiotic treatment.¹⁸⁷ The incubation period generally is seven to 14 days, with wide variations.

During the first week, high fever, headache, and abdominal pain are common. The pulse often is slower than would be expected for the degree of fever, a finding referred to as Faget’s sign. Abdominal pain is localized to the right lower quadrant in most cases but can be diffuse. In approximately 50% of patients, there is no change in bowel habits; in fact, constipation is more common than diarrhea in children with typhoid fever. Near the end of the first week, enlargement of the spleen is noticeable, and an evanescent classic rash (rose spots) becomes manifest, most commonly on the chest.

During the second week, the fever becomes more continuous, and the patient looks sick and withdrawn. During the third week, the patient’s illness evolves into the “typhoidal state,” with disordered mentation and, in some cases, extreme toxemia. It is from this altered mental state that the term typhoid derives. In this period there is often intestinal involvement, manifested clinically by greenish pea-soup–like diarrhea and the dire complications of intestinal perforation and hemorrhage. The fourth week brings slackening of the fever and improvement in the clinical status, if the patient survives and recovers.

Typhoid fever is a less-severe illness in previously healthy adults who seek medical attention for the earliest symptoms of fever, lassitude, and headache than it is in those who wait. Prompt diagnosis and appropriate therapy interrupt the classic four-week scenario and produce an aborted illness consisting of little more than a few days of fever and malaise.

Because the typhoid bacillus is disseminated widely through recurrent waves of bacteremia, many organ sites can be involved. Patients with typhoid fever can have pneumonia, pyelonephritis, and metastatic infections of bone, large joints, and the brain. The gallbladder and liver are involved with inflammatory changes. Acute cholecystitis can occur during the initial two to three weeks, and jaundice, resulting from diffuse hepatic inflammation, has been observed in some patients.

The preeminent complications are intestinal hemorrhage and perforation.¹⁸⁸ These events are most likely to occur in the third week and during convalescence and are not related to the severity of the disease; they tend to occur in the same patient, with bleeding serving as a harbinger of possible perforation. Bleeding may be sudden and severe or a slow ooze. Before the availability of antibiotics, the incidence of hemorrhage was as high as 20%; it is less common since specific treatment has become available. Approximately 3% of patients with typhoid fever experience intestinal perforation, most commonly in the ileum.¹⁸⁹ Onset of perforation may be sudden, with signs of an acute abdomen, or there may be a leak of intraluminal contents to form an abscess in the lower quadrant or pelvis, producing a more insidious course.

After defervescence has occurred and the patient has apparently ridden through the storm, a potential for recurrence remains. Relapse generally occurs eight to 10 days after cessation of drug therapy and consists of a reenactment of the major manifestations. The organism is the same as the one that caused the original infection, with the identical antimicrobial susceptibility pattern.

Carrier State

After six weeks, approximately 50% of typhoid victims still shed organisms in their feces. This figure declines progressively with time, and after three months only 5% to 10% are excreters; by one year the frequency is 1% to 3%.¹⁹⁰ The chronic carrier is identified by positive stool cultures for S. typhi at least one year after the acute episode or, in some cases, positive stool cultures without a documented history of disease. The probability of spontaneously aborting the carrier state is highly unlikely after this time. Chronic carriers are more common in older age groups, women (a 3 : 1 ratio of women to men), and persons with biliary disease. The organism usually is harbored in the gallbladder, although occasionally it is carried in the large intestine without involvement of the biliary tract.

Diagnosis

The diagnosis of typhoid fever is established by isolating the organism. Blood culture is the primary diagnostic test, is positive in 90% of patients during the first week, and remains positive for several weeks thereafter if the patient is not treated. Bone marrow culture also has a high yield, even in treated patients.¹⁹¹ Stool cultures become positive in the second and third weeks. Sampling duodenal contents by a string test yields a positive culture in 70% of patients. By the third week, urine cultures reveal the organism in approximately 25% of patients. The titer of agglutinins against somatic (O) antigen (Widal test) rises during the second and third weeks of illness. A serum O titer of 1 : 80 or more in a nonimmunized person is suggestive of typhoid fever, and a titer of 1 : 320 usually is diagnostic in the appropriate clinical setting; a fourfold rise in titer provides stronger evidence. Although the H antigen is less specific than the O titer and is likely to be elevated from prior immunization or by infection with other enteric bacteria, an initial H antigen titer of 1 : 640 is strongly suggestive of typhoid fever. There are many false-positive and occasional false-negative Widal reactions, so that diagnosis based on a rise in titer alone is tenuous. Other serologic tests have become available and permit rapid diagnosis of typhoid fever with a higher sensitivity and specificity relative to blood culture than the Widal test; PCR assays for the diagnosis of S. typhi have been developed.

Treatment

Drug resistance, mediated by plasmids, occurs among typhoid bacilli. Most strains are susceptible to chloramphenicol and ampicillin, although notable epidemics with strains resistant to either or both of these drugs have been reported in recent years. Hence, a great effort should be made in each case to isolate the organism and perform drug susceptibility tests. Antibiotic treatment is listed in Table 107-12.

Chloramphenicol has high activity against most clinical isolates of typhoid bacilli. The response to therapy is remarkably constant, and defervescence regularly occurs three to five days after treatment is begun.¹⁸⁸ The clinical condition improves within one to two days, with decreased toxemia and slowly declining fever. In adults, chloramphenicol should be given in a total daily dose of 2 g, administered in four equally divided doses by mouth. Occasionally, in very sick patients it may be necessary to give the drug by the intravenous route in the same total daily dose. Oral medication can be given upon clinical improvement. Chloramphenicol is well absorbed from the intestinal tract but is rather poorly absorbed from intramuscular sites; the intramuscular route is to be avoided. The duration of treatment is two weeks; prolongation of this treatment does not reduce the frequency of complications or carriers. Intestinal perforation and hemorrhage can occur during apparently successful treatment. Relapse can follow an otherwise uneventful course and should be treated with the same drug as was used initially.

Ampicillin has been recommended as alternative therapy but it has been disappointing in comparison with chloramphenicol.^187,192 Amoxicillin, a closely related drug, provides better absorption and increased efficacy. Several studies have shown that amoxicillin, in doses of 4 g/day in four divided doses, has good activity. TMP-SMX also has been used with good results in typhoid fever.

The advent of plasmid-mediated multidrug resistance and newer potentially more effective antimicrobial agents, such as the quinolones and third-generation cephalosporins, has led to a reevaluation of the treatment of typhoid fever. The fluoroquinolones, ciprofloxacin and ofloxacin, have been found to be highly effective therapy for infections caused by multidrug-resistant S. typhi and S. paratyphi. Long-term fecal carriage of S. typhi is rare in patients treated with quinolones. A 10- to 14-day course of a quinolone has proved highly effective for treating enteric fever, with cure rates consistently close to 100%.⁷² The only exception has been norfloxacin, which has provided slightly lower cure rates of 83% to 90%.⁷² Defervescence generally occurs within three to five days of initiating therapy.

The optimal length of fluoroquinolone therapy for typhoid fever has not been fully elucidated. A number of studies have shown that courses of therapy ranging from seven to 14 days provide a high degree of success. Courses of therapy shorter than five to six days have been associated with unacceptable levels of failure.⁷² The duration of fever before treatment, severity of infection at the time of presentation, and time to defervescence are factors that must be considered when determining the duration of fluoroquinolone therapy in a patient with enteric fever. The frequency of resistance of S. typhi to ciprofloxacin has been increasing gradually, especially in the Indian subcontinent, central Asia, and Vietnam.^185,193,194

Third-generation cephalosporins, such as cefotaxime, ceftriaxone, and cefoperazone, also have been used successfully to treat typhoid fever; courses as short as three days have been shown to be as effective as the usual 10- to 14-day regimens.^195,196 By contrast, one trial that compared a five-day course of ofloxacin with a seven-day course of cefixime found that the median fever clearance time was significantly longer and the rate of treatment failure was higher in children treated with cefixime.¹⁹⁷

Glucocorticoids are administered for severe toxemia and fever and can produce a dramatic response in patients with profound sepsis.¹⁹⁸ Glucocorticoids should be given in high doses—for example, 60 mg/day of prednisone divided into four doses—and tapered rapidly over the following three days. The wide experience with glucocorticoid treatment has failed to show any adverse effects, although the potential for masking intestinal perforation always is present. Glucocorticoids should be reserved for patients with severe toxicity.

Studies have emphasized the importance of aggressive surgical intervention in typhoid fever.^189,199,200 Indications for surgery include progressive peritoneal signs or formation of abscesses. Closure of the intestinal perforation coupled with broad-spectrum antibiotics has resulted in reduced mortality from this dreaded complication²⁰⁰; the ileum may be riddled with multiple perforations, and resection may be required.

A chronic carrier who has been discharging S. typhi for longer than one year can be treated with antimicrobials in an attempt to eliminate the infection. The quinolone antibiotics, including ciprofloxacin and norfloxacin, have become the treatment of choice in eradicating the carrier state.²⁰¹ Reappearance of the carrier state after such treatment generally is associated with gallbladder disease. Cholecystectomy eliminates the carrier state in 85% of carriers with gallstones or chronic cholecystitis, but it is recommended only for persons whose profession is incompatible with the typhoid carrier state, such as food handlers and health care providers.

Vaccines

Two types of vaccine are available currently. The acetone-inactivated injectable vaccine affords 55% to 85% protection for two to five years. A live attenuated S. typhi strain, Ty21a, which is given by mouth, produced 96% protection in an initial field trial in Alexandria, Egypt,²⁰² although subsequent studies showed less-impressive results.^203,204 Because of its low toxicity and ease of administration, this vaccine should be used for travelers to high-risk areas, particularly in the developing world.

Epidemiology

Transmission to humans appears to occur most commonly from infected animals and their food products. The reservoir for Campylobacter is enormous, because many animals can be infected, including cattle, sheep, swine, birds (poultry and others), and dogs. Most human infections are related to consumption of improperly cooked or contaminated foods. Just as for Salmonella, chicken seems to be the major source, accounting for 50% to 70% of infections.²⁰⁶

Clinical Features

The incubation period is 24 to 72 hours after organisms are ingested, but it can extend as long as 10 days. There is a wide spectrum of clinical illness, from frank dysentery to watery diarrhea to asymptomatic excretion.²⁰⁷ Diarrhea and fever are almost invariable (90%). Abdominal pain is usually present (70%), and the patient may note bloody stools (50%). Constitutional symptoms such as headache, myalgia, backache, malaise, anorexia, and vomiting are common. Stool examination suggests colitis on the basis of fecal leukocytes and occult blood.²⁰⁷ Endoscopy might reveal an inflammatory colitis (Figs. 107-7 and 107-8). The duration of illness usually is less than one week, although symptoms can persist for two weeks or more, and relapses occur in as many as 25% of patients. Prolonged carriage of Campylobacter for two to 10 weeks after the onset of illness occurs in 16% of patients.²⁰⁸

Figure 107-7. Colonoscopic appearance of Campylobacter proctitis. The proctitis is patchy, with areas of erythema and erosion (seen on the left). The rectal mucosa on the right is hyperemic, but without loss of the mucosal vascular pattern.

(From Wilcox CM. Atlas of Clinical Gastrointestinal Endoscopy. Philadelphia: WB Saunders; 1995.)

Figure 107-8. Photomicrograph of a patient with acute self-limited colitis, the cause of which could have been an infection with Campylobacter, Salmonella, Escherichia coli species, or any one of a number of other bacteria. The presence of inflammatory cells throughout the lamina propria and straight glands without architectural distortion or branching helps differentiate acute colitis from chronic colitis.

(Courtesy of Feldman’s Online Atlas, Current Medicine.)

Infections rarely are complicated by gastrointestinal hemorrhage, toxic megacolon, pancreatitis, cholecystitis, HUS, bacteremia, meningitis, and purulent arthritis.²⁰⁷ Postinfection complications include reactive arthritis, usually in patients with the HLA-B27 phenotype, Guillain-Barré syndrome,^207,209 and immunoproliferative small intestinal disease.²¹⁰

Diagnosis

Although certain clinical features suggest the diagnosis of Campylobacter rather than other pathogens, the diagnosis only can be established by culture. Features suggesting Campylobacter infection are a prodrome consisting of constitutional symptoms with coryza, headache, and generalized malaise; a prolonged, often biphasic diarrheal illness manifesting initially with diarrhea, followed by slight improvement and then by increasing severity; and many white and red blood cells on microscopic examination of the stool.

The most reliable way to diagnose Campylobacter gastroenteritis is by stool culture. A selective isolation medium containing antibiotics must be used because Campylobacter organisms grow more slowly than do other enteric bacteria; the plates are grown at 42°C under CO₂ and reduced oxygen conditions. Dark-field or phase-contrast microscopy of fresh diarrheal stool shows the organism as a curved, highly motile rod, with darting corkscrew movements.

Treatment

Antibiotic treatment is listed in Table 107-12.

Although C. jejuni is sensitive to erythromycin in vitro, three controlled therapeutic trials with this drug have shown no effect on the clinical course compared with placebo.²¹¹ One study showed some clinical benefit when the antibiotic was started within three days of the onset of symptoms. If therapy is delayed beyond four days, it yields no clinical improvement; fecal excretion of the organism, however, is reduced by erythromycin. The fluoroquinolone antibiotics, such as ciprofloxacin, also are active against these organisms.²¹²

Resistance to fluoroquinolones has been observed during the course of treatment for Campylobacter diarrhea,^213,214 and such resistance is a major problem in some parts of the developing world. A study of U.S. military personnel in Thailand found that 50% of isolates were resistant to ciprofloxacin, whereas none was resistant to azithromycin.²¹⁵ A large study of human Campylobacter isolates in Minnesota found a rise in quinolone resistance from 1.3% to 10.2% between 1992 and 1998.²¹⁶ Factors associated with quinolone resistance of Campylobacter species include foreign travel and local patterns of fluoroquinolone use, especially if these agents are used in animal husbandry.^216,217 In locales where quinolone resistance is common, azithromycin has been shown to be superior to ciprofloxacin in decreasing the excretion of Campylobacter species and equivalent in terms of reducing the duration of symptoms.²¹⁵

Mild cases of Campylobacter do not benefit from antibiotic therapy, but treatment should be given, early if possible, to patients with dysentery and those with high fever suggesting bacteremia. Because of the difficulty in making an etiologic diagnosis on clinical grounds, a quinolone antibiotic should be used empirically because of its activity against Campylobacter, Shigella, and other enteric pathogens.

Epidemiology

Yersinia gastroenteritis has been reported more often in Scandinavian and other European countries than in the United States. Several epidemics have been related to the consumption of contaminated milk and ice cream. The organism can be isolated from many animals, including puppies, cats, cows, pigs, chickens, and horses. Animals, either as pets or food sources, are believed to be involved in the transmission of this disease.²²⁰

The serotypes most commonly involved in Scandinavia and Europe are O:3 and O:9, and Canada has many serotype O:3 isolates.²²¹ Most of the isolates in the United States are serotype O:8. Serogroups O:8 and O:5,27 have been responsible for most episodes of invasive disease in the United States.

Clinical Features

Several clinical syndromes have been described with Yersinia and tend to vary with the age of the patient and the underlying disease state.^218,222 Enterocolitis is the most common clinical condition and accounts for two thirds of all reported cases. This illness occurs most commonly in children younger than five years.^219,222 Presentation is nonspecific, with fever, abdominal cramps, and diarrhea, usually lasting one to three weeks. Microscopic examination of stool reveals leukocytes and red blood cells in most instances. Profuse watery diarrhea, possibly related to the enterotoxin, also can occur. Diarrhea can persist for several weeks and can raise the possibility of inflammatory bowel disease. Radiologic findings, particularly in prolonged cases, are most intense in the terminal ileum and can resemble those of Crohn’s disease²²³; most patients, however, have normal findings on endoscopy, intestinal biopsy, and barium studies.²²¹

In children older than five years, mesenteric adenitis and associated ileitis have been described. Accompanying symptoms include nausea, vomiting, and oral aphthous ulcers. Affected children often undergo a laparotomy, at which time enlarged mesenteric nodes and an ulcerated ileitis are observed. Clinically, the condition may be confused with acute appendicitis, although ultrasonography can be useful in separating these processes.²²⁴ Yersinia is less likely to cause severe disease in adults, in whom acute diarrhea may be followed two to three weeks later by joint symptoms and rash (erythema nodosum or erythema multiforme), reminiscent of Reiter’s syndrome. Reactive polyarthritis occurs in 2% of patients with yersiniosis, usually in persons seropositive for HLA-B27. Yersinia antigens can be detected in synovial fluid cells²²⁵ and in intestinal mucosal biopsies, and specific IgA antibodies are found in the blood.²²⁶

Yersinia bacteremia is a relatively uncommon condition that is seen in patients with underlying diseases such as malignancy, diabetes mellitus, anemia, and liver disease. Metastatic foci can occur in bones, joints, and lungs.

The diagnosis of yersiniosis is established by culture of stool or body fluids. Because the organism resembles coliforms and therefore is easily misdiagnosed on the culture plate, the laboratory should be advised that this infection is clinically suspected. Serologic tests have proved useful in Europe and Canada²²⁷ but have not provided much help for the cases reported in the United States.²²⁸

Treatment

Y. enterocolitica strains are susceptible to several antimicrobial agents, including chloramphenicol, gentamicin, tetracycline, TMP-SMX, and fluoroquinolones, but they are resistant to penicillins and cephalosporins. There is no substantial evidence that antibiotics alter the course of the gastrointestinal infection^218,228; indeed, the diagnosis often is established late in the course when the patient is already improving spontaneously. Antibiotics should be used in more severe intestinal infections, particularly those masquerading as appendicitis. For the chronic relapsing form of diarrhea, antibiotic therapy has not proved useful. Septicemia in the immunocompromised patient is associated with a high mortality, with no apparent benefit from antibiotics, although treatment is mandatory in this setting. Antibiotic treatment is listed in Table 107-12.