Introduction

The gastrointestinal (GI) tract is a common and easily accessible portal of entry for microbes or their toxins, with the ability to cause infection, inflammation, and/or disease. Foodborne diseases are a major concern worldwide. Contaminated food, water, and fomites, if they gain access through the fecal–oral route, all are capable of infecting the gastrointestinal system. Moreover, microbial infections and diseases of the gastrointestinal tract are the second most common cause of illnesses in the United States, with respiratory illnesses being the most common. A summary of organisms causing disease of the digestive system is provided in Table 12.1.

Normal flora Dental caries Brushing, flossing, fluoride; limit sugars Treponema sp. Contaminated food or water; poor oral/dental hygiene Fever, bleeding and painful gums, foul odor (trench mouth) Metronidazole, penicillin, azithromycin Veillonella Normal flora Dental caries Penicillins Oral Cavity: Viruses Paramyxovirus family: Mumps virus Salivary and respiratory secretions Swelling of parotid glands, fever, loss of appetite, headache, damage to reproductive organs in patients past puberty Mumps vaccine for prevention; no antiviral therapy available Herpes simplex virus Direct contact, fomites, contaminated food or water Blisters/lesions of mouth or throat, fever Acyclovir Oral Cavity: Fungi Candida albicans Normal flora of lower GI tract Nausea, vomiting, pustules on tongue or in throat, skin rash, heart valve damage Amphotericin B and fluconazole Gastrointestinal Tract: Bacteria Bacillus cereus Contaminated food Nausea, diarrhea, vomiting No recommended antibiotic treatment; fluid replacement Bacteroides Normal flora Diarrhea, fever Clindamycin Campylobacter jejuni Contaminated food Fever, abdominal pain No recommended antibiotic treatment, erythromycin used in severe cases; fluid replacement Campylobacter fetus Contaminated food Bloody diarrhea No recommended antibiotic treatment; erythromycin used in severe cases; fluid replacement Clostridium perfringens Contaminated food Abdominal pain, diarrhea Electrolyte/fluid replacement Clostridium botulinum Contaminated food Loss of muscle control, slurring of speech, difficulty in swallowing and/or breathing, eventual muscle paralysis Botulism antitoxin, artificial respirator Clostridium difficile Contaminated food Colitis, nausea, vomiting Clindamycin Escherichia coli Contaminated food Nausea, diarrhea; certain strains will cause intestinal bleeding Fluid replacement; gentamicin, polymyxin; fluoroquinolone and bismuth preparations help prevent E. coli gastroenteritis Helicobacter pylori Contaminated food or water Abdominal pain, vomiting, belching, bleeding if ulcers form Combination of two antibiotics (metronidazole, tetracycline, amoxicillin, or clarithromycin) plus medication to reduce or suppress stomach acid Salmonella enteritidis Contaminated food Fever, nausea, cramps, diarrhea No recommended antibiotic treatment unless it invades other tissue or blood; fluid replacement Salmonella typhi Contaminated food Fever, continual headache, nausea, cramps, diarrhea Vaccine available Contaminated food Nausea, cramps, intestinal bleeding (S. dysenteriae only), diarrhea Ampicillin, co-trimoxazole; fluid replacement Staphylococcus aureus Contaminated food, direct contact Nausea, cramps, diarrhea   Vibrio cholerae Contaminated food or water Vomiting, muscle cramps, severe watery diarrhea Preventive vaccine; rapid fluid and electrolyte replacement Gastrointestinal Tract: Viruses Rotaviruses Contaminated food or water Vomiting, slight fever, watery diarrhea No antiviral treatment; fluid replacement Hepatitis A, E Contaminated food or water Nausea, abdominal pain, dark urine, jaundice Vaccine for type A Norwalk virus Contaminated food or water Nausea, vomiting No vaccine, no antiviral treatment Gastrointestinal Tract: Fungi Aspergillus flavus Contaminated food, usually peanuts Liver damage (cirrhosis) Amphotericin B, voriconazole Claviceps purpurea Contaminated food, usually cereal grains or mushrooms Reduced blood flow to extremities, causing gangrene; hallucinations, muscle spasms, and seizures Nifedipine for spasms, seizures—drug detoxification protocol Gastrointestinal Tract: Protozoa Balantidium coli Contaminated food or water Nausea, weakness, weight loss, abdominal pain, diarrhea, vomiting Tetracycline, iodoquinol, metronidazole Cryptosporidium parvum Contaminated food or water, oral–fecal contamination Nausea, weakness, weight loss, abdominal pain, persistent diarrhea, vomiting Spiramycin, nitazoxanide for children from 1 to 11 yr old Entamoeba histolytica Contaminated food or water Nausea, weakness, weight loss, abdominal pain, diarrhea, vomiting, bloody stool Metronidazole plus iodoquinol, paromomycin Giardia lamblia Contaminated water Malaise, nausea, weakness, weight loss, cramps, diarrhea Metronidazole, furazolidone, tinidazole, quinacrine Gastrointestinal Tract: Helminths Taenia saginata (tapeworm) Ingestion of contaminated meat Vague abdominal discomfort, few symptoms if any, anemia Niclosamide (praziquantel, paromomycin, or quinacrine) Echinococcus granulosus Ingestion of eggs of organism found in soil, dust, food, water, clothing, etc. Formation of hydatid cysts Surgical removal of cysts; mebendazole, albendazole Enterobius vermicularis (pinworm) Ingestion of eggs of organism found in soil, dust, food, water, clothing, air, etc. Itching of anal region, restlessness, irritability, poor sleep, nervousness Pyrantel pamoate, mebendazole Necator americanus (hookworm) Contact with contaminated soil Usually no symptoms, sometimes a cough, shortness of breath, diarrhea, nausea, vomiting; anemia can result in mental retardation in children Mebendazole, pyrantel pamoate Ascaris lumbricoides Ingestion of eggs of organism found in soil, dust, food, water, clothing, etc. Usually no symptoms, occasional bouts with fever, trouble breathing, coughing, and wheezing Mebendazole, piperazine Trichinella spiralis Ingestion of contaminated meat Abdominal pain, diarrhea, fever, muscle pain, swelling around the eyes, rash Mebendazole, steroids with thiabendazole Trichuris trichiura (whipworm) Fecal–oral route Small numbers—asymptomatic; large numbers—abdominal pain, weakness, bloody diarrhea, weight loss Mebendazole

Resident Microbial Flora

The GI tract resident (normal) microbial flora is a complex and diverse ecosystem containing a large collection of microorganisms that flourish in the nutrient-rich environment provided by the digestive system. The type and amount of flora vary within the different areas of the GI tract, with increasing numbers as one moves from the upper GI tract areas to the large intestine. The normal flora of the intestine may prevent infection by competing or interfering with pathogens or potentially pathogenic organisms. Treatment with antibiotics that upset the balance of the normal flora may favor infections by exogenous pathogens and also the overgrowth of endogenous pathogens.

Oral Cavity

The normal microbial flora of the oral cavity is complex, with bacteria representing the largest population. More than 700 bacterial species have been detected in the adult oral cavity, but more than 50% of these have not been cultured in the laboratory at this time. At birth the oral cavity is sterile but rapidly becomes colonized by organisms from the environment, especially from the mother during the first breastfeedings. At this time Streptococcus salivarius is dominant, but the complexity of the flora continues to increase with time. Oral bacteria include streptococci, lactobacilli, staphylococci, corynebacteria, aerobes, Bacteroides, various types of spirochetes, and anaerobes.

Stomach

The stomach contains few bacteria because of its high acidity, which makes it a relatively hostile environment for bacteria. Bacteria swallowed with food and those dislodged from the mouth will be drastically lowered in number because of this acidity. However, some bacteria do survive this extreme environment and are moved on into the small intestine. It is all about numbers: the more bacteria are ingested with food, the more will survive and be transported into the next part of the GI tract.

One organism that has been discovered to live in the human stomach is Helicobacter pylori, a gram-negative, microaerophilic, spiral-shaped bacterium (Figure 12.2). The organism is highly motile due to its flagella and moves through the stomach lumen until it burrows into the stomach’s mucosa to a depth where the pH is essentially neutral. It is estimated that about 30% to 50% of the earth’s population is colonized by H. pylori. Although colonization of the bacterium in the stomach is not necessarily a problem, H. pylori is the cause of most cases of gastritis and peptic ulcers (see Bacterial Infections later in this chapter).

Small Intestine

The small intestine provides a friendlier environment for microbial survival than does the stomach. However, because of rapid peristalsis and emptying of pancreatic juice and bile into the first portion of the duodenum, it is difficult for bacteria to colonize the small intestine because they get washed out. As a result the bacterial population, especially in the first portions of the small intestine, remains relatively low. Gram-positive bacteria, mainly lactobacilli and Enterococcus faecalis, are inhabitants of this part of the GI tract. The bacterial population increases at the end of the small intestine, and enteric rods and Bacteroides can also colonize this part of the GI tract. Intestinal bacteria are capable of metabolizing some compounds that the human digestive tract cannot, which leads to more efficient utilization of food.

Large Intestine

The flow rate in the large intestine is much slower than in the small intestine and bacteria have time to colonize and reproduce, reaching a high concentration. About 1000 species are present including enteric rods, streptococci, clostridia, and lactobacilli. The predominant species are anaerobic Bacteroides and bifidobacteria, which greatly outnumber Escherichia coli. The colon serves as a holding tank for bacteria that participate in the last stages of food digestion. Intestinal bacteria can metabolize polysaccharides that cannot be broken down by human enzymes (e.g., cellulose) and they also synthesize vitamins K and B, which then can be absorbed by the GI tract. Approximately 20% of the feces of a healthy person consists of bacteria, most of which come from the colon.

Dental Caries

The oral cavity provides a good environment for a variety of microorganisms to flourish. Teeth, the tongue, and the salivary glands are accessory organs of digestion in this area. Microbial life on teeth was first observed by van Leeuwenhoek (see Chapter 1, Scope of Microbiology), who noticed that even after washing his teeth with vinegar, only microbes on the outer layer were killed, not those in the deeper layers.

The enameled surface of the teeth (Figure 12.3) is hard and does not shed cells, which allows microorganism to attach and form a biofilm known as dental plaque. If not removed on a regular basis it can lead to cavities (caries) or other periodontal problems such as gingivitis (inflammation of the gums). The microorganisms responsible for the formation of dental plaque (Figure 12.4) are almost entirely bacteria, normally present in the oral cavity. Under usual circumstances, these bacteria (e.g., fusobacteria and actinomycetes) do not cause damage; however, failure to remove these biofilms by regular brushing of the teeth will cause the organisms closest to the tooth surface to change to anaerobic respiration. Anaerobic respiration of bacteria converts sucrose and other carbohydrates into lactic acid, which consequently leads to the demineralization of the adjacent tooth surface, resulting in dental caries.

Periodontal Disease

Periodontal diseases include gingivitis and periodontitis, serious infections that if not treated can lead to decay, subsequent tooth loss, and systemic infection. Although the main cause of periodontal disease is dental plaque and an overpopulation of established oral bacteria, other factors also affect the health of the oral cavity. These factors include the following:

If periodontal disease is restricted to the gums, the inflammation is called gingivitis and is characterized by bleeding of the gums, seen predominantly during brushing of the teeth. This condition can progress to a chronic condition and is then referred to as periodontitis. Although this condition usually does not cause much discomfort, with the increase in older people who wish to retain their own teeth, it has become a more common condition of concern. If the health of the gums continues to be untreated, both tissue and bone that support the teeth deteriorate, eventually leading to the loss of teeth. A more serious condition is acute necrotizing ulcerative gingivitis, which causes enough pain to make normal chewing an unpleasant, difficult task.

Gastroenteritis

Gastroenteritis is the term used to describe the inflammation of the stomach and intestine, caused by microorganisms or ingestion of chemical toxins. This condition is often referred to as “stomach flu”; however, it has absolutely no relation to influenza (flu), which is a respiratory tract infection. Gastroenteritis transpires all over the world and affects people of every age, race, and socioeconomic status, and is a leading cause of death among children of underdeveloped countries. Intestinal infections have a tendency to flourish wherever people congregate, such as schools, dormitories, and other living or working areas with high population density.

Although gastroenteritis causes discomfort and inconvenience, it is generally not serious in a healthy adult. At-risk groups include children in child care centers and the elderly living in nursing homes, as well as people with chronic diseases and/or a compromised immune system. In theses cases gastroenteritis can be life-threatening due to dehydration and electrolyte imbalance.

The main symptom of gastroenteritis is mild to severe diarrhea, but it also may include loss of appetite, nausea, vomiting, cramps, and other abdominal discomfort. On occasion muscle aches, headache, and a low-grade fever may also occur. Causes of gastroenteritis include, but are not limited to:

The infectious form of gastroenteritis can be transmitted from person to person, especially if someone with diarrhea does not thoroughly wash their hands after a bowel movement. Transmission also can occur through sharing of eating utensils, towels, or through contaminated food or water. Gastroenteritis can also be obtained through the ingestion of bacterial toxins, which results in toxemia and does not involve an active infection.

Bacterial Infections/Illnesses

Bacteria are the most common cause of foodborne illness, which can be caused by the bacteria themselves, their toxins, or both. A bacterial infection occurs when a pathogen enters the GI tract, adheres, and multiplies (see Chapter 9, Infection and Disease). A bacterial intoxication occurs when toxins produced by bacteria contaminate food or water and then are introduced to the human body.

Bacterial Infections

The term infection implies the invasion and colonization of cells or tissues by a microorganism. Therefore, discussions of the following conditions involve illnesses due to infection of the gastrointestinal system with specific bacteria.

Helicobacter Peptic Ulcer

Peptic ulcers are a common illness in the United States, and 1 in 10 Americans will experience an ulcer at some time during their lifetime. Ulcers are a form of damage to the gastrointestinal mucosa caused by long-term use of antiinflammatory agents; conditions such as cancer of the esophagus, stomach, pancreas; or by a bacterium, Helicobacter pylori. This bacterium is helical, microaerophilic, motile, and gram negative. Many scientists believe that the majority of peptic ulcers are caused by this organism. Although most infected people do not develop an ulcer, others do. The transmission of H. pylori is not fully understood, but food and water, as well as kissing, are likely to be responsible. Unlike other organisms, H. pylori can survive in the acidic environment of the stomach because it produces an enzyme that can neutralize acids, and allows the bacterium to attach to the intestinal mucosa, damage it, and cause an ulcer by allowing the stomach acids access to the underlying tissue. The symptoms of a peptic ulcer include weight loss, poor appetite, bloating, burping, nausea, and vomiting. Ulcers caused by H. pylori can be treated with antibiotics, H₂ blockers, proton pump inhibitors, and stomach-lining protector drugs.

Salmonellosis

Salmonellosis is a gastroenteritis caused by Salmonella species, which belong to the family Enterobacteriaceae. They are rod-shaped, gram-negative, facultative anaerobic, nonsporing microorganisms, and their normal habitat is the intestinal tracts of many animals, including humans. Salmonella species cause typhoid fever, paratyphoid fever, and foodborne illnesses in humans. Because of the complicated taxonomy of Salmonella, the disease-causing species have been reclassified into a single species, Salmonella enterica, which has numerous serovars (serotypes). Therefore, instead of the conventional name of Salmonella typhi the serotype is now often referred to as Salmonella enterica serotype Typhi. From the pathological standpoint Salmonella can be divided into serotypes that are typhoidal and nontyphoidal.

Most people infected with Salmonella develop diarrhea, fever, and abdominal cramps 12 to 72 hours after infection. The incubation time and severity of resulting illness are proportional to the number of organisms ingested. The illness usually subsides within 5 to 7 days and most people do not require treatment. However, if dehydration occurs because of severe diarrhea, the patient may need to be hospitalized, and if the infection spreads to the bloodstream prompt antibiotic treatment is necessary to prevent septicemia and death. Infants, the elderly, people with chronic disease, and immunocompromised patients are more likely to develop a severe illness. Although the infection is usually treatable, the World Health Organization (WHO, Geneva, Switzerland) has pointed out an increase in the incidence of antibiotic-resistant strains of Salmonella, potentially caused by the long-term use of antibiotics in both the poultry and beef industries.

Salmonellosis is usually caused by consumption of foods of animal origin that have been contaminated with Salmonella, such as raw or undercooked eggs, poultry, untreated milk products, and meat or other items contaminated with feces. People with salmonellosis should avoid food preparation until they no longer carry the bacterium. Preventive measures include, but are not limited to:

• Avoiding raw or undercooked eggs, poultry, or meat

• Careful monitoring of homemade foods that include raw eggs (e.g., hollandaise sauce, salad dressings, ice cream, mayonnaise, cookie dough, and frostings)

• No consumption of nonpasteurized dairy products

• Avoidance of any chance of cross-contamination between raw and cooked foods

• Separation of uncooked meats from produce, cooked foods, and ready-to-eat foods

• Appropriate washing of hands, cutting boards, countertops, knives, and other utensils after handling uncooked foods

• Washing of hands before and after handling food

• Hand washing after handling reptiles or birds, or after contact with pet feces

According to the Centers for Disease Control and Prevention (CDC, Atlanta, GA), approximately 40,000 cases of salmonellosis are reported annually in the United States alone. But because the milder cases are usually not reported or diagnosed, the actual number of infections may be 30 or more times greater than 40,000. Seasonally, salmonellosis is more common in the summer months than in winter. Also, children are at greater risk of contracting the illness.

MEDICAL HIGHLIGHTS

Salmonella: More than 2000 “Flavors”

The genus Salmonella includes a number of pathogenic organisms that have become a serious problem for the food industry. In 2004, almost 40,000 cases of nontyphoid Salmonella infections were reported in the United States alone, along with an estimated 600 deaths. Some health experts estimate that the number exceeds 2 million cases of salmonellosis, as many cases go unreported. The most common sources for these types of infections are poultry products, eggs, dairy products, and other food types that are prepared on work surfaces where uncooked poultry has also been prepared. Although there are four different forms of Salmonella infections, the most common form of salmonellosis is gastroenteritis.

According to most taxonomic classification schemes there are only two or three species of Salmonella; however, there are a large number of serotypes within each subspecies. Moreover, within the species containing most of the human pathogens, there are presently more than 2000 unique serotypes! When not causing gastroenteritis, the pathogenic serovars also can cause bacteremia, particularly in the very young, the very old, or in otherwise immunocompromised individuals.

Typhoid Fever

Typhoid fever is an acute, life-threatening illness caused by the most virulent serotype of Salmonella enterica, serotype Typhi, a rod-shaped, flagellated, gram-negative bacterium. Typhoid fever is transmitted through contaminated food or water, or through close contact with an infected person; it is not found in animals, and is transmitted only via human feces. Salmonella enterica serotype Typhi lives only in humans and persons with typhoid fever carry the bacteria in their bloodstream and intestinal tract. Some people exposed to and infected by the organism do not develop the disease, or might have a minor illness and subsequently recover and thus only be carriers of the disease (see Medical Highlights: Asymptomatic Disease Carriers: Typhoid Mary). Typhoid fever is more common in areas of the world where hand washing is not common, and where water is contaminated with human waste.

Symptoms of typhoid fever include a sudden onset of sustained fever as high as 39° C to 40° C (103° F to 104° F), severe headache, nausea, abdominal pain, either constipation or diarrhea, and occasionally is accompanied by hoarse cough. Antimicrobial therapy shortens the clinical course of typhoid fever and also reduces the risk of death, but antibiotic resistance is a frequent problem as with so many other microbes.

Although typhoid fever is rare in industrialized countries, it remains a serious health threat in the underdeveloped world. According to the CDC, worldwide an estimated 22 million cases of typhoid fever and 200,000 deaths occur each year. Reported cases in the United States are mostly from recent international travelers. At greatest risk are travelers to South Asia and developing countries in Asia, Africa, the Caribbean, and Central and South America. Visitors to South Asia are at the highest risk for infections that are multidrug resistant. Although typhoid vaccination is available it is not required for international travel, but recommended by the CDC for travelers to areas where there is a recognized risk of exposure to the organism.

Paratyphoid fever, also known as enteric fever, is similar to typhoid fever, but generally milder. It is caused by Salmonella enterica serovars Paratyphi. Three species of Salmonella are responsible for this illness: serovars A, B, and C, all of which are transmitted by means of contaminated water or food. Although the agent of typhoid fever is carried only by humans, paratyphoid fever can be transmitted from animals or animal products to humans, or from person to person. Enteric fever is rare in the United States and of the reported cases approximately 60% are acquired during travel to Mexico, India, or South America.

MEDICAL HIGHLIGHTS

Asymptomatic Disease Carriers: Typhoid Mary

Mary Mallon (September 23, 1869—November 11, 1938), a hard-working Irish immigrant, was born in County Tyrone, Ireland in poverty and worked her way up the ladder in the United States to a top-salaried position as chief household cook in the New York City area between the years 1900 and 1907. Because of prevalent prejudicial attitudes against the Irish at that time, working class immigrants in other household labor positions were paid barely a living wage. Although she was successful in her job, she was an unwitting asymptomatic carrier of typhoid fever, infecting 22 people, 1 of whom died during this period of time. During her lifetime as a cook and a healthy carrier of typhoid fever, it is estimated that she infected 53 people, 3 of whom died. She simply could not accept that she—a healthy, hard-working Irish girl who had no known history of illness could possibly be an asymptomatic carrier of a disease and refused to be tested when confronted by health authorities. Her stubborn, steadfast denials led to forced quarantines. Even her attempt to change her name to Mary Brown, so that she could continue to work in the higher salaried position of domestic cook, was unsuccessful. Eventually she was seized by the public health authorities and quarantined for life on North Brother Island outside New York City. Mallon died at the age of 69 years from pneumonia some 6 years after a paralytic stroke. Typhoid bacteria were identified in her gallbladder at autopsy.

Today, any asymptomatic carrier is referred to as a “Typhoid Mary”; a more recent example is the American Andrew Speaker, suspected as being a carrier of the drug-resistant XDR-TB tuberculosis strain. Mr Speaker flew on a commercial aircraft to Europe and back and was placed in quarantine upon his return. This was later found to be a less dangerous, less drug-resistant MDR-TB strain.

Shigellosis

Shigellosis, also called bacillary dysentery, is an infectious disease caused by four species of Shigella: boydii, dysenteriae, flexneri, and sonnei. These bacteria are gram-negative, non–spore-forming, rod-shaped bacteria present only in the gastrointestinal tracts of humans, apes, and monkeys.

The infective dose is small, as the organism is somewhat resistant to the pH of the stomach. Once in the pH environment of the small intestine the organisms proliferate rapidly and continue down the GI tract to adhere to epithelial cells of the large intestine, the primary site of action for the illness. Shigella then multiplies in the cells, spreads to neighboring cells, and produces Shiga toxin that destroys tissues, resulting in dysentery.

The infection causes diarrhea that is often bloody, fever, and stomach cramps starting within a day or two of exposure. Five to 7 days after onset, the infection usually subsides. As with other infectious diseases, the very young, the very old, and immunocompromised people are at highest risk. With children less than 2 years of age, the infection may also be associated with febrile seizures (seizures due to high fever). Many different organisms cause diarrhea and bloody diarrhea; therefore it is essential to perform laboratory tests for a definite diagnosis, before selecting the appropriate antibiotic for treatment.

The CDC monitors the frequency of occurrence of Shigella infections in the country, and assists local and state health departments investigating outbreaks, to determine means of transmission and to implement control measures. An estimated 18,000 cases of shigellosis occur annually in the United States and are responsible for less than 10% of reported outbreaks of foodborne illness. Many of the mild cases of shigellosis remain undiagnosed and are not reported. The actual number of infections is likely to be much greater. The most common species reported in the United States is S. sonnei, which causes relatively mild dysentery. In underdeveloped countries this infection is far more common and is present in the communities most of the time.

Shigella is passed from one infected person to the next, by stool or soiled fingers of one person to the mouth of another. This type of transmission occurs when basic hygiene and hand-washing habits are inadequate, and is particularly common among toddlers who are not fully toilet-trained. Shigella can also be transmitted through food as a result of unsanitary handling by people preparing food.

If many cases of Shigella occur at the same time it may be due to problems in a restaurant or in the food or water supply in that given area. Any type of community-wide outbreak needs to be addressed by the city, county, and state public health departments, to ensure the safety of the water and food supplies.

Campylobacteriosis

Campylobacteriosis in humans is usually caused by Campylobacter jejuni, C. fetus, and C. coli. These bacteria are vibrioid or helical, gram-negative, motile microaerophiles. According to the CDC, C. jejuni is the leading cause of bacterial diarrhea in the United States, affecting primarily children below the age of 5 years and also young adults between the ages of 15 to 29 years. Worldwide the bacterium is blamed for causing between 5% and 14% of all diarrheal illness. The illness usually lasts for 2 to 5 days but has been reported to last as long as 10 days.

Transmission can occur by handling raw poultry, eating undercooked poultry, drinking nonchlorinated water or raw milk, or handling human or animal feces without following with appropriate hand-washing procedures. In addition to poultry and cattle waste as sources of the bacteria, feces from puppies, kittens, and birds can also contain this microorganism.

As with so many gastrointestinal illnesses caused by microorganisms, the infection is self-limiting and does not require special treatment. However, in the more serious cases antibiotic treatment can be provided. Only a few people will develop complications to the infection.

Escherichia spp. Gastroenteritis

Escherichia coli is a member of the normal intestinal flora and is normally harmless or useful by suppressing the growth of pathogenic organisms. E. coli also synthesizes vitamins that are absorbed by the human large intestine. However, E. coli does have pathogenic strains. These strains have specialized fimbriae that enable the organism to adhere to the intestinal epithelial cells and disrupt the microvilli. Once the strain colonizes the small intestine, it produces enterotoxins responsible for the illness. Intestinal disease caused by E. coli include enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), enteropathogenic E. coli (EPEC), enteroaggregative E. coli (EAggEC), and enterohemorrhagic E. coli (EHEC).

• Enterotoxigenic E. coli (ETEC) is the cause of diarrhea in infants and travelers in underdeveloped countries or areas where sanitation is poor. It is generally acquired by ingestion of contaminated food and water and the organism colonizes the small intestine. The illness can vary from minor discomfort to severe, cholera-like symptoms. The enterotoxins produced are heat-labile (LT) and/or heat-stable (ST) toxins.

• Enteroinvasive E. coli (EIEC) penetrates and multiplies in the epithelial cells of the colon, resulting in destruction of these cells. The clinical symptoms are similar to those of Shigella dysentery (dysentery-like diarrhea) with fever. EIEC is an invasive organism but does not produce LT or ST toxins.

• Enteropathogenic E. coli (EPEC) is similar to EIEC and also does not produce LT or ST toxins, although it has been reported that the organism produces an enterotoxin similar to that of Shigella.

• Enteroaggregative E. coli (EAggEC) is accompanied by persistent diarrhea in young children. In addition to its toxin (enteroaggregative ST-like toxin) it also produces a hemolysin related to one made by strains of E. coli that commonly cause urinary tract infections.

• Enterohemorrhagic E. coli (EHEC) is currently represented by a single strain, the serotype O157:H7. This serotype produces large quantities of one or more related potent toxins, capable of producing severe damage to the intestinal lining. These toxins are closely related to the toxin produced by Shigella dysenteriae and cause diarrhea with abundant bloody discharge. It is frequently life-threatening because of its toxic effects on the kidneys (hemolytic uremia).

LIFE APPLICATION

Escherichia coli

Perhaps no other microorganism has received more attention in the media than Escherichia coli (E. coli). This fecal organism is found as part of the normal flora in the human intestine, from birth to death. Although it is a normal inhabitant of the gastrointestinal tract, E. coli has been found as a pathogen in numerous organ systems including the central nervous system, lower respiratory tract, circulatory system, and urinary tract. Of the five strains of E. coli that cause gastroenteritis, the enterohemorrhagic (EHEC) strain is the one that most commonly produces disease in developed countries. This strain is responsible for an estimated 73,000 infections and more than 60 deaths per year in the United States alone. Within the strain, the O157:H7 serotype is believed to cause the majority of E. coli EHEC cases in the United States. Hemolytic uremic syndrome (HUS) is a serious complication caused by O157:H7 and is arguably one of the most dangerous manifestations of E. coli infection. Death occurs in 3% to 5% of cases. E. coli is a difficult organism to eliminate. It has been known to survive in a dried condition on stainless steel for up to 2 months and for 2 to 5 months in natural bodies of water. However, with proper sanitation procedures, proper personal hygiene, and proper food handling and preparation, the threat from E. coli can be effectively eliminated.

Yersiniosis

Yersinia enterocolitica, the causative agent of yersiniosis, can cause a variety of symptoms depending on the age of the infected patient; those infected are most commonly the young. This organism is a gram-negative, facultatively anaerobic rod. Symptoms, typically starting 4 to 5 days after exposure, include fever, abdominal pain, and diarrhea that can last 3 weeks or longer. Although relatively infrequent, symptoms of the infection in adults and older children are sometimes confused with appendicitis, because of the right-sided abdominal pain and fever. Most infections resolve on their own, and only severe cases require antibiotic treatment. The infection is most often acquired by consuming contaminated food, unpasteurized milk, or untreated water. The preparation of raw pork intestines is particularly risky.

Listeriosis

Listeriosis is an illness caused by the microorganism Listeria monocytogenes. This gram-positive, nonsporing rod is a facultatively anaerobic intracellular pathogen that is acquired by ingesting contaminated food. The organism can spread to the bloodstream and the central nervous system (see Chapter 13, Infections of the Nervous System and Senses). It has the capacity to infect a variety of cells, and because it can spread to the circulatory system it is capable of causing infections in a number of different body systems.

The primary source of Listeria is believed to be soil and decaying plant matter; the bacterium also can be carried by many animals and may be present in animal feed as well. An estimated 2500 cases are reported annually in the United States, but much larger outbreaks are associated with contaminated food products.

In the 1980s, the U.S. government began to take measures to decrease the occurrence of listeriosis. And in 1999 an outbreak in the United States resulted in a recall of 30 million pounds of meat. At present, processed meats and dairy products are tested for the presence of Listeria monocytogenes. The U.S. Food and Drug Administration (FDA) and the U.S. Department of Agriculture Food Safety and Inspection Service (FSIS) monitor food regularly, and if Listeria is detected in processed meat and dairy products, the food shipments are halted and food recalls are ordered.

Most infections, whether an individual occurrence and/or a larger outbreak, are associated with the consumption of contaminated or unpasteurized milk; soft cheese; undercooked meat; processed meat such as deli meat, hot dogs, and canned meat; fish; poultry; unwashed vegetables; and cabbage. High-risk populations for this illness are neonates, the elderly, persons with a compromised immune system, and pregnant women because the organism is small enough to cross the placenta. The mortality rate of Listeria infections is about 20% to 30%, which is the highest among the foodborne diseases.

After consuming Listeria-contaminated food, symptoms of the infection appear anywhere from 11 to 70 days later. Symptoms include fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhea. If the infection reaches the nervous system headache, stiff neck, confusion, loss of balance, or convulsions may occur (see Chapter 13, Infections of the Nervous System and Senses). Listeriosis is treated with antibiotics; the duration of treatment depends on the host and the symptoms of the illness.

Bacterial Intoxications

Bacterial food intoxication is an illness caused by the consumption of bacterial toxins. Organisms that are capable of causing this type of toxemia include Clostridium botulinum, Staphylococcus aureus, Vibrio cholerae, Bacillus cereus, and possibly other toxin-producing Bacillus species.

Botulism

Botulism (see also Chapter 21, Pharmacology) is a rare disease in the United States and healthcare providers report an average of 110 cases per year. Although rare, this mostly foodborne illness can be fatal if not treated quickly. Four forms of botulism have been identified: (1) classic or foodborne botulism, (2) infant botulism, (3) wound botulism, and (4) inhalation botulism. The illness is caused by the toxin produced by Clostridium botulinum, an anaerobic organism that flourishes in sealed containers, and is commonly isolated from soil and water samples throughout the world. The organism produces a neurotoxin most commonly acquired by a patient through consumption of contaminated food. Foodborne botulism often can be blamed on home-canned food with low acid content, such as asparagus, green beans, beets, and corn. However, baked potatoes wrapped in aluminum foil but not kept hot have also been blamed for outbreaks. Exposure to the toxin in an aerosolized form can quickly be fatal and is a concern in the defense against its use as a bioweapon (see Bioterrorism in Chapter 24, Microorganisms in the Environment and Environmental Safety).

Symptoms of foodborne botulism generally occur within 18 to 36 hours after ingesting contaminated food, but they can occur as early as 6 hours or as long as 10 days after consumption. The patients become weak and dizzy, and early symptoms also can include blurred vision, dry mouth, constipation, and abdominal pain. With progression of the disease, weakness of the peripheral muscles develops. Death is due primarily to respiratory paralysis.

HEALTHCARE APPLICATION
Organisms Responsible for Food Poisoning

Organism	Transmission	Toxins	Symptoms
Staphylococcus aureus	Ingestion: Mayonnaise and dairy products Infective dose is 100,000 organisms per gram of food; a toxin dose of less than 1 µg in contaminated food will produce symptoms

Heat-stable enterotoxins (A, B, C, D, and E) Nausea, vomiting, retching, abdominal cramping, little or no diarrhea, no fever Bacillus cereus Ingestion: Meats and cream sauces Diarrheal: heat-labile enterotoxin Diarrhea, little vomiting, no fever Ingestion: Starchy foods Emetic: heat-stable enterotoxin Vomiting, little diarrhea, no fever Clostridium perfringens Ingestion of toxin Single heat-labile protein Diarrhea, little or no vomiting, no fever Clostridium botulinum Ingestion of toxin, or contaminated food (as in cases of infant botulism) Neurotoxic protein (neurotoxin); lethality from as little as nanogram quantities (1 ng = 10^–9 g) Botulism: A progressive, descending, flaccid paralysis of skeletal muscles, beginning at the head (i.e., ptosis of eyelids, facial muscle paralysis; death from diaphragmatic muscle paralysis)

Staphylococcal Intoxication

Any food prepared and not appropriately chilled or refrigerated after preparation is a potential source of food poisoning. Staphylococcus aureus is one of the leading causes of gastroenteritis as a result of contaminated food ingestion, with the illness being caused by the absorption of staphylococcal enterotoxins present in the food. Staphylococcus aureus does not form endospores, as do C. perfringens, C. botulinum, and B. cereus. Staphylococcus is somewhat heat resistant and even the vegetative organism can tolerate 60° C for up to half an hour. The organism also has high resistance to osmotic changes, allowing it to grow in environments in which other bacteria cannot survive, such as high salt concentrations. S. aureus is often present in the normal flora of the nasal cavity; it can also survive on the skin, can be transmitted by hands into food, and, because of its resistance to environmental changes, thrives to produce disease-causing toxins (Figure 12.5). Normally, the infective dose (see Chapter 21, Pharmacology) is reached when the population of S. aureus exceeds 100,000 bacteria per gram of food.

The onset of S. aureus food intoxication is usually rapid and in many cases acute, but it depends on the individual’s susceptibility to the toxin, the amount of contaminated food/toxin ingested, and the general health of the individual. The most common symptoms are nausea, vomiting, retching, abdominal pain, and prostration. They may also include headache, muscle cramping, and changes in blood pressure and pulse rate.

Bacillus Intoxication

Food poisoning caused by Bacillus cereus can occur when prepared foods are held without adequate refrigeration for several hours before serving. Bacillus cereus is a gram-positive, spore-forming, aerobic organism capable of producing enterotoxins. It is commonly found in soil, on vegetables, and in many raw as well as processed foods. Consumption of foods containing more than 10⁶ B. cereus per gram may result in one of two possible types of food poisoning: an emetic illness or a diarrheal illness.

Outbreaks of the illnesses have been associated with cooked meat and vegetables, boiled or fried rice, vanilla sauce, custards, soups, and raw vegetable sprouts. Moreover, raw foods of plant origin are the major source of B. cereus. It is widely distributed because the spores can survive drying and are heat resistant. Most ready-to-eat foods can contain B. cereus and require various measures to prevent growth. Strains that produce emetic toxin grow well in rice dishes and other starchy foods; strains producing the diarrheal toxin grow in a wide variety of foods, such as vegetables, salads, meats, and casseroles.

Other Bacillus species that have been implicated in food poisoning episodes are B. subtilis and B. licheniformis, both of which can produce a highly heat-stable toxin similar to the emetic type produced by B. cereus.

Cholera

One of the most serious gastrointestinal infections is an acute, diarrheal illness caused by the bacterium Vibrio cholerae. Although the infection can be mild or without symptoms, about 1 in 20 infected people develop profuse watery diarrhea, vomiting, and leg cramps. The resulting rapid loss of body fluids leads to dehydration and shock, and without treatment death can follow within hours, when approximately 10% to 15% of the total body weight is lost.

V. cholerae is a gram-negative, vibrioid or rod-shaped, facultatively anaerobic bacterium that produces cholera toxin and enterotoxin, acting on the mucosal epithelium of the small intestine, which is responsible for the characteristic massive diarrhea.

Although cholera can be life-threatening it is easily prevented by appropriate water treatment and sanitation systems and is rare in industrialized nations. Still, more than 60 countries report outbreaks of the disease each year. In the United States cholera epidemics presented a major problem in the 1800s but basically have been eliminated through the use of modern sewage and water treatment systems. However, U.S. travelers to parts of Africa, Asia, or Latin America may be exposed to the pathogen and can bring contaminated seafood back to the States, causing foodborne outbreaks.

Infections are usually acquired by ingesting contaminated water or food, although person-to-person transmission is possible. Persons infected with cholera have massive diarrhea, often described as “rice water stools,” which is loaded with the pathogen and can spread under unsanitary conditions. Any contaminated water, food washed in contaminated water, or shellfish living in contaminated water can cause an outbreak of cholera. Furthermore, V. cholerae, both toxic and nontoxic strains, are present in zooplankton of fresh, brackish, and salt water. Often coastal cholera outbreaks occur after zooplankton blooms.

The main treatment for cholera is aggressive rehydration and replacement of electrolytes. Antibiotics may play a role in reducing the duration and severity of the disease; however, drug resistance has been reported.

Viral Infections

The rotavirus and norovirus families of viruses are the leading causes of viral gastroenteritis. In addition, a number of other viruses have been implicated in outbreaks of gastroenteritis, including astroviruses, caliciviruses, and enteric adenoviruses. Viral gastroenteritis is usually a mild illness, but it can be lethal if rehydration is not maintained. As with other cases of gastroenteritis, it is transmitted by the fecal–oral route, by person-to-person contact or ingestion of contaminated foods and water.

Rotaviruses

Rotaviruses are the most common cause of infectious diarrhea in infants and children worldwide and are responsible for more than 50% of hospitalizations of children due to diarrhea. According to the CDC, approximately 55,000 children are hospitalized in the United States each year and more than 600,000 children worldwide die annually from rotavirus infections, with 80% of these occurring in underdeveloped countries.

In general, children obtain at least one rotavirus infection before the age of 5 years. Immunity after infection is incomplete, but repeat infections are less severe, and therefore adults infected with the rotavirus often do not develop symptoms but can be carriers and are able to spread the infection. The incubation period is about 2 days and the gastroenteritis lasts for 3 to 8 days, with fever and abdominal pain being common symptoms.

Several vaccines have been developed and continue to be developed to help countries in which this illness is a major problem. The Rotavirus Vaccine Program (RVP), a partnership between the WHO, PATH (Program for Appropriate Technology in Health), and the CDC, and funded by the Global Alliance for Vaccines and Immunization, has been established to help eliminate deaths from rotavirus infection.

Astroviruses and Caliciviruses

Astroviridae and Caliciviridae are positive-sense single-stranded RNA [(+) ssRNA] viruses and belong to group IV according to the Baltimore classification system. Astroviruses and caliciviruses have been isolated from birds, cats, dogs, pigs, sheep, cows, and humans. They were first described in 1975 with the help of electron microscopy during an outbreak of diarrhea. The viruses are considered to be a major cause of gastroenteritis in children and adults. The infections occur throughout the year with apparent peaks in the winter months. Infections occur worldwide and the symptoms include diarrhea, headache, malaise, and nausea; although vomiting may occur it is not frequent. The incubation period is reported to be 3 to 4 days, with symptoms lasting approximately 5 days if there are no complications.

Noroviruses

Noroviruses (family Caliciviridae), previously described as “Norwalk-like viruses,” are a group of related single-stranded RNA, nonenveloped viruses, all of which cause acute gastroenteritis in humans. In addition to diarrhea, nausea, and vomiting, these viruses may cause muscle aches, headache, fatigue, and even a low-grade fever. Noroviruses affect children and adults and often infect families as well as communities, especially in confined, crowded spaces. The illness usually occurs within 18 to 72 hours of exposure, and although symptoms have the tendency to fade within a day or two, infected persons are still contagious for at least 3 days to 2 weeks after recovery from the illness.

Adenoviruses

Adenoviruses (group I double-stranded DNA) are a group of viruses that can infect the membranes of the respiratory tract, the conjunctiva, the gastrointestinal tract, and the urinary tract. Most commonly adenoviruses cause respiratory illness; however, depending on the infecting serotype they can cause different infections and symptoms. Adenoviruses 40 and 41 are a major cause of acute viral gastroenteritis in children, and have been grouped together as enteric adenoviruses (group F). Although the epidemiologic characteristics of adenoviruses vary, they are all transmitted by direct contact, the fecal–oral route, or sometimes through contaminated water.

Hepatitis

Hepatitis is an inflammation of the liver that can be caused by a number of hepatitis viruses, namely A to E. Hepatitis A and E are transmitted by the fecal–oral route and neither one results in a chronic infection. Both infections result in gastroenteritis and can be transmitted through contaminated food or water.

• Hepatitis A, caused by the hepatitis A virus (HAV), can affect anyone and in the United States; infections can be isolated or widespread. Good personal hygiene and appropriate sanitation will curb the spread of the disease. Vaccines are available for long-term prevention among people 12 months of age and older.

• Hepatitis E is rare in the United States. It is caused by the hepatitis E virus (HEV), and is transmitted in the same way as hepatitis A.

Fungal Infections

Mycoses that affect the gastrointestinal system are generally opportunistic and typically do not affect healthy humans. Opportunistic mycoses are limited to people with a compromised immune system, but certain fungi can also cause problems in otherwise healthy people. These species include Candida albicans, Aspergillus flavus, and Claviceps purpurea.

Candidiasis

Candidiasis is an infection caused by a variety of opportunistic organisms of the genus Candida. Candida albicans is part of the normal gastrointestinal flora of about 80% of the human population, and does not usually cause any problems. Only the overgrowth of Candida due to changes in the normal environment will cause candidiasis, which can occur in many different body systems. Oral candidiasis, called “thrush,” is rare in healthy individuals, occurring in about 5% of newborns and 10% of the elderly, but is common in immunocompromised patients. Thrush presents itself as creamy white or bluish-white patches on the tongue, on the lining of the mouth, and/or in the throat (Figure 12.6). Certain medications such as antibiotics, corticosteroids, and the birth control pill may upset the normal flora of the mouth and allow Candida to overgrow. Certain medical conditions such as uncontrolled diabetes, HIV infection, cancer, dry mouth, or pregnancy may also provide a favorable condition for the development of candidiasis. Several antifungal preparations are available for the treatment of this condition (see Chapter 22, Antimicrobial Drugs).

Aspergillosis

Aspergillosis is a term used for illnesses caused by organisms in the genus Aspergillus. Aspergillus is found in soil, food, compost, agricultural buildings, and air vents of homes and offices worldwide. Aspergillus flavus is a mold common on corn and peanuts, and is one of several species of mold known to produce aflatoxin, a carcinogenic substance. Its toxic effects include acute hepatitis, immunosuppression, and hepatocellular carcinoma. Because of these toxic effects, the FDA monitors and regulates the allowable aflatoxin concentration in food and feed.

Aspergillus spp. can be opportunistic pathogens in almost all tissues of the body. They are mainly responsible for three distinct pulmonary diseases (see Chapter 11, Infections of the Respiratory System), but the systemic illness can produce abscesses in the gastrointestinal tract, especially in patients with AIDS.

Ergotism

The fungus Claviceps purpurea contaminates rye and wheat and produces substances called alkaloids. Ergotism is a result of ingestion of alkaloids (ergotamines) produced by the fungus. In excess, ergotamines can cause symptoms of hallucinations, severe gastrointestinal upset, a type of dry gangrene, and a painful burning sensation in the limbs and extremities that is called “St. Anthony’s fire.” Because of the effect of alkaloids on the central nervous system, ergotism is discussed in more detail in Chapter 13 (Infections of the Nervous System and Senses).

Parasitic Infections

Although all infectious agents of humans are parasites, parasitic diseases are often defined as those caused by protozoans or helminths, and this section focuses on protozoan and helminthic infections of the human gastrointestinal system.

Protozoans

Protozoans are unicellular eukaryotes (see Chapter 8, Eukaryotic Microorganisms) found in a wide range of habitats; most are free living and not harmful to humans. However, some are parasites and capable of causing debilitating and deadly diseases. They enter the human body either as a trophozoite, the active feeding and reproductive stage, or as a cyst, the dormant stage, becoming active under the appropriate environmental conditions.

Giardia intestinalis (Giardiasis)

Giardiasis is a common waterborne gastrointestinal disease in the United States, with the infective agents present in both drinking and recreational water. The causative agent is Giardia lamblia, also known as G. intestinalis, an organism that lives in the intestinal tracts of animals and humans worldwide. Once a human or other animal has been infected, the parasite will reside in the intestine and will pass into the feces (see Chapter 8, Eukaryotic Microorganisms). Therefore, Giardia is found in soil, food, water, or any surfaces that have been contaminated with infected feces. The symptoms of Giardia infection include diarrhea, flatulence, greasy stools, stomach cramps, and nausea, and normally begin 1 to 2 weeks after infection, lasting 2 to 6 weeks. Chronic infections lasting months to years have also been reported. Prescription drugs are available to treat giardiasis, but in addition plenty of fluid intake is necessary to counteract the dehydration due to diarrhea.

Balantidium coli

Balantidium coli is the largest protozoan parasite of humans. It is prevalent in the tropics but is also present in temperate regions. Balantidium coli lives in the cecum or colon of pigs and less commonly in monkeys. Transmission occurs by the fecal–oral route and by person-to-person contact via food handlers. Symptoms of the illness include abdominal pain, tenderness, nausea, anorexia, and bloody, watery stools. Risk factors for humans include contact with swine and/or substandard hygienic conditions. For treatment, the drug of choice is tetracycline, or metronidazole and iodoquinol as alternatives.

Entamoeba histolytica (Amebiasis)

Asymptomatic Entamoeba histolytica is present in the digestive tracts of about 10% of the human population. These carriers are found predominantly in underdeveloped countries, especially in rural areas where water and sanitation practices are deficient. When the disease develops, it can be fatal and the worldwide annual mortality is about 100,000. Infections most commonly occur through ingestion of water contaminated with human feces that contain cysts. There is no animal reservoir. Another way of transmission is by fecal contamination of hands or food or during any other activities that will allow the organism to be spread from the fecal material of one person to the GI tract of another.

Once ingested, the cysts undergo excystment in the small intestine of the new host. New trophozoites form and migrate to the large intestine to multiply. As a result, both cysts and trophozoites are released to the environment via the feces of an infected person; the trophozoites will die rapidly, but the cysts will remain infective. In as many as 90% of infections the trophozoites reencyst and the infection remains asymptomatic and self-limiting; but disease symptoms may recur. Acute amoebic colitis has a gradual onset of 1 to 2 weeks, involving abdominal pain and diarrhea. Several different antibiotics are available to treat amebiasis (see Chapter 23, Human Age and Microorganisms).

Cryptosporidiosis

This parasitic diarrheal disease cryptosporidiosis, also known as “crypto,” is caused by the protozoan Cryptosporidium and is spread via the fecal–oral route, through contaminated water, and via uncooked or cross-contaminated food. After infection the parasite lives in the intestine of the animal or human and is released with the feces (Figure 12.7). Because of the parasite’s outer shell it can survive outside the body of the host for extended periods of time. Furthermore, the organism is rather resistant to chlorine and chlorine-based disinfectants and can live for days in chlorine-treated swimming pools.

According to the CDC, cryptosporidiosis is one of the most common waterborne diseases in the United States, because the organism can be found in drinking water and recreational water bodies. In addition to watery diarrhea, the symptoms include dehydration, weight loss, stomach cramps or pain, fever, nausea, and vomiting. Some people do not develop symptoms, but in the rest, the symptoms generally begin 2 to 10 days after infection and in people with a healthy immune system will subside in 1 to 2 weeks.

Helminths

As indicated previously, helminths are not microorganisms, they are macroscopic, multicellular, eukaryotic worms found throughout the globe. What is interesting to microbiologists is the fact that their eggs and larvae are microscopic. The distinguishing traits, life cycle, and classification are described in Chapter 8 (Eukaryotic Microorganisms). This chapter highlights some of the most common human intestinal parasitic helminths.

Taeniasis

Taeniasis, the term for human tapeworm infection, is normally acquired by consuming raw or undercooked meat of infected animals. Taenia saginata is common in beef, and Taenia solium is common in pigs (Figure 12.8). The larvae present in the infected meat will then grow into an adult tapeworm in the human intestine, where it can live for years and grow very large—longer than 12 feet. Humans are the only definitive host for the two organisms (see Chapter 8) and both species are distributed worldwide. Tapeworm infestation usually does not cause any symptoms, and the infection is normally recognized by the presence of moving worm segments in the stool. Infected individuals can expose other individuals via food handling with contaminated hands, and can also self-infect if personal hygiene is deficient. Tapeworm infestation can be treated and eradication of the tapeworm is expected after treatment.

Pinworm Infection

The best known human pinworm, also known as the “seatworm,” is Enterobius vermicularis, a small, white intestinal worm (nematode) found in soil, dust, food, and water. It lives in the rectum of humans (Figure 12.9). Pinworm infections are the most common worm infections in the United States, with school-age children and preschoolers being at highest risk. The female pinworm leaves the intestines through the anus, usually during the night, and deposits the eggs on the surrounding skin. This causes itching around the anus, and disturbed sleep in the infected person. Pinworm eggs can survive up to 2 weeks on clothing, bedding, or other objects. Infection can occur after accidentally ingesting pinworm eggs from contaminated surfaces or fingers. Pinworm infections can be treated with either a prescription drug such as mebendazole, or over-the-counter drugs such as pyrantel pamoate, involving a two-dose course.

Ascariasis

Ascaris infection, or ascariasis, caused by Ascaris lumbricoides (Figure 12.10) is the most common nematode infection of humans worldwide. The infection is most common in tropical and subtropical areas but is also endemic in rural areas of the southeastern United States. Ascaris eggs are found in human feces and when soil becomes contaminated, the eggs become infectious after a few weeks. Infection occurs after accidental ingestion of infectious Ascaris eggs (see Chapter 8 [Eukaryotic Microorganisms] for its life cycle). The parasite can enter the soil when contaminated with human feces, and the eggs remain viable in the soil for long periods of time. Subsequently, vegetables and fruits grown in this soil become contaminated; when humans consume them they become infected.

Most infected people are asymptomatic, but the illness may cause slower weight gain and growth. In the United States infections are generally treated for 1 to 3 days with antiparasitic medication. The drugs most commonly used against ascariasis are mebendazole, albendazole, and pyrantel pamoate. Heavy infestation may require surgery to repair the intestinal damage and to remove the worms.

Hookworm Infections (Necatoriasis)

Necatoriasis is caused by hookworms, parasitic nematodes that live in the small intestine of mammals such as dogs, cats, and humans. The two species that are capable of causing human infections are Ancylostoma duodenale and Necator americanus. Once the adult hookworm reaches the intestine (see Chapter 8 [Eukaryotic Microorganisms] for the life cycle) it attaches itself to the villi of the intestinal wall and sucks blood from its host to obtain nutrition. A single Necator americanus can take about 30 µl of blood daily; the larger Ancylostoma duodenale can take up to 260 µl/day. Slight infections may appear asymptomatic, but others may cause abdominal discomfort, diarrhea, cramps, anorexia, and weight loss. Heavy infections generally lead to iron-deficient anemia. Hookworm can be treated when it is still on the skin, in the migrating stage, and during the intestinal stage.

Summary

Review Questions