Epidemiology

Helminth infections are estimated to affect 1 billion people in developing countries, namely, sub-Saharan Africa, Latin America, and Asia. Most commonly seen in resource-poor areas with poor sanitation, helminthiases are increasingly recognized in refugees, immigrants, rural populations, and international travelers.

Perspective

Parasite morbidity includes anemia, growth stunting, undernutrition, disfigurement, reduced work capacity, and increased susceptibility to other infections. Complications can include elephantiasis, blindness, gastroenterologic and urinary obstruction, and bladder cancer. Despite the widespread distribution of helminthic disease, research funding comprises less than 1% of global dollars spent.¹

Classification

Two major phyla are responsible for human disease: Nemathelminthes (roundworms) and Platyhelminthes (flatworms). Nematodes are categorized by their migration route. Enterobius and Trichuris are found intraintestinally, whereas filarial worms live in tissue and lymphatics. Platyhelminthes is further divided into two classes: cestodes, or tapeworms; and trematodes, or flukes.

Epidemiology

Ascariasis infects approximately 25% of the world’s population.^1,2 It is prevalent in warm countries and areas of poor sanitation and is endemic to the southern United States. Ova can remain infective for several years. They are sensitive to temperatures higher than 65° C or lower than 20° C, direct sunlight, and organic solvents. Ascaris lumbricoides reaches up to 40 cm and is characterized by a constricted area at the junction of the first and middle thirds. The golden brown ovoid eggs measure 60 by 40 mcm.

Transmission and Life Cycle

Ascariasis is transmitted by ingesting mature eggs from contaminated soil or in conjunction with human feces used as fertilizer. Infection may cross into nonendemic areas through vegetable transport and tainted water. Children frequently contract the disease by playing in dirt.

Fertilized eggs become infective in soil 3 to 4 weeks after excretion. When the eggs are swallowed, the larvae hatch in the duodenum, penetrate the intestinal wall, and migrate through the portal venous system to the liver. The larvae migrate through the right side of the heart, the lungs, and into the tracheobronchial tree. Over 2 weeks, the larvae are carried up the trachea to the larynx, where they move into the esophagus and are swallowed a second time, to reach the small intestine, where they mature. Female Ascaris worms lay eggs 2 to 3 months after ingestion and produce up to 240,000 eggs per day. The life span of Ascaris is 6 months to 1 year. Ascaris coexists with Trichuris trichiura in the United States, predominantly in the Appalachian Mountains.

Presenting Signs and Symptoms

Many patients are asymptomatic or have mild symptoms, and the infection ends after expulsion of the adult worms. Ectopic migration is possible, and it causes severe clinical manifestations. The pulmonary larval migration occurs 9 to 12 days after egg ingestion and may manifest with cough, bloody or mucoid sputum, pleurisy, and fever.

Eosinophilia develops during this pulmonary migration and may manifest as eosinophilic pneumonitis, known as Loeffler syndrome. Chest radiographs may demonstrate small, rounded infiltrates that are transient and usually resolve after several weeks. Loeffler syndrome is seen mainly in ascariasis, but it also may occur in other parasitic infections such as hookworm infestation and strongyloidiasis. Allergic reaction can occur with reinfection. Two to 3 months after ingestion, the parasite matures in the small intestine and can cause abdominal pain, nausea, vomiting, anorexia, diarrhea, constipation, volvulus, or intussusception. Biliary obstruction and intestinal inflammation can manifest as an acute surgical abdomen, mimicking acute appendicitis or cholecystitis. Liver abscesses and pancreatitis are possible when the female worm migrates up the common bile duct. Protein malnutrition and intestinal obstruction are more likely to occur in heavily infected children rather than in adults.

Diagnosis and Medical Decision Making

The diagnosis can be made from the passage of worms in the stool or by finding eggs in the feces. A single stool specimen sent for ova and parasite examination is usually sufficient. As the worm transverses the esophagus, it may be coughed out. In eosinophilic pneumonitis, larvae may be isolated from sputum or gastric aspirate.

Gastrointestinal radiographic examination may visualize adult worms on contrast studies, ultrasound scans of the pancreatic-biliary system, plain abdominal films, and endoscopic retrograde cholangiopancreatography (ERCP). ERCP can be therapeutic in extracting worms. Serologic antibody tests include complement fixation, precipitin, agar gel diffusion, immunoelectrophoresis, and the radioallergosorbent test.

Epidemiology

Hookworm infestations of Ancylostoma duodenale prevail in southern Europe, northern Asia, and North Africa, whereas Necator americanus is the main species affecting the Western Hemisphere and equatorial Africa. Worldwide, 1 billion persons are believed to be infected with hookworms.³ The frequency of infection is a general indication of the local level of hygiene and sanitation. Disease burden from hookworms results in iron-deficiency anemia and hypoproteinemia.

Transmission and Life Cycle

Hookworm infection is contracted by penetration of the skin or oral mucosa by filariform larvae. Persons who walk barefoot on contaminated soil or who eat contaminated vegetables become infected. Transplacental infection and transmammary infection are also possible. Eggs are passed in the stools to the soil, where they hatch into rhabditiform larvae that develop into filariform larvae. The larvae penetrate the skin or mucosa directly and reach the alveoli through the circulation. The larvae then ascend the airway, enter the esophagus, are swallowed, and reach the small intestine, where they mature.

Adult worms use hooks to attach to the intestinal mucosa, and they suck blood directly from the host. The time frame from skin invasion to egg appearance is 6 to 8 weeks. Ancylostoma adults live on average for 6 to 8 years, and the life span of Necator ranges from 2 to 5 years, but both species of worms can survive for more than a decade.

Presenting Signs and Symptoms

Presentation includes rash, cough, low-grade fever, abdominal pain, diarrhea, generalized weakness, weight loss, heme-positive stools, and eosinophilia. Early larval invasion of the skin causes pruritus and erythematous maculopapular or vesicular dermatitis, known as “ground itch.” Sensitized hosts may have serpiginous tracts as larvae migrate through the subcutaneous tissue, similar to cutaneous larva migrans.

Transient pneumonitis can occur as larvae travel through the lungs. Epigastric pain, inflammatory diarrhea, and eosinophilia may develop with early intestinal invasion. Loss of plasma protein results from malabsorption and increased intestinal permeability and may cause anasarca.

Chronic hookworm infection leads to iron deficiency. In malnourished patients, infection can cause severe anemia and growth delay. Infection in pregnancy may lead to low fetal birth weights and birth defects. Pica and geophagy are observed in infected children.

Diagnosis and Medical Decision Making

Multiple stool specimens for ova and parasite studies or concentration techniques may be necessary to confirm the diagnosis. The parasite burden may be estimated using the Beaver stool or Kato 50 adult slide smear method. The polymerase chain reaction (PCR) technique can identify a hookworm from a single egg.

Hookworm infection may be confused with pneumonia, anemia, and malnutrition from other causes. Hypochromic microcytic anemia, coupled with eosinophilia or hypoproteinemia, indicates a heavy infection.

Treatment

Oral iron supplementation is usually sufficient to correct mild anemia, although blood transfusion may be needed to correct more severe anemia. Vaccine development is currently under way, based on canine and sheep models.³

Epidemiology

Strongyloides stercoralis is found in the tropics, subtropics, and in temperate areas, and it affects an estimated 55 million to 100 million persons.⁴ Strongyloides is endemic to Southeast Asia, Latin America, the West Indies, Bangladesh, Pakistan, Africa, Spain, and the Appalachian region of the United States. S. stercoralis is an unusual helminth in its ability to replicate within the human host, thus resulting in continuous autoinfection. The infection is often difficult to eradicate, especially in immunocompromised hosts (Box 177.1). The larvae can disseminate and can cause systematic disease and mortality.

Transmission and Life Cycle

In addition to replication within the human host, Strongyloides can survive in a free-living cycle in soil. The rhabditiform larvae can directly transform into infective filariform larvae or a free-living soil form. The filariform larvae penetrate skin or mucosa, travel to the alveoli, and ascend the airway. The larvae are then swallowed and reach the small intestine. The host has no adult male worms, and the 2-mm female worms reproduce by parthenogenesis, hatching eggs within the intestinal mucosa. Rhabditiform larvae migrate to the intestinal lumen and either pass into the feces or directly transform into filariform larvae and cause autologous reinfection.

Presenting Signs and Symptoms

Patients may be asymptomatic or have mild abdominal discomfort, cough, dyspnea, wheezing, and peripheral eosinophilia. Ten percent of patients present with wheezing as their primary complaint.⁵ If adult worms invade the duodenojejunal mucosa, the symptoms can mimic those of peptic ulcer disease, and small bowel obstruction may occur.

Fluctuating eosinophilia is a common finding. Strongyloidiasis may also manifest with urticaria, usually recurrent along the buttocks and wrists. These urticarial serpiginous eruptions are called larva currens, and they are pathognomonic findings.

Diagnosis and Medical Decision Making

Stool examinations need to be repeated 3 to 5 times. An immunosorbent assay for aspiration or biopsy of duodenojejunal contents is also available. Serum immunoglobulin G (IgG) antibodies can be detected with enzyme-linked immunosorbent assay (ELISA). In disseminated strongyloidiasis, sputum, bronchoalveolar lavage samples, and surgical drainage fluids should be examined for larvae. Eosinophilia greater than 5% or a finding of more than 400 eosinophils/microliter is consistent with Strongyloides infection.⁵

Strongyloidiasis can mimic gastroenteritis, gastritis, colitis, irritable bowel syndrome, asthma, and pneumonia. It may also cause meningitis or sepsis.

Treatment

Strongyloidiasis must be treated to avoid hyperinfection. In patients coinfected with human T-cell lymphotropic virus-1 (HTLV-1), treatment failures are common. A regimen of ivermectin plus thiabendazole or ivermectin plus albendazole is recommended for patients with disseminated disease.

Epidemiology

Enterobiasis is worldwide in distribution and is commonly found in young children. Pinworms are most common in developed countries with temperate or colder climates. The adult worms are 6 to 12 mm in length and 0.3 to 0.5 mm in diameter. The eggs are 50 by 25 mcm and are asymmetrically flattened on one side.

Transmission and Life Cycle

Eggs are ingested orally, hatch in the stomach, and pass through to the intestine, where they invade the glandular crypts. The adult worms live in the cecum and appendix and survive for approximately 2 months. Whether pinworms cause appendicitis is not known. The female pinworm migrates out through the rectum and onto the perianal skin to deposit her eggs. Rarely, the worms invade the abdominal cavity and cause threadworm granulomas of the liver, ovary, kidney, spleen, and lung. Transmission of pinworms occurs by direct anus-to-mouth spread from contact with an infected person. It can also be caused by airborne eggs that are shaken free from contaminated clothing or bed linens. Autoinfection is common because patients scratch the pruritic anal area and then bite their nails or put their fingers in their mouth. The whole cycle takes 2 to 4 weeks.

Presenting Signs and Symptoms

Pruritus ani is the main symptom and varies from mild itching to acute pain, generally worse at night. Pinworm disease is essentially an allergic reaction to the release of eggs and other secreted materials from the gravid female. Associated scratching and excoriation can cause secondary bacterial infection. Vulvitis can occur when pinworms enter the vulva and cause a mucoid discharge with pruritus vulvae. Insomnia, restlessness, loss of appetite, weight loss, irritability, and enuresis may be associated with pinworm infection.

Diagnosis and Medical Decision Making

The eggs are rarely seen in the feces, but they are observed when the adult worms migrate to the anus or vulvar area, particularly at night. The “Scotch tape test” is done by pressing a piece of clear sticky tape against the perianal region and then mounting the tape on a slide. The eggs are identified with light microscopy. Patients usually have no eosinophilia or associated anemia.

Treatment

The entire family is treated simultaneously, to avoid reinfection. All bedding and contaminated clothing should be washed. Fingernails should be kept short. Frequent hand washing and bathing may reduce reinfection. Eradication of the parasite may necessitate repeated courses of treatment.

Epidemiology

Trichinosis has a worldwide distribution. Eight species of Trichinella infect humans. Trichinella spiralis and Trichinella pseudospiralis are found worldwide. Trichinella nativa is found in the Arctic; Trichinella nelsoni occurs in eastern Africa; Trichinella britovi is found in Europe, Asia, and western Africa; and Trichinella murrelli occurs in North America. Trichinella papuae and Trichinella zimbabwensis have also been identified. Human infection results from ingestion of poorly cooked infected meat, particularly pork. Cattle, horse, dog, and wild game meat are also potential sources.

Transmission and Life Cycle

The encysted larvae are released from their capsule by digestive enzymes in the intestine. The larvae then penetrate the intestinal mucosa and mature into adults. After mating, the male worms die, and the female worms discharge larvae to the tissues. After travelling through the circulation, the larvae encyst in muscle and may calcify. Cysts can be seen in the diaphragm, masseter and intercostal muscles, musculature of the larynx, tongue, eye, and heart, and the brain.

The main methods of infection prevention are thorough cooking and regular meat inspection. Trichinella larvae in meat may be killed by heating to a temperature of 77° C or freezing to 15° C for 3 weeks. Both encysted and free Trichinella larvae remain viable for years.

Presenting Signs and Symptoms

The presenting symptoms depend on the level of infection and the location of the larvae. Two phases are recognized: intestinal and muscular. Enteric invasion may cause diarrhea, fever, nausea, vomiting, and abdominal pain during the first week of infection and is often confused with food poisoning. During the second week, the larvae migrate into the muscles and cause hypersensitivity reactions with fever, eosinophilia, rash, headache, cough, and myalgias.

Cardinal clinical findings of trichinellosis include cachexia, edema, splinter hemorrhages, dehydration, ongoing fever, and pruritus. Myocarditis, encephalitis, nephritis, congestive heart failure, pneumonitis, hemiplegia, severe pain, psychiatric disturbances, and epilepsy may develop and lead to death.

Diagnosis and Medical Decision Making

Eosinophilia is a hallmark of trichinellosis. No relationship exists between the level of eosinophilia and the clinical course of disease. Other laboratory manifestations of trichinosis include leukocytosis and elevated creatine phosphokinase, lactate dehydrogenase, and myokinase levels. The diagnosis is made by identification of larvae in blood or tissue. Serologic assays are available, including ELISA, indirect immunofluorescence, and latex agglutination. The circulating antibody can be detected by serologic tests between 2 and 4 weeks after infection. Immunofluorescence is positive 2 to 3 weeks after infection. If serologic test results are equivocal, larvae may be found in muscle by trichinoscopy. This procedure is performed during the muscular phase approximately 7 days from onset of symptoms with samples taken from the deltoid, biceps, gastrocnemius, or pectoralis major muscle.

Trichinosis resembles many conditions: typhoid, encephalitis, myositis, and tetanus. Trichinosis may also be confused with collagen disorders such as periarteritis nodosa and rheumatoid arthritis.

Treatment

For light infections, supportive therapy with analgesics, rest, and antipyretics usually suffices. Patients with severe hypersensitivity reactions such as myositis, myocarditis, and central nervous system (CNS) disease may benefit from corticosteroids tapered over 2 to 3 weeks. Corticosteroids are not indicated for mild disease because the use of these agents can increase the number of circulating larvae.

Epidemiology

Filarial nematodes are estimated to infect 170 million persons.¹ Four main species cause most serious infections: Wucheria bancrofti, Brugia malayi, Onchocerca volvulus, and Loa loa. W. bancrofti is found in Africa, South America, Asia, the Pacific islands, and the Caribbean. Brugia species are found mainly in Southeast Asia, Asia, and Indonesia.

Transmission and Life Cycle

Mosquitoes transmit infective Wucheria and Brugia larvae during a blood meal. Onochocerca and Loa loa larvae are carried by the black and tabanid flies, respectively. The larvae mature into adults that remain alive in the lymphatics or subcutaneous tissues for years. Microfilariae are produced by the adult worms, which are ingested by mosquitoes and develop into infective larvae over a period of weeks.

Presenting Signs and Symptoms

Most infected persons remain asymptomatic, and few progress to acute or chronic disease. Common presentations include hydrocele, adenolymphangitis, microscopic hematuria and proteinuria, and lymphangiectasia. Acute adenolymphangitis is a form of retrograde lymphangitis accompanied with high fevers, lymphadenitis, and often thrombophlebitis.

W. bancrofti infection involves the genital lymphatics, and it often manifests with epididymitis and scrotal pain. Travelers to endemic regions may develop acute edema, urticaria, and local lymphadenitis, especially of the femoral, axillary, inguinal, or epitrochlear lymph nodes.

Diagnosis and Medical Decision Making

Parasite detection is key for definitive diagnosis. Blood and hydrocele fluid may demonstrate microfilariae, although blood draws should be timed based on the periodicity of the endemic species suspected. ELISA and rapid-format immunochromatographic card tests are available for W. bancrofti antigens. In addition, PCR assays for W. bancrofti and Brugia DNA are available.

High-frequency ultrasound with Doppler imaging of the breast and scrotum may reveal dilated lymphatics containing moving adult worms, known as the filarial dance sign. The broad cross-reactivity of filarial antigens with other helminths often confounds the serologic test results. In addition, endemic populations may become sensitized through mosquito exposure without actual infection.

Treatment

Wolbachia bacterial endosymbionts are present in most filarial worms, with the exception of L. loa. When patients are treated with diethylcarbamazine, a hypersensitivity reaction to the intracellular Wolbachia may occur and may progress to encephalopathy. Diehtylcarbamazine should not be used as treatment, except for Loa loa, because the hypersensitivity reaction can lead to hypotension or angioedema. Chemotherapy and vaccine development directed against Wolbachia are under investigation.⁶

Epidemiology

O. volvulus is responsible for river blindness, a leading cause of infectious blindness in Africa, South America, and the Middle East.

Transmission and Life Cycle

Blackfly vectors bite humans and transmit larvae into the skin. Nodules form in a few months to years, and the developed adults release microfilariae into the dermis. Adult worms can live for a decade or two.

Presenting Signs and Symptoms

Onchocerciasis results from inflammatory reactions to the microfilaria that can affect skin, eyes, and lymph nodes. An intensely pruritic papular rash is common. Atrophy and pigmentation changes can occur in long-term infection. Subcutaneous nodules, or onchocercomas, contain adult worms and are common over bony prominences. Conjunctivitis, uveitis, chorioretinitis, and optic atrophy result from ongoing optic inflammation; neovascularization and scarring of the cornea lead to corneal opacity and eventual blindness. Lymphadenopathy is also common.

Diagnosis and Medical Decision Making

The removal of an adult worm from onchocercomas or of microfilariae from skin snips leads to the definitive diagnosis. The Mazzotti test, involving a small dose of diethylcarbamazine (50 mg/kg) and subsequent observation for worsening rash or pruritus, can aid in diagnosis. Topical diethylcarbamazine (10% in lotion) can be applied to the skin as a localized Mazotti test; it is considered safer than an oral test. If there are Onchocerca microfilariae present in the ocular chamber, diethylcarbamazine treatment can lead to ocular damage.

Epidemiology

Taenia has a worldwide distribution and is highly endemic in Latin America, Africa, Eastern Europe, Central and South Asia, and the Middle East.⁷ Approximately 45 Taenia species and subspecies are currently recognized.⁸ T. solium infection of the CNS, or neurocysticercosis, is a leading cause of acquired epilepsy and may result in more than 50,000 deaths annually.⁹

The most distinctive feature of Taenia infection is that humans can serve as both the definitive host and the intermediary host. When humans are infected by the larval stage, the infection is known as cysticercosis. Infection with the adult tapeworm is associated with taeniasis. For T. saginata and T. solium, humans serve as the final obligatory host. Infection is uncommon in infants and vegetarians.

Transmission and Life Cycle

Humans acquire the infection by ingestion of undercooked pork containing cysticerci (larval cysts). The cysts contain protoscolices. After ingestion, the protoscolices are released and attach to the intestinal wall. Each protoscolex can become an adult tapeworm. The head of the worm generally resides in the jejunum. Adult worms contain 800 to 900 proglottids. Maturation occurs over 2 to 4 months. The mature proglottids are hermaphroditic, become gravid, and contain 1000 to 2000 eggs. These eggs are passed in the stool. Pigs acquire the infection from soil contaminated with human feces, and then larvae are activated, penetrate tissues, and encyst within 2 to 3 months.

Presenting Signs and Symptoms

Most patients are infected by a single worm. The symptoms are usually mild, or the patient may be asymptomatic. Because tapeworms can survive for years in an otherwise healthy host, symptomatic patients may have a protracted clinical course. Complaints are nonspecific and include indigestion, anorexia, diarrhea, constipation, and vague abdominal pain.

Severe signs and symptoms may include intestinal obstruction, appendicitis, and perforation. The worms occasionally migrate to the biliary system, respiratory tract, uterine cavity, or nasopharynx. If cysts are in the CNS, the patient may present with seizures, meningitis, stroke, or signs of increased intracranial pressure. Approximately 30% of patients with neurocysticercosis have residual calcification, and less than 20% have seizures.⁷ Parasite invasion of the ventricles can result in obstructive hydrocephalus. Extraparenchymal disease leads to a worse prognosis, secondary to invasive cyst growth and increased cerebrospinal fluid (CSF) debris and inflammation.⁷ Classic calcified lesions from prior infections are often seen on computed tomography (CT) or magnetic resonance imaging (MRI).

Diagnosis and Medical Decision Making

Infection is confirmed by finding the eggs or proglottids in the feces. The mobile proglottids often move with discharge, and some patients may feel their passage. Because the eggs are often eliminated intermittently, several stool specimens may need to be obtained. ELISA, DNA probes, and co-proantigen assays are available. The enzyme-linked immunoelectrotransfer blot (EITB) assay is quite specific and sensitive.

Neurocysticercosis is usually diagnosed by the characteristic CNS cystic lesions, which are nodular calcifications 5 to 20 mm in diameter, with or without ring enhancement on CT or MRI. Although calcified lesions are identified more easily on CT, MRI is more sensitive for ring enhancement and cystic lesions. The CSF may show pleocytosis, elevated protein, and occasionally low glucose concentrations.

Treatment

Calcified cysts do not warrant antiparasitic therapy.¹⁰ Patients with seizures may be treated with antiepileptics, and occasionally endoscopic surgery or ventriculoperitoneal shunting is necessary to relieve obstructive hydrocephalus. Corticosteroids may help mitigate cerebrovascular complications and may be used in the treatment of cysticercotic encephalitis.

Epidemiology

Echinococcosis is caused by the larval invasion of Echinococcus granulosus, Echinococcus multilocularis, and Echinococcus vogeli. These cestodes are ubiquitous, with particularly heavy endemicity in Central Asia, the Mediterranean, the Middle East, South America, and eastern Africa. E. multilocularis is confined to the Northern Hemisphere. E. granulosus causes cystic echinococcosis, with 3 million cases, and E. multilocularis is responsible for the alveolar variant, with half a million cases.¹¹ Canines are the definitive hosts and pass echinococcal eggs in feces. Intermediate mammalian hosts then ingest the eggs, and eventually cysts develop. The cycle is continued as canines eat infected meat.

Transmission and Life Cycle

E. granulosus has three proglottids, one of which is gravid and releases eggs. After ingestion of the parasite, embryos penetrate the intestinal mucosa and enter the portal circulation. Fluid-filled hydatid cysts are formed in organs, commonly the lungs and liver. Within these hydatid cysts, brood capsules that house new larvae develop.

Presenting Signs and Symptoms

The cysts grow over several years, and patients are usually asymptomatic until large cyst size or encroachment causes abdominal pain, palpable masses, biliary colic, or, if in the lungs, dyspnea, cough, pleurisy, or hemoptysis. Cyst rupture may provoke an anaphylactic or allergic reaction with eosinophilia, fever, pruritus, and rash. Less commonly, the cysts can invade bone, CNS, cardiac tissue, and other intraabdominal structures.

Diagnosis and Medical Decision Making

Radiographs, CT, ultrasound, and MRI can all reveal cysts. Additional testing of aspirated fluids may reveal protoscolices. In addition, immunoblotting techniques are available for serologic antigen testing.

Treatment

Treatment depends on cyst size, location, and symptoms. Ultrasound staging is recommended. Percutaneous aspiration, infusion of scolicidal agents, and reaspiration (PAIR) is less invasive and is preferred over surgical removal. If cysts are superficial, honeycombed, connected to the biliary systems, or intrapulmonary, PAIR is not recommended. For complicated cysts, surgical intervention is warranted.

Epidemiology

Diphyllobothrium species are estimated to infect more than 20 million people. Although an overall decline in North America, Europe, and Asia has been reported, outbreaks have occurred in Russia, Japan, South Korea, Italy, France, Switzerland, and South America.¹² Diphyllobothrium also enjoys a range of hosts, from humans to foxes, bears, sea birds, and other fish-eating mammals.

Transmission and Life Cycle

The fish tapeworm, or Diphyllobothrium latum, holds the dubious honor for longest tapeworm known to humankind, with adult worms measuring up to 10 to 25 m. The adult tapeworms consist of 3000 to 4000 proglottids and produce up to 1 million eggs a day; they may survive for decades in the human host. Egg-laden feces enter lakes, rivers, and deltas, and the eggs hatch into embryos.

Crustaceans, usually Cyclops or Diaptomus species, ingest the free-swimming embryos, known as coracidia. The coracidia penetrate the crustacean intestinal wall and develop into a procercoid. The crustacean host is eaten by a freshwater or marine fish, and additional development takes place. The procercoid larva migrates into fish tissue and organs and sometimes encysts. Multiple predatory fish, such as perch, pike, burbot, walleye, snook, Alaska blackfish, salmon, whitefish, trout, and Japanese anchovy have been implicated as hosts.¹² After the host ingests a meal of undercooked or raw fish, Diphyllobothrium matures in the human gastrointestinal tract within a few weeks.

Presenting Signs and Symptoms

Most Diphyllobothrium infections are asymptomatic or are characterized by mild gastrointestinal upset. Acute abdominal pain, cholecystitis, cholangitis, and intestinal obstruction are rare. Approximately 2% of infected persons demonstrate megaloblastic anemia secondary to tapeworm absorption of vitamin B₁₂. In older patients and in patients with chronic infections, neurologic manifestations of vitamin B₁₂ deficiency may arise.

Diagnosis and Medical Decision Making

Stool examination reveals eggs and segment chains. After ingestion by the human host, adult worms usually begin producing eggs within 15 to 45 days, so results of early stool studies may be negative.

Treatment

Intraduodenal Gastrografin has been used to remove large cestodes. Heating fish to 54° C for 5 minutes or freezing to −18° C for 24 hours kills Diphyllobothrium. Placing fish in a 12% brine solution kills the eggs.¹²

Epidemiology

Trematodes represent a diverse group of flatworms, or flukes, that cause human infection through invasion of blood, intestines, biliary system, and lungs. Schistosoma is perhaps the best known of the flukes. Clonorchis and Opisthorchis species cause biliary inflammation and obstruction and predispose patients to cholangiocarcinoma. Paragonimus and Fasciolopsis species cause granulomatous disease in the pulmonary and intestinal systems, respectively.

Transmission and Life Cycle

The life cycle of most trematodes involves a mammalian definitive host and intermediate snail hosts. Sexual reproduction occurs in the mammalian host, with subsequent asexual reproduction in snail hosts. Ova are excreted in feces or sputum, and they become miracidia that then undergo asexual reproduction in often multiple intermediate hosts. Free-living cercariae larvae and encysted metacercariae infect the definitive host. Transmission to humans is through ingestion or direct dermal penetration.

Epidemiology

Human schistosomiasis, also known as bilharziasis, remains a serious health threat in Africa, Southeast Asia, South America, and the Middle East.¹ Schistosomiasis is a complex of acute and chronic parasitic infections caused by digenetic blood nematodes. Infections with Schistosoma haematobium (bladder fluke), Schistosoma mansoni, Schistosoma japonicum, Schistosoma mekongi, and Schistosoma intercalatum (intestinal flukes) cause illness in humans. Schistosomes have been documented to infect humans for thousands of years and are associated with agricultural civilizations of the great river valleys. Hematuria, most likely caused by S. haematobium, occurred in ancient Egypt and Mesopotamia.¹³

Transmission and Life Cycle

The adult fluke lays eggs that leave the definitive host through the feces or urine, depending on the species and the host location. A freshwater species of snail is required as an intermediate host for each species of fluke. In general, the miracidium (larval stage) emerges from the egg in freshwater, enters the snail host, and then undergoes multiplication. The cercaria (final larval state) leaves the snail.

These larvae penetrate the tissues of humans through contact in infested freshwater. The cercaria then loses its tail, becomes a schistosomulum, and migrates to blood vessels to become an adult. Adults migrate into intestinal veins and vesical veins (S. haematobium) and lay eggs in the vasculature. The ova transmigrate through the lumen and are voided in feces or urine. Ova that reach freshwater complete the life cycle in intermediate hosts.

Signs and Symptoms

Patients may be asymptomatic initially, or they may show mild pruritic maculopapular skin lesions within hours to days after exposure to cercariae. Four to 8 weeks after invasion, some patients may present with fever, eosinophilia, lymphadenopathy, and hepatosplenomegaly. Acute schistosomiasis or Katayama fever resembles serum sickness, and it appears to be related to antigen excess during sexual maturation of the parasite. It is associated with a mortality rate of up to 25%.¹³

Chronic schistosomiasis persisting for months to years after primary exposure is more common. A granulomatous reaction to Schistosoma eggs, coupled with host cell–mediated inflammatory response lead to organomegaly and obstruction. Patients may present with abdominal pain, hematemesis, ascites, hematuria, dysuria, vulvar or perianal lesions, dyspnea on exertion, fatigue, cough, chest pain, or seizures, depending on the site of infection and the involvement of the body system.

Eggs induce an immune response as they travel to the liver, intestine, bladder, and rarely, to the brain or spinal cord. Granuloma formation in the bowel wall with S. mansoni or S. japonicum may cause bloody diarrhea, cramping, and colonic polyposis. Schistosoma can also cause pulmonary and CNS disease, with sequelae including pulmonary hypertension, cor pulmonale, epilepsy, and transverse myelitis.¹³

Egg retention and granuloma formation in the urinary tract with S. haematobium can lead to hematuria, urinary tract infections, glomerulonephritis, obstructive uropathy, dysuria, and bladder polyps and ulcerations. S. haematobium is considered a carcinogen in squamous cell bladder cancer.¹³

Diagnosis and Medical Decision Making

Geographic history and clinical presentation are key considerations in diagnosing schistosomiasis. The diagnosis can also be aided by recovery of eggs in feces or urine, but Kato thick smear microscopy must be ordered specifically. Hatching assays on fresh stool specimens help to distinguish active from treated infection. Dead eggs may be shed for up to a year. Peripheral eosinophilia supports the diagnosis. Gross and microscopic hematuria is common in patients infected by S. haematobium.

Imaging techniques such as ultrasonography, echocardiography, and radiography can help pinpoint organ system involvement. ELISA testing is available and confirms past exposure, although it cannot discriminate between acute and chronic infection. The EITB test also yields good results. In some cases, colonic biopsy is necessary to visualize parasite eggs in the bowel wall.

Acute schistosomiasis should be suspected in patients recently returning from endemic areas who present with fever, headache, malaise, arthralgias or myalgias, bloody diarrhea, and right upper quadrant abdominal pain, especially if they have a history of freshwater swimming or bathing.

Treatment

Early treatment with cidal drugs may exacerbate Katayama fever, and concomitant steroid therapy is recommended. Patients with long-term infection may require procedures such as bladder stents, endoscopic treatment, or surgery.

Helminths cause a large burden of disease, yet their coevolution with humans and adaptation to our immune defenses may be beneficial (Box 177.2). Numerous epidemiologic and experimental studies have noted the low prevalence of allergy and autoimmune disease among populations with chronic helminth infections. Helminth infections generally cause a skewed type 1 helper T-cell (Th2) response. The Th1 arm of the immune system is often suppressed or downregulated. Populations with chronic helminthiases may not clear microbial infections adequately or respond optimally to vaccination.^14,15 Schistosoma and Onchocerca infections appear to decrease tetanus and tuberculosis vaccine efficacy, and Ascaris infection similarly dampens the immune response to Mycoplasma pneumonia vaccination.¹⁶