Flukes are parasitic flatworms that infect various parts of the body (eg, blood vessels, gastrointestinal tract, lungs, liver) depending on the species.
Etiology of Schistosomiasis
Schistosomiasis is by far the most important trematode infection. Schistosoma is the only trematode that invades through the skin; all other trematodes infect only via ingestion. Over 230 million people are infected worldwide.
Five species of schistosomes infect humans; all have similar life cycles involving freshwater snails. S. haematobium causes urinary tract disease; the other Schistosoma species cause intestinal disease.
Geographic distribution of schistosomes that infect humans differs by species:
S. haematobium: Widely distributed over the African continent with smaller foci in the Middle East, Turkey, and India
S. mansoni: Widespread in Africa, foci in Middle East, and the only species in the Western Hemisphere in parts of South America and some Caribbean islands
S. japonicum: Asia, mainly in China, the Philippines, Thailand, and Indonesia
S. mekongi: Southeast Asia
S. intercalatum: Central and West Africa
Humans are the main reservoir of infection. Dogs, cats, rodents, pigs, horses, and goats are reservoirs for S. japonicum, and dogs are reservoirs for S. mekongi. Transmission of these species does not occur within the US (including Puerto Rico) and Canada, but the disease may be present in travelers and immigrants from endemic areas.
Pathophysiology of Schistosomiasis
Adult Schistosoma worms live and copulate within venules of the mesentery (typically S. mekongi, S. intercalatum, S. japonicum and S. mansoni) or bladder (typically S. haematobium). Some eggs penetrate the intestinal or bladder mucosa and are passed in stool or urine; other eggs remain within the host organ or are transported through the portal system to the liver and occasionally to other sites (eg, lungs, central nervous system, spinal cord). Excreted eggs hatch in freshwater, releasing miracidia (first larval stage), which enter snails. After multiplication, thousands of free-swimming, forked-tailed cercariae are released.
Cercariae penetrate human skin within a few minutes after exposure. When they penetrate the skin, they lose their forked tail and transform into schistosomula, which travel through the bloodstream to the liver, where they mature into adults. The adults then migrate to their ultimate home in the intestinal veins or the venous plexus of the genitourinary tract.
Eggs appear in stool or urine 1 to 3 months after cercarial penetration.
Estimates of the adult worm life span range from 3 to 7 years. The females range in size from 7 to 20 mm; males are slightly smaller.
Simplified Schistosoma life cycle
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Symptoms and Signs of Schistosomiasis
Acute schistosome dermatitis
Most infections are asymptomatic. A pruritic papular rash (cercarial dermatitis Dermatitis Caused by Avian and Animal Schistosomes Schistosomiasis is infection with blood flukes of the genus Schistosoma, which are acquired transcutaneously by swimming or wading in contaminated freshwater. The organisms infect the vasculature... read more ) can develop where cercariae penetrate the skin.
Acute Katayama fever
Katayama fever is a systemic hypersensitivity reaction that may occur with onset of egg laying, typically 2 to 4 weeks after heavy exposure. Symptoms include fever, chills, cough, nausea, abdominal pain, malaise, myalgia, urticarial rashes, and marked eosinophilia, resembling serum sickness. Manifestations are more common and usually more severe in visitors than in residents of endemic areas and typically last for several weeks.
Chronic schistosomiasis
Chronic infection is most commonly due to repeated exposure in endemic areas but can also occur after brief exposure such as occurs in travelers. Chronic schistosomiasis results primarily from granulomatous host responses to eggs retained in tissues.
Intestinal schistosomiasis: Early on, intestinal mucosal ulcerations caused by S. mansoni, S. japonicum, S. mekongi, or S. intercalatum may bleed and result in bloody diarrhea. As lesions progress, focal fibrosis, strictures, fistulas, and papillomatous growths may develop in the intestine.
Hepatosplenic schistosomiasis: Granulomatous reactions to eggs of S. mansoni, S. japonicum, S. mekongi, and S. intercalatum in the liver usually do not compromise liver function, but they may cause fibrosis Hepatic Fibrosis Hepatic fibrosis is overly exuberant wound healing in which excessive connective tissue builds up in the liver. The extracellular matrix is overproduced, degraded deficiently, or both. The trigger... read more and cirrhosis Cirrhosis Cirrhosis is a late stage of hepatic fibrosis that has resulted in widespread distortion of normal hepatic architecture. Cirrhosis is characterized by regenerative nodules surrounded by dense... read more , which can lead to portal hypertension Portal Hypertension Portal hypertension is elevated pressure in the portal vein. It is caused most often by cirrhosis (in developed countries), schistosomiasis (in endemic areas), or hepatic vascular abnormalities... read more , resulting in splenomegaly Splenomegaly Splenomegaly is abnormal enlargement of the spleen. (See also Overview of the Spleen.) Splenomegaly is almost always secondary to other disorders. Causes of splenomegaly are myriad, as are the... read more , and esophageal varices. Esophageal varices may bleed, causing hematemesis.
Eggs in the lungs may produce granulomas and focal obliterative arteritis, which may ultimately result in pulmonary hypertension Pulmonary Hypertension Pulmonary hypertension is increased pressure in the pulmonary circulation. It has many secondary causes; some cases are idiopathic. In pulmonary hypertension, pulmonary vessels become constricted... read more and cor pulmonale Cor Pulmonale Cor pulmonale is right ventricular enlargement secondary to a lung disorder that causes pulmonary artery hypertension. Right ventricular failure follows. Findings include peripheral edema, neck... read more 
.
Bladder involvement with S. haematobium produces ulcerations in the bladder wall that may cause dysuria, hematuria, and urinary frequency. Over time, chronic cystitis develops. Strictures may lead to hydroureter and hydronephrosis. Papillomatous masses in the bladder are common, and squamous cell carcinoma of the bladder Bladder Cancer Bladder cancer is usually transitional cell (urothelial) carcinoma. Patients usually present with hematuria (most commonly) or irritative voiding symptoms such as frequency and/or urgency; later... read more 
may develop.
Blood loss from both gastrointestinal and genitourinary tracts frequently results in iron-deficiency anemia Iron Deficiency Anemia Iron deficiency is the most common cause of anemia and usually results from blood loss; malabsorption, such as with celiac disease, is a much less common cause. Symptoms are usually nonspecific... read more 
.
Secondary bacterial infection of the genitourinary tract is common with S. haematobium, and persistent or recurrent Salmonella septicemia may occur. Several species, notably S. haematobium, can cause genital disease in both men and women, resulting in numerous symptoms including infertility. Neurologic complications can occur even in light Schistosoma infections. Eggs or adult worms lodged in the spinal cord can cause transverse myelitis Acute Transverse Myelitis Acute transverse myelitis is acute inflammation of gray and white matter in one or more adjacent spinal cord segments, usually thoracic. Causes include multiple sclerosis, neuromyelitis optica... read more , and those in the brain can produce focal lesions and seizures.
Diagnosis of Schistosomiasis
Microscopic examination of stool or urine (S. haematobium) for eggs
Antigen tests
Serologic tests
Diagnostic testing is indicated for patients with symptoms of schistosomiasis and relevant epidemiologic exposure. Screening of asymptomatic people may be warranted for those exposed to fresh water in endemic areas.
Schistosomiasis is diagnosed and parasite burden is estimated by microscopic examination of stool or urine (S. haematobium) for eggs. Repeated examinations using concentration techniques may be necessary. Geography is a primary determinant of species, so the location of exposure should be communicated to the laboratory. If the clinical picture suggests schistosomiasis but no eggs are found after repeated examination of urine or feces, intestinal or bladder mucosa can be biopsied to check for characteristic granulomas around embedded eggs.
Tests for schistosomal antigens or DNA in blood, urine, or stool are particularly useful for schistosome eradication programs and in returning travelers with suspected infection. Most antigen detection tests are quantitative and antigen levels are correlated to parasite burden. Some antigen tests, such as the commercially available urine dipstick for S. mansoni, are qualitative.
Serologic tests are sensitive and specific for infection, but do not provide information about worm burden, clinical status, or prognosis and do not distinguish active from resolved infection. Antibody tests thus are most useful for detecting infection in returning travelers and not helpful in patients who are residents of endemic areas. With returning travelers, serologic tests should be done ≥ 6 to 12 weeks after the last exposure to fresh water to allow time for maturation of the schistosomes into adults and for development of antibodies.
Hepatosplenic schistosomiasis can be diagnosed by finding eggs in stool, intestinal tissue, or liver samples taken for biopsy with variable sensitivity as egg shedding can be intermittent in such patients. Typically, liver blood tests are normal. Ultrasonography may show periportal fibrosis and splenomegaly.
Neuroschistosomiasis is diagnosed if there is infection at an extraneural site along with clinical and radiographic evidence of neurologic involvement. Schistosomes in biopsied central nervous system lesions, and/or a positive antibody test or polymerase chain reaction in cerebrospinal fluid are also diagnostic.
Treatment of Schistosomiasis
Praziquantel
Single-day oral treatment with praziquantel (20 mg/kg twice a day for S. haematobium, S. mansoni, and S. intercalatum; 20 mg/kg 3 times a day for S. japonicum and S. mekongi) is recommended. Praziquantel is effective against adult schistosomes, but not developing schistosomula, which are present early in infection. Thus, for asymptomatic travelers who have had exposure in potentially contaminated fresh water, treatment is delayed for 6 to 8 weeks after the last exposure. Adverse effects of praziquantel are generally mild and include abdominal pain, diarrhea, headache, and dizziness. Therapeutic failures have been reported, but it is difficult to determine whether they are due to reinfection, the relative resistance of immature schistosomes, or praziquantel-resistant adult schistosomes.
If eggs are present at the time of diagnosis, follow-up examination 1 to 2 months after treatment is suggested to help confirm cure. Treatment is repeated if eggs are still present.
Treatment of acute schistosomiasis (Katayama fever) is based on limited data. Corticosteroids can ameliorate severe symptoms; treatment with prednisone 20 to 40 mg daily for 5 days in adults is usually effective. Once symptoms have subsided, treatment with praziquantel is given as detailed above and repeated 4 to 6 weeks later after parasites have reached adulthood.
Patients with eggs in stool or urine at the time acute or chronic schistosomiasis is diagnosed should be examined for living eggs 1 to 2 months after treatment. An experienced microscopist can distinguish viable eggs from empty shells based on the presence of living miracidium. Retreatment is indicated if viable eggs are present.
Prevention of Schistosomiasis
Scrupulously avoiding contact with contaminated fresh water prevents schistosomiasis.
Schistosomiasis is not transmitted by swallowing contaminated water; however, mouth and lip contact with contaminated water could lead to infection.
Fresh water used for bathing should be boiled for at least 1 minute and then cooled before bathing. However, water that has been held in a storage tank for at least 1 to 2 days should be safe without boiling.
People who are accidentally exposed to possibly contaminated water (eg, by falling into a river) should vigorously dry off with a towel to attempt to remove any parasites before they penetrate the skin.
The sanitary disposal of urine and feces reduces the likelihood of infection.
Adult residents of endemic areas are more resistant to reinfection than children, suggesting the possibility of acquired immunity.
Mass community-based or school-based treatment with praziquantel, education programs, and molluscicides to reduce snail populations are used to control schistosomiasis in endemic areas.
Vaccine development is under way.
Key Points
Schistosoma is the only trematode that invades through the skin; over 230 million people are infected worldwide.
When cercariae penetrate the skin, they lose their forked tail and become schistosomula, which travel through the bloodstream to the liver, where they mature; as adults, they migrate to their ultimate home in the intestinal veins or the venous plexus of the genitourinary tract.
Ova in the liver trigger a granulomatous reaction that can lead to fibrosis and portal hypertension, resulting in splenomegaly, esophageal varices, and hematemesis.
Organisms in the intestine can cause bloody diarrhea, and organisms in the bladder can cause hematuria and chronic cystitis.
Treat with praziquantel.
To prevent infection, avoid contact with fresh water in endemic areas.
Dermatitis Caused by Avian and Animal Schistosomes
Cercariae of Schistosoma species that infect birds and mammals other than humans can penetrate human skin. Although the organisms do not develop in humans, humans may become sensitized and develop pruritic maculopapular or vesicular skin lesions at the site of penetration. Skin lesions may be accompanied by a systemic febrile response that runs for 5 to 7 days and resolves spontaneously.
Cercarial dermatitis occurs worldwide. In North America, ocean-related schistosome dermatitis (clam digger's itch) occurs on all Atlantic, Gulf, Pacific, and Hawaiian coasts. It is common in muddy flats off Cape Cod. Freshwater schistosome dermatitis (swimmer's itch) is common in the Great Lakes region.
Diagnosis of cercarial dermatitis is based on clinical findings. Most cases do not require medical attention.
Treatment of cercarial dermatitis is symptomatic with cool compresses, baking soda, or antipruritic lotions. Topical corticosteroids can also be used.
