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 of the GI or GU system. Acute symptoms are dermatitis, followed several weeks later by fever, chills, nausea, abdominal pain, diarrhea, malaise, and myalgia. Chronic symptoms vary with species but include bloody diarrhea (eg, with S. mansoni) or hematuria (eg, with S. haematobium). Diagnosis is by identifying eggs in stool, urine, or biopsy specimens. Serologic tests may be sensitive and specific but do not provide information about the worm burden or clinical status. Treatment is with praziquantel.

Etiology

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. About 200 million people are infected worldwide. The risk of infection is spreading as new dams are built in endemic areas.

Five species of schistosomes infect humans; all have similar life cycles involving freshwater snails. S. haematobium causes urinary tract disease; the other Schistosoma sp cause intestinal disease. Geographic distribution differs by species:

• S. haematobium: Widely distributed over the African continent with smaller foci in the Middle East and India
• S. mansoni: Widespread in Africa and the only species in the Western Hemisphere, endemic in Brazil, Surinam, and Venezuela and on some Caribbean islands
• S. japonicum: Only in Asia, mainly in China and the Philippines
• S. mekongi: Laos and Cambodia
• S. intercalatum: Central Africa

The disease may be imported in travelers and immigrants from endemic areas, but transmission does not occur within the US and Canada.

Pathophysiology

Adult worms live and copulate within the veins of the mesentery (typically S. japonicum and S. mansoni) or bladder (typically S. haematobium—see Fig. 1: Trematodes (Flukes): Simplified Schistosoma life cycle.). 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, CNS, spinal cord). Excreted eggs hatch in freshwater, releasing miracidia (first larval stage), which enter snails. After multiplication, thousands of free-swimming cercariae are released. Cercariae penetrate human skin within a few minutes after exposure 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 GU tract.

Eggs appear in stool or urine 1 to 3 mo after cercarial penetration.

Estimates of the adult worm life span range from 3 to 7 yr. The females range in size from 7 to 20 mm; males are slightly smaller.

Fig. 1
Simplified Schistosoma life cycle. In the human host, eggs containing miracidia are eliminated with feces or urine into water. In water, the eggs hatch and release miracidia. The miracidia swim and penetrate a snail (intermediate host). Within the snail, the miracidia progress through 2 generations of sporocysts to become cercariae. The free-swimming cercariae are released from the snail and penetrate the skin of the human host. During penetration, the cercariae lose their forked tail, becoming schistosomula. The schistosomula are transported through the vasculature to the liver; there, they mature into adults. The paired (male and female) adult worms migrate (depending on their species) to the intestinal veins in the bowel or rectum or to the venous plexus of the GU tract, where they reside and begin to lay eggs.

Symptoms and Signs

Schistosome dermatitis

This pruritic papular rash (see also Dermatitis Caused by Avian and Animal Schistosomes on see Trematodes (Flukes): Dermatitis Caused by Avian and Animal Schistosomes) develops where the cercariae penetrate the skin in previously sensitized people.

Acute schistosomiasis

Acute schistosomiasis (Katayama fever) occurs with onset of egg laying, typically 2 to 4 wk 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 schistosomiasis results mostly from host responses to eggs retained in tissues. Early on, intestinal mucosal ulcerations caused by S. mansoni or S. japonicum may bleed and result in bloody diarrhea. As lesions progress, focal fibrosis, strictures, fistulas, and papillomatous growths may develop. With S. haematobium, ulcerations in the bladder wall 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 may develop. Blood loss from both GI and GU tracts frequently results in anemia.

Secondary bacterial infection of the GU tract is common, and persistent Salmonella septicemia may occur with S. mansoni. Several species, notably S. haematobium, can cause genital disease in both men and women, resulting in numerous symptoms including infertility.

Granulomatous reactions to eggs of S. mansoni and S. japonicum in the liver usually do not compromise liver function but may cause fibrosis and cirrhosis, which can lead to portal hypertension and subsequent hematemesis due to esophageal varices. Eggs in the lungs may produce granulomas and focal obliterative arteritis, which may cause pulmonary hypertension and cor pulmonale. Eggs lodged in the spinal cord can cause transverse myelitis, and those in the CNS can cause seizures.

Diagnosis

• Microscopic examination of stool

Stool (S. japonicum, S. mansoni, S. mekongi, S. intercalatum) or urine (S. haematobium, occasionally S. japonicum) is examined for eggs. Repeated examinations using concentration techniques may be necessary. Geography is a primary determinant of species, so a history 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 eggs.

Depending on the antigens used, serologic tests may be sensitive and specific for infection, but they do not provide information about worm burden, clinical status, or prognosis.

Treatment

• Praziquantel

Single-day oral treatment with praziquantel (20 mg/kg bid for S. haematobium, S. mansoni, and S. intercalatum; 20 mg/kg tid for S. japonicum and S. mekongi) is recommended. However, treatment does not affect developing schistosomula and thus may not abort an early infection. Adverse effects 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 or drug-resistant strains.

Oxamniquine (not available in the US) has been effective in treating infection due to S. mansoni in some areas where praziquantel has been less effective. African strains are more resistant to this drug than South American strains and require higher doses.

Patients should be examined for living eggs 3 and 6 mo after treatment. Retreatment is indicated if egg excretion has not decreased markedly.

Prevention

Scrupulously avoiding contact with contaminated freshwater prevents infection. 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. Vaccine development is under way.

Dermatitis Caused by Avian and Animal Schistosomes

(Cercarial Dermatitis; Clam Diggers' Itch; Swimmers' Itch)

Cercarial dermatitis, a skin condition, occurs when Schistosoma sp that cannot develop in humans penetrate the skin during contact with contaminated freshwater or brackish water.

Cercariae of Schistosoma sp that infect birds and mammals other than humans can penetrate the skin. Although the organisms do not develop in humans, humans may become sensitized and develop pruritic maculopapular, then 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.

Ocean-related schistosome dermatitis (clam diggers' itch) occurs on all Atlantic, Gulf, Pacific, and Hawaiian coasts. It is very common in muddy flats off Cape Cod. Freshwater schistosome dermatitis (swimmers' itch) is common in the lakes of northern Michigan, Wisconsin, and Minnesota.

Diagnosis is based on clinical findings. Most cases do not require medical attention.

Treatment is symptomatic with cool compresses, baking soda, or antipruritic lotions. Topical corticosteroids can also be used.

Last full review/revision December 2009 by Richard D. Pearson, MD