Author(s): Susan Montgomery, W. Evan Secor

Infectious Agent

Schistosomiasis (also known as bilharzia and snail fever) is caused by helminth parasites of the genus Schistosoma. Other helminth infections are discussed in Sec. 5, Part 3, Ch. 13, Soil-Transmitted Helminths.

Transmission

Waterborne transmission occurs when larval cercariae, found in contaminated bodies of freshwater, penetrate the skin. Bathing, swimming, or wading in contaminated freshwater can result in infection; people of all ages are at risk. Human schistosomiasis is not acquired by contact with brackish or saltwater (oceans or seas). Schistosomiasis distribution is very focal and determined by the presence of competent snail intermediate hosts, inadequate sanitation, and infected humans. The specific snail intermediate hosts can be difficult to identify, and laboratory testing is the only way to determine whether snails are infected with human schistosome species.

Epidemiology

An estimated 85% of the world’s cases of schistosomiasis are in Africa, where prevalence rates can exceed 50% in local populations. Schistosoma mansoni and S. haematobium are distributed throughout Africa. Only S. haematobium is found in areas of the Middle East, and only S. mansoni in parts of Brazil, Suriname, and Venezuela. In the Caribbean, risk is very low, but S. mansoni is found Guadeloupe, Martinique, and Saint Lucia, and previously in the Dominican Republic. S. japonicum is found in parts of China, in Indonesia, and in the Philippines. Although schistosomiasis had been eliminated in Europe for decades, transmission of S. haematobium was reported in Corsica in 2014, where cases were identified among travelers who had bathed in the Cavu River. Two other species can infect humans: S. mekongi, found in Cambodia and Laos, and S. intercalatum, found in parts of Central and West Africa. These 2 species are rarely reported causes of human infection.

Many but not all countries endemic for schistosomiasis have established control programs. Countries where development has led to widespread improvements in sanitation and water safety, especially where successful schistosomiasis control programs have been implemented, likely have eliminated this disease. No international guidelines currently exist for verification of elimination, however.

Travelers and expatriates potentially at increased risk for infection include adventure travelers and ecotourists, missionaries, Peace Corps volunteers, and soldiers. Outbreaks of schistosomiasis have occurred among adventure travelers on river trips in Africa. The geographic distribution of schistosomiasis acquired by travelers reflects travel and immigration patterns.

Most travel-associated cases of schistosomiasis are acquired in sub-Saharan Africa. Some African transmission sites frequently visited by travelers include rivers and water sources in the Banfora region (Burkina Faso) and areas populated by the Dogon people (Mali), Lake Malawi, Lake Tanganyika, Lake Victoria, the Nile River, the Omo River (Ethiopia), and the Zambezi River. As travel to more remote areas increases, travelers should remember that most freshwater surface water sources in Africa are potentially contaminated and can be sources of infection. Travelers should view with skepticism any local claim that a body of freshwater is free from schistosomiasis.

Clinical Presentation

The incubation period is typically 14–84 days for acute schistosomiasis, and chronic asymptomatic infection can persist for years. Penetration of cercariae can cause a rash that develops within hours or up to a week after contaminated water exposure. Acute schistosomiasis (Katayama syndrome) is characterized by diarrhea, fever, headache, myalgia, and respiratory symptoms. Eosinophilia often is present; painful hepatomegaly or splenomegaly also can occur.

Clinical manifestations of chronic schistosomiasis are the result of host immune responses to schistosome eggs. Eggs secreted by adult worm pairs living in the bloodstream become lodged in the capillaries of organs and cause granulomatous reactions. S. japonicum and S. mansoni eggs most commonly lodge in the blood vessels of the liver or intestine and can cause blood in the stool, constipation, and diarrhea. Chronic inflammation can lead to bowel wall ulceration, hyperplasia, polyposis, and, with heavy infections, to periportal liver fibrosis and splenomegaly.

S. haematobium eggs typically lodge in the urinary tract and can cause dysuria and hematuria. Calcifications in the bladder might appear late in the disease. S. haematobium infection can cause genital symptoms and has been associated with increased risk for bladder cancer. As with acute schistosomiasis, eosinophilia might be present during chronic infection with any species.

Rarely, central nervous system manifestations of schistosomiasis develop; these are thought to result from aberrant migration of adult worms or eggs depositing in the spinal cord or brain. Signs and symptoms are related to ectopic granulomas in the central nervous system and can present as transverse myelitis.

Diagnosis

Diagnosis is made by microscopic identification of parasite eggs in stool (S. japonicum or S. mansoni) or urine (S. haematobium). Serologic tests are useful to diagnose light infections, because egg shedding might not be consistent in travelers and in others who have not had schistosomiasis previously. Antibody tests do not distinguish between past and current infection but are useful for identifying infection in asymptomatic people who might have been exposed during travel and could benefit from treatment. Clinicians can obtain diagnostic assistance and confirmatory testing from the Centers for Disease Control and Prevention (CDC)’s Division of Parasitic Diseases and Malaria DPDx laboratory (dpdx@cdc.gov), and from the Parasitic Diseases Hotline for Healthcare Providers (404-718-4745; parasites@cdc.gov).

Treatment

Schistosomiasis is uncommon in the United States; clinicians unfamiliar with management of the condition should consult an infectious disease or tropical medicine specialist for assistance with diagnosis and treatment. Praziquantel is used to treat schistosomiasis. Praziquantel is most effective against adult forms of the parasite and requires a host immune response to the adult worm to be fully effective. Although a single course of treatment is usually curative, in lightly infected patients the immune response can be less robust and repeat treatment might be needed after 2–4 weeks to increase effectiveness.

Prevention

No vaccine or drugs are available to prevent infection. Travelers can prevent schistosomiasis by avoiding bathing, swimming, wading, or other contact with freshwater in disease-endemic countries. Untreated piped water coming directly from freshwater sources could contain cercariae; travelers should use fine-mesh filters, heat bathing water to 122°F (50°C) for 5 minutes, or allow water to stand for ≥24 hours before exposure to help prevent infection (see Sec. 2, Ch. 9, Water Disinfection).

Swimming in adequately chlorinated swimming pools is safe, even in disease-endemic countries, although confirming adequate levels of chlorination is difficult. Vigorous towel-drying after accidental exposure to water has been suggested as a method of removing cercariae before they can penetrate, but this should not generally be recommended as a preventive measure. Topical applications of insect repellents (e.g., DEET) can block penetrating cercariae, but the effect depends on the repellent formulation, could be short-lived, and does not provide adequate coverage to prevent infection reliably.

CDC website: Schistosomiasis