Schistosomiasis

Description

Schistosomiasis is caused by flukes, which have complex life cycles involving specific fresh-water snail species as intermediate hosts. Infected snails release large numbers of minute, free-swimming larvae (cercariae) that are capable of penetrating the unbroken skin of the human host. Even brief exposure to contaminated fresh water, such as wading, swimming, or bathing, can result in infection. Human schistosomiasis cannot be acquired by wading or swimming in salt water (oceans or seas). The cercariae of birds and aquatic mammals can penetrate the skin of human beings who enter infested fresh or salt water in many parts of the world, including cool temperate areas. The cercariae die in the skin but may elicit a puritic rash ("swimmer's itch" or "clam-digger's itch").

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Occurrence

This infection is estimated to occur worldwide, affecting some 200 million persons. Schistosomiasis is most prevalent in sub-Saharan Africa, southern China, the Philippines, and Brazil.

Risk for Travelers

Exposure to schistosomiasis is a health hazard for U.S. citizens who travel to endemic areas (see Map 3-8). Outbreaks of schistosomiasis have occurred among adventure travelers on river trips in Africa, as well as among resident expatriates, such as Peace Corps volunteers in high-risk areas. Those at greatest risk are travelers who wade or swim in or bathe with fresh water in areas where poor sanitation and appropriate snail hosts are present.

Clinical Presentation

Clinical manifestations of acute infection can occur within 2-3 weeks of exposure to cercariae-infested water, but most acute infections are asymptomatic. The most common acute syndrome is Katayama fever. Symptoms, which include fever, lack of appetite, weight loss, abdominal pain, hematuria, weakness, headaches, joint and muscle pain, diarrhea, nausea, and cough, may develop several weeks after exposure. Rarely, the central nervous system can be involved, producing seizures or transverse myelitis as a result of mass lesions of the brain or spinal cord. Chronic infections can cause disease in the liver, intestinal tract, bladder (including bladder cancer), kidneys, or lung. Many persons with chronic infections recall no symptoms of acute infection. Diagnosis of infection is usually confirmed by serologic studies or by finding schistosome eggs on microscopic examination of stool or urine. Schistosome eggs can be found as soon as 6-8 weeks after exposure, but are not invariably present.

Prevention

No vaccine is available, nor are any drugs recommended as chemoprophylactic agents at this time. Because there is no practical way for the traveler to distinguish infested from noninfested water, travelers should be advised to avoid fresh-water wading or swimming in rural areas of endemic countries. In such areas, heating bathing water to 50° C (122° F) for 5 minutes or treating it with iodine or chlorine in a manner similar to the precautions recommended for preparing drinking water will destroy cercariae and make the water safe. Thus, swimming in adequately chlorinated swimming pools is virtually always safe, even in endemic countries. Filtering water with paper coffee filters can also be effective in removing cercariae from bathing water. If these measures are not feasible, travelers should be advised to allow bathing water to stand for 3 days because cercariae rarely survive longer than 48 hours. Vigorous towel drying after accidental exposure to water has been suggested as a way to remove cercariae in the process of skin penetration; however, this may only prevent some infections and should not be recommended to travelers as a preventive measure.

Upon return from foreign travel, those who might have been exposed to schistosome-infested fresh water should be advised to undergo screening tests. Because serologic tests are often more sensitive than microscopic examination of stool and urine for eggs, previously uninfected but potentially exposed travelers should be tested for antibodies to schistosomes if microscopic examination of stool and urine for eggs is negative or not available. CDC performs a screening ELISA that is 99%, 90%, and 50% sensitive for Schistosoma mansoni, haematobium, and japonicum, respectively, and a confirmatory, species-specific immunoblot that is at least 95% sensitive and 99% specific for all three species. Serologic tests performed in commercial laboratories may not be as sensitive or specific.

Treatment

Safe and effective oral drugs are available for the treatment of schistosomiasis. Praziquantel is the drug of choice for all species of Schistosoma. Oxamniquine has been effective in treating infections caused by S. mansoni in some areas where praziquantel is less effective. Travelers should be advised to contact an infectious disease or tropical medicine specialist.

Health Information for International Travel 2003-2004