Schistosomiasis

Description

Schistosomiasis (also known as bilharzia) is a parasitic infection caused by Schistosoma flukes that have complex life cycles involving specific freshwater 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 freshwater, such as wading, swimming, or bathing, can result in infection. Human schistosomiasis cannot be acquired by contact with salt water (oceans or seas). However, 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. These cercariae die in the skin but may elicit a pruritic rash ("swimmer's itch" or "clam-digger's itch").

Occurrence

This infection occurs widely throughout the tropics and subtropics, affecting some 200 million persons. Schistosomiasis is most prevalent in sub-Saharan Africa. In highly endemic areas, prevalence rates can exceed 50% among the local population, and high rates have been reported among expatriates living in such areas.

Risk for Travellers

Exposure to schistosomiasis is a health hazard for persons who travel to endemic areas (see Map 4-10). Outbreaks of schistosomiasis have occurred among adventure travellers 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 travellers who wade, swim, or bathe in fresh water in areas where sanitation is poor and the snail hosts are present.

Map 4-10. Geographic distribution of schistosomiasis

Click on the map to enlarge

Note that the prevalence of schistosomiasis is changing rapidly. Control programs have eliminated or greatly reduced transmission of schistosomiasis in most countries in Asia and the Americas. On the other hand, the prevalence of schistosomiasis has increased in sub-Saharan Africa, and water resource development projects and population movements have led to introduction of schistosomiasis into regions and countries that were not endemic previously. Japan and Monteserrat have eliminated schistosomiasis. At present, there is believed to be extremely low (or no risk) of schistosomiasis in the shaded countries as indicated on the map. In Brazil, China, Egypt, Philippines, Iran, Morocco, Venezuela, and elsewhere national control programs have reduced morbidity due to schistosomiasis. Organisms causing hepatic or intestinal schistosomiasis include S. mansoni, S. Mekongi, S. intercalatum, and S. malayanensis. S. haematobium causes urinary schistosomiasis.

Clinical Presentation

Clinical manifestations of acute infection can occur within 2-12 weeks of exposure to cercariae-infested water, but most acute infections are asymptomatic. The most common acute syndrome is Katayama fever. Symptoms include fever, loss of appetite, weight loss, abdominal pain, haematuria, weakness, headaches, joint and muscle pain, diarrhoea, nausea, and cough. 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 always detectable.

Prevention

No vaccine is available, nor are any drugs recommended as chemoprophylactic agents at this time. Because there is no practical way for the traveller to distinguish infested from noninfested water, travellers should be advised to avoid wading, swimming or other fresh-water contact in endemic countries. Untreated piped water coming directly from canals, lakes, rivers, streams or springs may contain cercariae, but heating bathing water to 50°C (122°F) for 5 minutes or filtering water with fine-mesh filters can eliminate the risk of infection. If such measures are not feasible, travellers should be advised to allow bathing water to stand for 2 days because cercariae rarely remain infective longer than 24 hours. Swimming in adequately chlorinated swimming pools is virtually always safe, even in endemic countries. 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 prevent only some infections and should not be recommended to travellers as a preventive measure. Although topical application of the insect repellent DEET can block penetrating cercariae, the effect is short lived and cannot reliably prevent infection.

Upon return from foreign travel, those who may have been exposed to schistosome-infested freshwater should be advised to undergo screening tests. Because serologic tests are more sensitive than microscopic examination of stool and urine for eggs, previously uninfected but potentially exposed travellers 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, S. haematobium, and S. 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. Travellers who suspect they may have schistosomiasis should be advised to contact an infectious disease or tropical medicine specialist.

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- James Maguire and Brian Blackburn

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