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Schistosomiasis

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Description

Schistosomiasis (also known as bilharzia) is a parasitic infection caused by Schistosoma flukes, which 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 (e.g., during 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"). Organisms causing hepatic or intestinal schistosomiasis include Schistosoma mansoni, S. mekongi, S. intercalatum, and S. malayanensis. S. haematobium causes urinary schistosomiasis (1,2).

Occurrence

This infection occurs widely throughout the tropics and subtropics, affecting some 200 million persons (see Map 4-11). Schistosomiasis is most prevalent in sub-Saharan Africa. In highly disease-endemic areas, prevalence rates can exceed 50% among the local population, and high rates have been reported among expatriates living in such areas and even among short-term travelers to these areas (1-3).

Risk for Travelers

Exposure to schistosomiasis is a health hazard for persons who travel to disease-endemic areas. 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 (4,5). Those at greatest risk are travelers who wade, swim, or bathe in freshwater in areas where sanitation is poor and the snail hosts are present.

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, hematuria, weakness, headaches, joint and muscle pain, diarrhea, 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. Serologic testing in the exposed, yet asymptomatic traveler should ideally be performed several months following exposure (6-8).

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 wading, swimming or other contact with freshwater in disease-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, travelers 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 disease-endemic countries. Vigorous towel-drying after accidental exposure to water has been suggested as a way to remove cercariae before they can penetrate the skin; however, this may prevent only some infections and should not be recommended to travelers 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 (6).

Upon return from foreign travel, persons 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 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, 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 (9). Serologic tests performed in commercial laboratories may not be as sensitive or specific.

MAP 4-11 Geographic distribution of schistosomiasis.

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. Water resource development projects and population movements have led to introduction of schistosomiasis into regions and countries that had not been endemic for the disease. Japan and Montserrat have eliminated schistosomiasis, and the risk is believed to be extremely low or nonexistent in the shaded countries indicated on the map. In Brazil, China, Egypt, the Philippines, Iran, Morocco, Venezuela, and elsewhere, national control programs have reduced morbidity due to schistosomiasis. (Updated August 30, 2007)

Symptoms

What are the symptoms of schistosomiasis?

Within days after becoming infected, you may develop a rash or itchy skin. Fever, chills, cough, and muscle aches can begin within 1-2 months of infection. Most people have no symptoms at this early phase of infection.

Eggs travel to the liver or pass into the intestine or bladder, causing inflammation or scarring. Children who are repeatedly infected can develop anemia, malnutrition, and learning difficulties. After years of infection, the parasite can also damage the liver, intestines, lungs, and bladder. Rarely, eggs are found in the brain or spinal cord and can cause seizures, paralysis, or spinal cord inflammation.

Symptoms of schistosomiasis are caused by the body's reaction to the eggs produced by worms, not by the worms themselves.

Diagnosis

How is schistosomiasis diagnosed?

Microscopic identification of eggs in stool or urine is the most practical method for diagnosis. Stool examination should be performed when infection with S. mansoni or S. japonicum is suspected, and urine examination should be performed if S. haematobium is suspected.

  • Eggs can be present in the stool in infections with all Schistosoma species. The examination can be performed on a simple smear (1 to 2 mg of fecal material). Since eggs may be passed intermittently or in small amounts, their detection will be enhanced by repeated examinations and/or concentration procedures.
  • Eggs can be found in the urine in infections with S. haematobium (recommended time for collection: between noon and 3 PM) and with S. japonicum.

A blood test has been developed and is available at  the Center for Disease Control (CDC). For accurate results, the delay is 6-8 weeks after the last exposure to contaminated water before the blood sample is taken.

Tissue biopsy (rectal biopsy for all species and biopsy of the bladder for S. haematobium) may demonstrate eggs when stool or urine examinations are negative.

Prevention

How can I prevent schistosomiasis?

  • Avoid swimming or wading in freshwater when you are in countries in which schistosomiasis occurs. Swimming in the ocean and in chlorinated swimming pools is generally thought to be safe.
  • Drink safe water. Because there is no way to make sure that water coming directly from canals, lakes, rivers, streams or springs is safe, you should either boil water for 1 minute or filter water before drinking it. Boiling water for at least 1 minute will kill any harmful parasites, bacteria, or viruses present. Iodine treatment alone WILL NOT GUARANTEE that water is safe and free of all parasites.
  • Bath water should be heated for 5 minutes at 150°F. Water held in a storage tank for at least 48 hours should be safe for showering.
  • Vigorous towel drying after an accidental, very brief water exposure may help to prevent the Schistosoma parasite from penetrating the skin. You should NOT rely on vigorous towel drying to prevent schistosomiasis.

 

 

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