Schistosomiasis, also known as bilharzia or snail fever, is an acute and chronic disease caused by parasitic flatworms called schistosomes or blood flukes. Most human infections are caused by Schistosoma mansoni, S. haematobium, or S. japonicum. The parasites spend part of their lifecycle in freshwater snails. The infectious, larval form emerges from the snail and contaminates water.

People who contact contaminated water during recreational, agricultural, or domestic activities become infected when the fork-shaped larval worms penetrate their skin, travel to blood vessels, and develop into adults. The release of eggs from female worms triggers symptoms in infected people, which can include abdominal pain, diarrhea, and blood in the urine. In children, repeated infections may cause anemia and stunted growth. Chronic infections frequently result in serious damage to the liver, intestines, and bladder. Women who are exposed to infested water through such routine tasks as clothes washing can develop urogenital schistosomiasis, which may result in tissue damage that increases the risk of HIV transmission.

Causal Agents

Schistosomiasis (Bilharziasis) is caused by some species of blood trematodes (flukes) in the genus Schistosoma. The three main species infecting humans are Schistosoma haematobium, S. japonicum, and S. mansoni. Three other species, more localized geographically, are S. mekongi, S. intercalatum, and S. guineensis (previously considered synonymous with S. intercalatum). There have also been a few reports of hybrid schistosomes of cattle origin (S. haematobium, x S. bovis, x S. curassoni, x S. mattheei) infecting humans. Unlike other trematodes, which are hermaphroditic, Schistosoma spp. are dioecous (individuals of separate sexes).

In addition, other species of schistosomes, which parasitize birds and mammals, can cause cercarial dermatitis in humans but this is clinically distinct from schistosomiasis.

Life Cycle

Schistosoma eggs are eliminated with feces or urine, depending on species (1). Under appropriate conditions the eggs hatch and release miracidia (2), which swim and penetrate specific snail intermediate hosts (3). The stages in the snail include two generations of sporocysts (4) and the production of cercariae (5). Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host (6), and shed their forked tails, becoming schistosomulae (7). The schistosomulae migrate via venous circulation to lungs, then to the heart, and then develop in the liver, exiting the liver via the portal vein system when mature, (8) (9). Male and female adult worms copulate and reside in the mesenteric venules, the location of which varies by species (with some exceptions) (10). For instance, S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine, and S. mansoni occurs more often in the inferior mesenteric veins draining the large intestine. However, both species can occupy either location and are capable of moving between sites. S. intercalatum and S. guineensis also inhabit the inferior mesenteric plexus but lower in the bowel than S. mansoni. S. haematobium most often inhabitsin the vesicular and pelvic venous plexus of the bladder, but it can also be found in the rectal venules. The females (size ranges from 7–28 mm, depending on species) deposit eggs in the small venules of the portal and perivesical systems. The eggs are moved progressively toward the lumen of the intestine (S. mansoni,S. japonicum, S. mekongi, S. intercalatum/guineensis) and of the bladder and ureters (S. haematobium), and are eliminated with feces or urine, respectively (1).

Hosts

Various animals such as cattle, dogs, cats, rodents, pigs, horses, and goats, serve as reservoirs for S. japonicum, and dogs for S. mekongi. S. mansoni is also frequently recovered from wild primates in endemic areas but is considered primarily a human parasite and not a zoonosis.

Intermediate hosts are snails of the genera Biomphalaria, (S. mansoni), Oncomelania (S. japonicum), Bulinus (S. haematobium, S. intercalatum, S. guineensis). The only known intermediate host for S. mekongi is Neotricula aperta.

Geographic Distribution

Schistosoma mansoni is found primarily across sub-Saharan Africa and some South American countries (Brazil, Venezuela, Suriname) and the Caribbean, with sporadic reports in the Arabian Peninsula.

S. haematobium is found in Africa and pockets of the Middle East.

S. japonicum is found in China, the Philippines, and Sulawesi. Despite its name, it has long been eliminated from Japan.

The other, less common human-infecting species have relatively restricted geographic ranges. S. mekongi occurs focally in parts of Cambodia and Laos. S. intercalatum has only been found in the Democratic Republic of the Congo; S. guineensis is found in West Africa. Instances of infections with hybrid/introgressed Schistosoma (S. haematobium x S. bovis, x S. curassoni, x S. mattheei) have occurred in Corsica, France, and some West African countries.

Clinical Presentation

Symptoms of schistosomiasis are not caused by the worms themselves but by the body’s reaction to the eggs. Many infections are asymptomatic. A local cutaneous hypersensitivity reaction following skin penetration by cercariae may occur and appears as small, itchy maculopapular lesions. Acute schistosomiasis (Katayama fever) is a systemic hypersensitivity reaction that may occur weeks after the initial infection, especially by S. mansoni and S. japonicum. Manifestations include systemic symptoms/signs including fever, cough, abdominal pain, diarrhea, hepatosplenomegaly, and eosinophilia.

Occasionally, Schistosoma infections may lead to central nervous system lesions. Cerebral granulomatous disease may be caused by ectopic S. japonicum eggs in the brain, and granulomatous lesions around ectopic eggs in the spinal cord may occur in S. mansoni and S. haematobium infections. Continuing infection may cause granulomatous reactions and fibrosis in the affected organs (e.g., liver and spleen) with associated signs/symptoms.

Pathology associated with S. mansoni and S. japonicum schistosomiasis includes various hepatic complications from inflammation and granulomatous reactions, and occasional embolic egg granulomas in brain or spinal cord. Pathology of S. haematobium schistosomiasis includes hematuria, scarring, calcification, squamous cell carcinoma, and occasional embolic egg granulomas in brain or spinal cord.

What is schistosomiasis?

Schistosomiasis, also known as bilharzia, is a disease caused by parasitic worms. Infection with Schistosoma mansoni, S. haematobium, and S. japonicum causes illness in humans; less commonly, S. mekongi and S. intercalatum can cause disease. Although the worms that cause schistosomiasis are not found in the United States, more than 200 million people are infected worldwide.

How can I get schistosomiasis?

Infection occurs when your skin comes in contact with contaminated freshwater in which certain types of snails that carry schistosomes are living.

Freshwater becomes contaminated by Schistosoma eggs when infected people urinate or defecate in the water. The eggs hatch, and if certain types of freshwater snails are present in the water, the parasites develop and multiply inside the snails. The parasite leaves the snail and enters the water where it can survive for about 48 hours. Schistosoma parasites can penetrate the skin of persons who are wading, swimming, bathing, or washing in contaminated water. Within several weeks, the parasites mature into adult worms and live in the blood vessels of the body where the females produce eggs. Some of the eggs travel to the bladder or intestine and are passed into the urine or stool.

What are the signs and 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.

When adult worms are present, the eggs that are produced usually travel to the intestine, liver 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, intestine, 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.

What should I do if I think I have schistosomiasis?

See your health care provider. If you have traveled to countries where schistosomiasis is found and had contact with freshwater, describe in detail where and for how long you traveled. Explain that you may have been exposed to contaminated water.

How is schistosomiasis diagnosed?

Your health care provider may ask you to provide stool or urine samples to see if you have the parasite. A blood sample can also be tested for evidence of infection. For accurate results, you must wait 6-8 weeks after your last exposure to contaminated water before samples are taken.

What is the treatment for schistosomiasis?

Safe and effective drugs are available for the treatment of schistosomiasis. Praziquantel is the recommended treatment drug. See your doctor for appropriate diagnosis and treatment.

Am I at risk?

If you live in or travel to areas where schistosomiasis occurs and your skin comes in contact with freshwater from canals, rivers, streams, ponds, or lakes, you are at risk of getting schistosomiasis.

In what areas of the world does schistosomiasis occur?

• Africa: contact with any freshwater in southern and sub-Saharan Africa–including the great lakes and rivers as well as smaller bodies of water– should be considered a risk for schistosomiasis transmission. Transmission also occurs in the Mahgreb region of North Africa and the Nile River valley in Egypt and Sudan .
• South America: Brazil, Suriname, Venezuela
• Caribbean: Dominican Republic, Guadeloupe, Martinique, Saint Lucia (risk in Caribbean is very low)
• The Middle East: Iran, Iraq, Saudi Arabia, Yemen
• Southern China
• Parts of Southeast Asia and the Philippines, Laos
• A recent focus of ongoing transmission has been identified in Corsica.

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 safe.
• Drink safe water. Although schistosomiasis is not transmitted by swallowing contaminated water, if your mouth or lips come in contact with water containing the parasites, you could become infected. Because water coming directly from canals, lakes, rivers, streams, or springs may be contaminated with a variety of infectious organisms, 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 to a rolling boil for at least 1 minute. Water held in a storage tank for at least 1-2 days should be safe for bathing.
• Vigorous towel drying after an accidental, very brief water exposure may help to prevent the Schistosoma parasite from penetrating the skin. However, you should NOT rely on vigorous towel drying to prevent schistosomiasis.

Schistosomiasis is an important cause of disease in many parts of the world, most commonly in places with poor sanitation. School-age children who live in these areas are often most at risk because they tend to spend time swimming or bathing in water containing infectious cercariae.
If you live in, or travel to, areas where schistosomiasis is found and are exposed to contaminated freshwater, you are at risk.

Areas where human schistosomiasis is found include:

• Schistosoma mansoni
  • Distributed throughout Africa: There is risk of infection in freshwater in southern and sub-Saharan Africa–including the great lakes and rivers as well as smaller bodies of water. Transmission also occurs in the Nile River valley in Sudan and Egypt.
  • South America: Including Brazil, Suriname, and Venezuela.
  • Caribbean (risk is very low): Dominican Republic, Guadeloupe, Martinique, and Saint Lucia.
• S. haematobium
  • Distributed throughout Africa: There is risk of infection in freshwater in southern and sub-Saharan Africa–including the great lakes and rivers as well as smaller bodies of water. Transmission also occurs in the Nile River valley in Egypt and the Mahgreb region of North Africa.
  • Found in areas of the Middle East.
  • A recent focus of ongoing transmission has been identified in Corsica.
• S. japonicum
  • Found in Indonesia and parts of China and Southeast Asia.
• S. mekongi
  • Found in Cambodia and Laos.
• S. intercalatum
  • Found in parts of Central and West Africa.

Infection occurs when skin comes in contact with contaminated freshwater in which certain types of snails that carry the parasite are living. Freshwater becomes contaminated by schistosome eggs when infected people urinate or defecate in the water. The eggs hatch, and if the appropriate species of snails are present in the water, the parasites infect, develop and multiply inside the snails. The parasite leaves the snail and enters the water where it can survive for about 48 hours. Larval schistosomes (cercariae) can penetrate the skin of persons who come in contact with contaminated freshwater, typically when wading, swimming, bathing, or washing. Over several weeks, the parasites migrate through host tissue and develop into adult worms inside the blood vessels of the body. Once mature, the worms mate and females produce eggs. Some of these eggs travel to the bladder or intestine and are passed into the urine or stool.

Symptoms of schistosomiasis are caused not by the worms themselves but by the body’s reaction to the eggs. Eggs shed by the adult worms that do not pass out of the body can become lodged in 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, intestine, spleen, lungs, and bladder.

Common symptoms

Most people have no symptoms when they are first infected. However, within days after becoming infected, they may develop a rash or itchy skin. Within 1-2 months of infection, symptoms may develop including fever, chills, cough, and muscle aches.

Chronic schistosomiasis

Without treatment, schistosomiasis can persist for years. Signs and symptoms of chronic schistosomiasis include: abdominal pain, enlarged liver, blood in the stool or blood in the urine, and problems passing urine. Chronic infection can also lead to increased risk of liver fibrosis or bladder cancer.

Rarely, eggs are found in the brain or spinal cord and can cause seizures, paralysis, or spinal cord inflammation.

Safe and effective medication is available for treatment of both urinary and intestinal schistosomiasis. Praziquantel, a prescription medication, is taken for 1-2 days to treat infections caused by all schistosome species.

Prevention

No vaccine is available.

The best way to prevent schistosomiasis is to take the following steps if you are visiting or live in an area where schistosomiasis is transmitted:

• 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 safe.
• Drink safe water. Although schistosomiasis is not transmitted by swallowing contaminated water, if your mouth or lips come in contact with water containing the parasites, you could become infected. Because water coming directly from canals, lakes, rivers, streams, or springs may be contaminated with a variety of infectious organisms, you should either bring your water to a rolling boil for 1 minute or filter water before drinking it. Bring your water to a rolling boil 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.
• Water used for bathing should be brought to a rolling boil for 1 minute to kill any cercariae, and then cooled before bathing to avoid scalding. Water held in a storage tank for at least 1 – 2 days should be safe for bathing.
• Vigorous towel drying after an accidental, very brief water exposure may help to prevent parasites from penetrating the skin. However, do not rely on vigorous towel drying alone to prevent schistosomiasis.

Those who have had contact with potentially contaminated water overseas should see their health care provider after returning from travel to discuss testing.

Control

In countries where schistosomiasis causes significant disease, control efforts usually focus on:

• Reducing the number of infections in people and/or
• Eliminating the snails that are required to maintain the parasite’s life cycle.

For all species that cause schistosomiasis, improved sanitation could reduce or eliminate transmission of this disease. In some areas with lower transmission levels, elimination of schistosomiasis is considered a “winnable battle” by public health officials.

Control measures can include mass drug treatment of entire communities and targeted treatment of school-age children. Some of the problems with control of schistosomiasis include:

• Chemicals used to eliminate snails in freshwater sources may harm other species of animals in the water and, if treatment is not sustained, the snails may return to those sites afterwards.
• For certain species of the parasite, such as S. japonicum, animals such as cows or water buffalo can also be infected. Runoff from pastures (if livestock are infected) can contaminate freshwater sources.