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Schistosomiasis, also known as bilharziasis or snail fever, is a primarily tropical parasitic disease caused by the larvae of one or more of five types of flatworms or blood flukes known as schistosomes. The name bilharziasis comes from Theodor Bilharz, a German pathologist, who identified the worms in 1851.


Infections associated with worms present some of the most universal health problems in the world. In fact, only malaria accounts for more diseases than schistosomiasis. The World Health Organization (WHO) estimates that 200 million people are infected and 120 million display symptoms. Another 600 million people are at risk of infection. Schistosomes are prevalent in rural and outlying city areas of 74 countries in Africa, Asia, and Latin America. In Central China and Egypt, the disease poses a major health risk.
There are five species of schistosomes that are prevalent in different areas of the world and produce somewhat different symptoms:
  • Schistosoma mansoni is widespread in Africa, the Eastern-Mediterranean, the Caribbean, and South America and can only infect humans and rodents.
  • S. mekongi is prevalent only in the Mekong river basin in Asia.
  • S. japonicum is limited to China and the Philippines and can infect other mammals, in addition to humans, such as pigs, dogs, and water buffalos. As a result, it can be harder to control disease caused by this species.
  • S. intercalatum is found in central Africa.
  • S. haematobium occurs predominantly in Africa and the Eastern Mediterranean.
Intestinal schistosomiasis, caused by Schistosoma japonicum, S. mekongi, S. mansoni, and S. intercalatum, can lead to serious complications of the liver and spleen. Urinary schistosomiasis is caused by S. haematobium.
It is difficult to know how many individuals die of schistomiasis each year because death certificates and patient records seldom identify schistosomiasis as the primary cause of death. Mortality estimates vary related to the type of schistosome infection but is generally low, for example, 2.4 of 100,000 die each year from infection with S. mansoni.

Causes and symptoms

All five species are contracted in the same way, through direct contact with fresh water infested with the free-living form of the parasite known as cercariae. The building of dams, irrigation systems, and reservoirs, and the movements of refugee groups introduce and spread schistosomiasis.
Eggs are excreted in human urine and feces and, in areas with poor sanitation, contaminate freshwater sources. The eggs break open to release a form of the parasite called miracidium. Freshwater snails become infested with the miracidium, which multiply inside the snail and mature into multiple cercariae that the snail ejects into the water. The cercariae, which survive outside a host for 48 hours, quickly penetrate unbroken skin, the lining of the mouth, or the gastrointestinal tract. Once inside the human body, the worms penetrate the wall of the nearest vein and travel to the liver where they grow and sexually mature. Mature male and female worms pair and migrate either to the intestines or the bladder where egg production occurs. One female worm may lay an average of 200 to 2,000 eggs per day for up to twenty years. Most eggs leave the blood stream and body through the intestines. Some of the eggs are not excreted, however, and can lodge in the tissues. It is the presence of these eggs, rather than the worms themselves, that causes the disease.

Early symptoms of infection

Many individuals do not experience symptoms. If present, it usually takes four to six weeks for symptoms to appear. The first symptom of the disease may be a general ill feeling. Within twelve hours of infection, an individual may complain of a tingling sensation or light rash, commonly referred to as "swimmer's itch," due to irritation at the point of entrance. The rash that may develop can mimic scabies and other types of rashes. Other symptoms can occur two to ten weeks later and can include fever, aching, cough, diarrhea, or gland enlargement. These symptoms can also be related to avian schistosomiasis, which does not cause any further symptoms in humans.

Katayama fever

Another primary condition, called Katayama fever, may also develop from infection with these worms, and it can be very difficult to recognize. Symptoms include fever, lethargy, the eruption of pale temporary bumps associated with severe itching (urticarial) rash, liver and spleen enlargement, and bronchospasm.

Intestinal schistosomiasis

In intestinal schistosomiasis, eggs become lodged in the intestinal wall and cause an immune system reaction called a granulomatous reaction. This immune response can lead to obstruction of the colon and blood loss. The infected individual may have what appears to be a potbelly. Eggs can also become lodged in the liver, leading to high blood pressure through the liver, enlarged spleen, the buildup of fluid in the abdomen (ascites), and potentially life-threatening dilations or swollen areas in the esophagus or gastrointestinal tract that can tear and bleed profusely (esophageal varices). Rarely, the central nervous system may be affected. Individuals with chronic active schistosomiasis may not complain of typical symptoms.

Urinary tract schistosomiasis

Urinary tract schistosomiasis is characterized by blood in the urine, pain or difficulty urinating, and frequent urination and are associated with S. haematobium. The loss of blood can lead to iron deficiency anemia. A large percentage of persons, especially children, who are moderately to heavily infected experience urinary tract damage that can lead to blocking of the urinary tract and bladder cancer.


Proper diagnosis and treatment may require a tropical disease specialist because the disease can be confused with malaria or typhoid in the early stages. The healthcare provider should do a thorough history of travel in endemic areas. The rash, if present, can mimic scabies or other rashes, and the gastrointestinal symptoms may be confused with those caused by bacterial illnesses or other intestinal parasites. These other conditions will need to be excluded before an accurate diagnosis can be made. As a result, clinical evidence of exposure to infected water along with physical findings, a negative test for malaria, and an increased number of one type of immune cell, called an eosinophil, are necessary to diagnose acute schistosomiasis.
Eggs may be detected in the feces or urine. Repeated stool tests may be required to concentrate and identify the eggs. Blood tests may be used to detect a particular antigen or particle associated with the schistosome that induces an immune response. Persons infected with schistosomiasis may not test positive for six months, and as a result, tests may need to be repeated to obtain an accurate diagnosis. Blood can be detected visually in the urine or with chemical strips that react to small amounts of blood.
Sophisticated imaging techniques, such as ultrasound, computed tomography scan (CT scan), and magnetic resonance imaging (MRI), can detect damage to the blood vessels in the liver and visualize polyps and ulcers of the urinary tract, for example, that occur in the more advanced stages. S. haematobium is difficult to diagnose with ultrasound in pregnant women.


The use of medications against schistosomiasis, such as praziquantel (Biltricide), oxamniquine, and metrifonate, have been shown to be safe and effective. Praziquantel is effective against all forms of schistososmiasis and has few side effects. This drug is given in either two or three doses over the course of a single day. Oxamniquine is typically used in Africa and South America to treat intestinal schistosomiasis. Metrifonate has been found to be safe and effective in the treatment of urinary schistosomiasis. Patients are typically checked for the presence of living eggs at three and six months after treatment. If the number of eggs excreted has not significantly decreased, the patient may require another course of medication.

Key terms

Ascites — The condition that occurs when the liver and kidneys are not functioning properly and a clear, straw-colored fluid is excreted by the membrane that lines the abdominal cavity (peritoneum).
Cercariae — The free-living form of the schistosome worm that has a tail, swims, and has suckers on its head for penetration into a host.
Miracidium — The form of the schistosome worm that infects freshwater snails.


If treated early, prognosis is very good and complete recovery is expected. The illness is treatable, but people can die from the effects of untreated schistomiasis. The severity of the disease depends on the number of worms, or worm load, in addition to how long the person has been infected. With treatment, the number of worms can be substantially reduced, and the secondary conditions can be treated. The goal of the World Health Organization is to reduce the severity of the disease rather than to completely stop transmission of the disease. There is, however, little natural immunity to reinfection. Treated individuals do not usually require retreatment for two to five years in areas of low transmission. The World Health Organization has made research to develop a vaccine against the disease one of its priorities.


Prevention of the disease involves several targets and requires long term community commitment. Infected patients require diagnosis, treatment, and education about how to avoid reinfecting themselves and others. Adequate healthcare facilities need to be available, water systems must be treated to kill the worms and control snail populations, and sanitation must be improved to prevent the spread of the disease.
To avoid schistosomiasis in endemic areas:
  • contact the CDC for current health information on travel destinations.
  • upon arrival, ask an informed local authority about the infestation of schistosomiasis before being exposed to freshwater in countries that are likely to have the disease.
  • do not swim, stand, wade, or take baths in untreated water.
  • treat all water used for drinking or bathing. Water can be treated by letting it stand for three days, heating it for five minutes to around 122°F (around 50°C), or filtering or treating water chemically, with chlorine or iodine, as with drinking water.
  • Should accidental exposure occur, infection can be prevented by hastily drying off or applying rubbing alcohol to the exposed area.



Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435, (404) 639-3311.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.


 [shis″-, skis″to-so-mi´ahsis]
infection with flukes of the genus Schistosoma; called also bilharziasis. The disease is rare in North America, but is a significant health problem in many parts of the world, including the Middle East, Africa, East Asia, South America, and the Caribbean. Different species cause different forms of the disease: S. mansoni and S. japonicum produce intestinal symptoms, while S. haematobium produces hematuria and other urinary symptoms. The early stages of acute infection with S. japonicum produce katayama fever, a serum sickness–like illness. Initial treatment includes correction of anemia and other nutritional disorders caused by the parasites; definitive treatment may be delayed until acute symptoms subside. Anthelmintic medications are administered, such as oxamniquine or praziquantel. Improvement in sanitation and snail control are the chief preventive measures.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


(shis', skis'tō-sō-mī'ă-sis),
Infection with a species of Schistosoma; manifestations of this often chronic and debilitating disease vary with the infecting species but depend in large measure on tissue reaction (granulation and fibrosis) to the eggs deposited in venules and in the hepatic portals, the latter resulting in portal hypertension and esophageal varices, as well as liver damage leading to cirrhosis. See: tropic diseases.
See also: schistosomal dermatitis, Symmers clay pipestem fibrosis.
Farlex Partner Medical Dictionary © Farlex 2012


n. pl. schistosomia·ses (-sēz′)
Any of various chiefly tropical diseases caused by infestation with schistosomes and usually acquired through contact with contaminated water, characterized by gradual destruction of the tissues of the kidneys, liver, and other organs. Also called bilharzia, bilharziasis, snail fever.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.


Bilharziasis A parasitic infection by a trematode acquired from infested water, which can live in humans, producing liver, bladder, and GI disease
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.


Infection with a species of Schistosoma; manifestations of this often chronic and debilitating disease vary with the infecting species but depend in large measure on tissue reaction (granulation and fibrosis) to the eggs deposited in venules and in the hepatic portal system, the latter resulting in portal hypertension and esophageal varices, as well as liver damage leading to cirrhosis.
See also: schistosomal dermatitis
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


A parasitic worm disease common in most tropical countries and acquired during immersion in water contaminated with the larval forms of schistosome worms, such as Schistosoma mansoni . These penetrate the skin and travel to the veins of the bowel, bladder, liver and other organs where they grow to adult size-1 to 2.5 cm. Worm eggs are excreted in urine and faeces. Late complications of schistosomiasis include CYSTITIS, loss of normal bladder function, bladder stones, cancer of the bladder, kidney failure and CIRRHOSIS of the liver. Some species of schistosomes can cause bowel and liver upset and may involve the nervous system to cause epilepsy, paralysis, coma and death. Schistosomiasis can be avoiding by staying out of water which might be contaminated with human excreta. The drug praziquantel will kill the adult worms. The name derives from the fact that the body of the adult male worm is effectively split longitudinally to form a ‘gynaecophoric canal’ in which the female worm usually lies.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005


Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005


Infection with a species of Schistosoma can elicit portal hypertension and esophageal varices, as well as liver damage leading to cirrhosis.
Medical Dictionary for the Dental Professions © Farlex 2012
References in periodicals archive ?
At the end of 2014, Merck founded the Global Schistosomiasis Alliance together with partners such as the Bill & Melinda Gates Foundation, the U.S.
According to him, schistosomiasis control focuses more on reducing its spread through periodic and large scale population treatment with praziquantel.
mansoni schistosomes was presented recently (9); given autochthonous transmission of intestinal schistosomiasis in Lake Malawi (10), there may be sufficient epidemiologic opportunity for other introgression events to occur with the hybrids we report.
According to the World Health Organization, schistosomiasis is an acute and chronic disease caused by parasitic worms that infect people during routine agricultural, domestic, occupational, and recreational activities that expose them to infested water.
At baseline, the participants were interviewed face-to-face in isiZulu on water contact, schistosomiasis infection (local name: isichenene) and if they had received prior treatment.
At the study area (six basic schools) were selected according to the burden of schistosomiasis in White Nile state, the number of pupils in these schools was 1696 individuals, and the sample size (382) pupils were determined using the formula.
In Nigeria, urinary schistosomiasis is a serious health problem with about 29 million infected cases and 101 million people at risk of infection [7-9].
Prof Hoffmann said: "These findings open up the possibility of controlling schistosomiasis by encouraging or engineering these natural microbes to turn against the snail, thereby reducing snail numbers and parasite transmission in endemic areas".
The GHIT's latest round of funding will go towards its Phase III clinical trials for treating schistosomiasis in paediatric patients.