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.
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.
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.
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.
Schistosomiasis (shis-tuh-so-MY-uh-sis) is an illness caused by parasitic* worms. The worms must spend part of their life cycle growing in freshwater snails before they enter and cause infestations* in humans.
- (pair-uh-SIH-tik) refers to organisms such as protozoa (one-celled animals), worms, or insects that can invade and live on or inside human beings and may cause illness. An animal or plant harboring a parasite is called its host.
- refer to illnesses caused by multi-celled parasitic organisms, such as tapeworms, roundworms, or protozoa.
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Schistosomiasis is a parasitic disease that is not directly contagious from person to person. Five types of Schistosoma worm, also called blood flukes, can infest people and cause schistosomiasis: S. mansoni, S. japonicum, S. mekongi, S. intercalatum, and S. haematobium. These parasites have a complex life cycle; they have to go through several separate stages on their way to adulthood, and both snails and humans play important roles in that cycle. Another name for the disease is bilharziasis (bil-har-ZYE-uh-sis) or “snail fever.”
The worm starts life as an egg in a freshwater source such as a pond, lake, or stream. It hatches into a larva*, and if the right type of aquatic snails live in that water, the larva will find and enter a snail. There it passes through several stages of development. During the last phase in the snail, the parasite turns into a larva that can swim. It then leaves the snail and returns to the water, where it may come into contact with a person; the larva can survive in the water for up to 2 days without a human host. When people bathe, wade, swim, or wash clothes in the water, the parasite can burrow into bare skin and enter the bloodstream. Once it is in the blood, it matures into an adult worm.
- (LAR-vuh) is the immature form of an insect or worm that hatches from an egg.
Depending on the species, the female adult worms lay their eggs within blood vessels near the person’s bladder or liver. The eggs gradually move to the urinary tract, liver, and intestines*. Over time, some eggs pass through the urinary tract and intestines and are excreted when the person urinates or has a bowel movement. If feces (excreted waste) from an infested person contaminate a freshwater source such as a pond, the eggs can enter the water and begin the parasite life cycle all over again.
- are the muscular tubes that food passes through during digestion after it exits the stomach.
Schistosomiasis is not seen in the United States. However, the disease has a major impact on millions of people around the world in developing countries. According to the World Health Organization, more than 200 million people worldwide are infested with the worms, with 20 million of those having serious symptoms.
The disease is most common in tropical parts of the world, where it is a leading cause of illness. The parasites that cause schistosomiasis can be found in southern China, parts of the Middle East, and some countries in the Caribbean, South America, Africa, and southeast Asia. People from the United States who travel to those areas sometimes develop schistosomiasis if they swim or wade in tainted water, but they rarely get the severe, chronic* form of the disease.
- (KRAH-nik) means continuing for a long period of time.
A rash and itchy skin, particularly at the spot where the parasite burrowed into the body, may develop within a few days. The worms then mature and spread through the bloodstream, and 1 to 2 months later patients may have muscle aches, fever, chills, and cough. It is not uncommon, however, for people to not show any symptoms during this early stage of infestation. Over time, as the worms spread into the liver and intestines, patients can experience diarrhea (dye-uh-REE-uh), liver enlargement, vomiting, and abdominal* pain.
- (ab-DAH-mih-nul) refers to the area of the body below the ribs and above the hips that contains the stomach, intestines, and other organs.
If the doctor suspects schistosomiasis, he or she will collect a urine or stool (bowel movement) sample to look for the worm’s eggs. Several samples may need to be examined before the worms can be identified. The doctor also may take a sample of blood for testing, although the blood test may not show evidence of the infestation unless it is done 6 to 8 weeks after the patient’s contact with the parasite. Occasionally, a tissue biopsy* will be done to check for signs of the parasite in organs such as the liver.
- (BI-op-see) is a test in which a small sample of skin or other body tissue is removed and examined for signs of disease.
Doctors can prescribe medicine to treat the infestation. Patients usually need to take pills for only 1 to 2 days. Without treatment, and with continued use of the same tainted water source, the illness can last for years.
People who become re-infested with schistosomiasis again and again over many years can develop damage to the bladder, lungs, intestines, and liver; the disease is one of the leading causes of cirrhosis* in the world. In some cases, scarring of the liver is so severe that blood flowing through the organ becomes partly blocked, causing a condition known as portal hypertension. Severe portal hypertension can make veins in the esophagus* and stomach swell and bleed, sometimes to the point that the bleeding is fatal.
- (sir-O-sis) is a condition that affects the liver, involving long-term inflammation and scarring, which can lead to problems with liver function.
- (eh-SAH-fuh-gus) is the soft tube that, with swallowing, carries food from the throat to the stomach.
Other complications of the disease arise when the worm’s eggs travel through the bloodstream to the spinal cord or brain, where they can cause seizures*, inflammation of the spinal cord, or paralysis*.
- (SEE-zhurs) are sudden bursts of disorganized electrical activity that interrupt the normal functioning of the brain, often leading to uncontrolled movements in the body and sometimes a temporary change in consciousness.
- (pah-RAH-luh-sis) is the loss or impairment of the ability to move some part of the body.
Experts advise that travelers visiting countries where schistosomiasis occurs avoid wading, swimming, or bathing in any body of fresh water such as ponds, rivers, or lakes. Filtering or boiling drinking water for at least 1 minute will kill parasites, including the Schistosoma worms. The U.S. Centers for Disease Control and Prevention also recommends heating bathing water to 150 degrees Fahrenheit for at least 5 minutes to make sure it is free of potential parasites.
To reduce the spread of schistosomiasis, health officials focus on educating people who live in areas where the worms are found. They teach the public how the parasites spread and encourage people not to urinate or have bowel movements in rivers and ponds.
U.S. Centers for Disease Control and Prevention (CDC), 1600 Clifton Road, Atlanta, GA 30333. The CDC is the U.S. government authority for information about infectious and other diseases. The organization provides information about schistosomiasis at its website.
Telephone 800-311-3435 http://www.cdc.gov
Telephone 011-41-22-791-2111 http://www.who.int
Schistosomiasis, also known as bliharzia or bilharziasis, is a parasitic infection caused by trematodes, also known as flatworms or flukes, of the genus Schistosoma. There are many species of animal schistosomes worldwide, with five responsible for the majority of human infections: S. haematobium, S. mansoni, S. japonicum, S. intercalatum, and S. mekongi. Schistosomiasis is the second most common human parasitic disease, malaria being the most common.
The World Health Organization estimates that 600 million people worldwide are at risk of infection, with 200 million already infected. Of these, over 120 million have a symptomatic infection. The disease is endemic in over seventy-five countries.
Schistosomes are blood flukes that have two distinct life-cycle stages: a sexual stage in mammals and an asexual stage in freshwater snails. Humans acquire infection when they come into contact with freshwater lakes and rivers containing infective schistosome larvae, called "cercariae." Cercariae penetrate the skin, migrate through the bloodstream and, in the case of S. mansoni, S. japonicum, S. intercalatum, and S. mekongi, come to rest in the mesenteric venous plexus. S. haematobium cercariae end up in venous plexus surrounding the urinary bladder. The male and female worms mature into adults and form a permanent mating pair that lives up to five years. Approximately six weeks after the initial infection, the females begin to produce between 200 and 3,000 eggs a day, depending on the species. Approximately half of the eggs are excreted in the urine or stool. If the excreted eggs reach freshwater, they hatch into free swimming miracidiae that will infect the appropriate snail species. Further development occurs within the snail and, after three to five weeks, a new generation of cercariae emerge from the snail ready to infect other mammalian hosts.
Most infections are asymptomatic. In a minority of cases, a transient illness may occur several weeks after the initial infection, known as Katayama fever, characterized by fever, cough, abdominal pain, and diarrhea.
Many eggs are not excreted and end up trapped in tissues. The host's granulomatous inflammatory response to these eggs is responsible for most of the damage associated with chronic schistosomiasis. S. haematobium eggs, which are mainly found around the bladder, can result in hematuria, ureteric obstruction, and bladder cancer. Eggs of the other species usually lodge in mesenteric vessels draining to the liver and cause periportal fibrosis, with the subsequent development of portal hypertension, splenomegaly, esophageal varices, and progressive liver dysfunction. Eggs in the bowel mucosa cause ulcerations and polyp formation leading to diarrhea and abdominal pain. When portal hypertension occurs, eggs are shunted to the lungs, where pulmonary hypertension may occur.
Diagnosis is made by finding the characteristic eggs in stool or urine. Because eggs may be excreted intermittently, several specimens should be examined. Occasionally, a rectal or bladder biopsy may be necessary. Serology is the most sensitive diagnostic tool and is particularly useful for detecting light infections. However, the antibody test does not distinguish between past and current infection, so it is not clinically useful in areas of high prevalence where individuals may have been successfully treated and then reinfected.
The drug of choice for treatment is praziquantel. Other options include oxamniquine for treatment of S. mansoni and metrifonate for S. haematobium. Treatment may reverse some of the long-term sequelae of infection, including fibrosis, especially in children.
Infection control is based on two strategies: reduction of transmission and reduction of morbidity. Reduction of transmission is accomplished by providing safe water supplies and proper sanitation facilities. Snail eradication is not an effective long-term strategy. Much of the focus of current schistosomiasis control strategies is to minimize the morbidity caused by the infection through mass treatment of at risk populations with praziquantel. This approach also leads to the reduction of egg output and transmission.
(see also: Communicable Disease Control; Tropical Infectious Diseases )
Ali El-Garem, A. (1998). "Schistosomiasis." Digestion 59:589–605.
Bica, I.; Hamer, D. H.; and Stadecker, M. J. (2000). "Hepatic Schistosomiasis." Infectious Disease Clinics of North America 14(3):583–604.
Dunne, D. W.; Hagan, P.; and Abath, F. G. C. (1995). "Prospects for Immunological Control of Schistosomiasis." Lancet 345:1488–1492.
Elliot, D. E. (1996). "Schistosomiasis, Pathophysiology, Diagnosis and Treatment." Gastroenterology Clinics of North America 25(3):599–625.
Lucey, D. R., and Maguire, J. H. (1993). "Schistosomiasis." Infectious Disease Clinics of North America 7(3):635–653.
Mostafa, M. H.; Sheweita, S. A.; and O'Connor, P. J. (1999). "Relationship between Shistosomiasis and Bladder Cancer." Clinical Microbiology Reviews 12(1): 97–111.
World Health Organization (1993). "The Control of Schistosomiasis: Second Report of the WHO Expert Committee." WHO Technical Report Series 803:1–86.
—— (1996). "Schistosomiasis." (Fact Sheet No. 115). Geneva: Author.
Human blood fluke disease, also called schistosomiasis or bilharziasis, is a major parasitic disease affecting over 200 million people worldwide, mostly those in the tropics. Although sometimes fatal, schistosomiasis more commonly results in chronic ill-health and low energy levels. The disease is caused by small parasitic flatworms of the genus Schistosoma. Of the three species , two (S. haematobium and S. mansoni ) are found in Africa and the Middle East, the third (S. japonicum ) in the Orient. Schistosoma haematobium lives in the blood vessels of the urinary bladder and is responsible for over 100 million human cases of the disease a year. Schistosoma mansoni and Schistosoma japonicum reside in the intestine; the former species infect 75 million people a year and the latter 25 million.
Schistosomiasis is spread when infected people urinate or defecate into open waterways and introduce parasite eggs that hatch in the water. Each egg liberates a microscopic free-living larva called the miracidium which bores into the tissues of a water snail of the genus Biomphalaria, Bulinus, or Onchomelania, the intermediate host. Inside the snail the parasite multiplies in sporocyst sacs to produce masses of larger, mobile, long-tailed larvae known as cercariae. The cercariae emerge from the snail into the water, actively seek out a human host, and bore deep into the skin. Larvae that reach the blood vessels are carried to the liver where they develop into adult egg-producing worms that settle in the vessels of the urinary bladder or intestine. Adult Schistosoma live entwined in mating couples inside the small veins of their host. Fertilized females release small eggs (0.2 mm long), at the rate of 3,500 per day, which are carried out of the body with the urine or the feces.
The symptoms of schistosomiasis correlate with the progress of the disease. Immediately after infection migrating cercariae cause itching skin. Subsequent establishment of larvae in the liver damages this organ. Later, egg release causes blood in the stool (dysentery), damage to the intestinal wall, or blood in the urine (hematuria), and damage to the urinary bladder.
Schistosomiasis is increasing in developing countries due in part to rapidly increasing human populations. In rural areas, attempts to increase food production that include more irrigation and more dams also increase the habitat for water snails. In urban areas the combination of crowding and lack of sanitation ensures that increasingly large numbers of people become exposed to the parasite.
Most control strategies for schistosomiasis target the snail hosts. One strategy kills snails directly by adding snail poisons (molluscicides) to the water. Another strategy either kills or removes vegetation upon which snails feed. Biological methods of snail control include the introduction of fish that feed on snails, of snails that kill schistosome snail hosts, of insect larvae that prey on snails, and of flukes that kill schistosomes inside the snail. Some countries, such as Egypt, have attempted to eliminate the parasite in humans through mass treatment with curative drugs including ambilar, niridazole, nicolifan, and praziquantel. Total eradication programs for schistosomiasis focus both on avoiding contact with the parasite through education, better sanitation, and on breaking its life cycle through snail control and human treatment.
[Neil Cumberlidge Ph.D. ]
Bullock, W. L. People, Parasites, and Pestilence: An Introduction to the Natural History of Infectious Disease. Minneapolis: Burgess Publishing Company, 1982.
Malek, E. A. Snail-Transmitted Parasitic Diseases. Boca Raton: CRC Press, 1980.
Markell, E. K., M. Voge, and D. T. John. Medical Parasitology. 7th ed. Philadelphia: Saunders, 1992.