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.
What Is Schistosomiasis?
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. This larva, known as a miracidium (meer-uh-SID-ce-um), swims around until it locates a certain type of aquatic snail that lives in that water (the type of snail depends on the particular species of the parasite). The miracidium then penetrates the tissuc of the snail. Once inside the snail, it passes through several stages of development, eventually multiplying into numerous larvae, called cercariae (sir-CARE-ee-ay),that leave the snail and swim through the water for up to two days. During that time, one or more may come into contact with a person who is bathing, wading, swimming, or washing clothes in the water. At that point, the parasite may burrow into bare skin and enter the bloodstream. Once it is in the person's blood, it matures into an adult worm.
The female adult worms lay their eggs within blood vessels. Where this occurs depends on the species of the worm. For instance, S. japonicum usually lays its eggs in vessels near the small intestine; S. mansoni usually lays them near the large intestine; and S. baematobium prefers blood vessels near the bladder. However, this pattern of behavior is not always followed, and eggs from the different species sometimes show up elsewhere in the human body. The eggs from all species gradually move to the urinary tract, liver, or intestines, and finally leave the body in the person's urine or feces. If feces (excreted waste) or urine from an infested person contaminate a freshwater source such as a pond, the eggs can enter the water and begin the life cycle all over again.
How Common Is Schistosomiasis?
Schistosomiasis does not occur in the United States, but it does have a major impact on millions of people who live in developing countries around the world. According to the World Health Organization, 200 million people worldwide are infested with the worms, with perhaps 20 million of those having serious symptoms, and an estimated 200,000 dying every year as a result.
The discase is most common in tropical regions, where it is a leading cause of illness. S. japonicum occurs in China, the Philippines and other parts of the Far East; S. mansoni in certain regions of Africa, the Middle East, South America and the Caribbean; S. haematobium in Africa and the Middle East; S. mekongi in Cambodia and other parts of Southeast Asia; and S. intercalatum mainly in the Democratic Republic of Congo and other parts of West Africa. People from the United States who travel to these areas sometimes develop schistosomiasis if they swim or wade in tainted water, but they rarely get the severe, chronic form of the disease.
What Are the Signs and Symptoms of Schistosomiasis?
A rash and itchy skin, particularly at the spot where the parasite burrowed into the body, may develop within a few days. Symptoms vary from person to person and may not even occur in some infected individuals. The symptoms also vary depending on the species of worm that has infected the person. The most common initial symptoms, however,appear about one to two months after the initial infestation and include muscle aches, fever, chills, and cough.
Certain other symptoms are associated with infections from different species. Infection with Schistosoma mansoni, for instance, sometimes causes such symptoms as nausca with blood in the vomit, lesions on the spinal cord, and/or an enlarged liver and spleen. S. haematobium infections can cause spinal cord lesions, frequent and/or painful urination, blood in the urine, and pain in the area of the anus and genital organs. Individuals infected with S. japonicum may experience nausea with blood in the vomit, a certain type of seizure known as focal epilepsy, and an enlarged liver and spleen. People who are infected with S. japonicum often also develop a fever, called Katayama fever, that can cause the body temperature to rise to 105 degrees Fahrenheit. The fever, which is sometimes also seen in S. mansoni infestations, may last several weeks, but it usually subsides on its own.
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