SCHISTOSOMIASIS

also called bilharziasis group of chronic disorders caused by small, parasitic flatworms, species of trematodes of the family Schistosomatidae, which are commonly called blood flukes (see fluke). They live in the blood vessels of man and other mammals, releasing eggs that produce tissue damage. There are three main types of schistosomiasis, caused by closely related organisms: (1) Japonica, or Eastern, schistosomiasis is caused by Schistosoma japonicum, found in Japan, Central and South China, the Philippines, Thailand, and Indonesia. (2) Manson's, or intestinal, schistosomiasis is caused by S. mansoni, found in Africa, Arabia, Puerto Rico, the West Indies, and northern South America. (3) Vesical, or urinary, schistosomiasis is caused by S. haematobium, found throughout Africa and the Middle East and in southern Europe. Depending on the species of worm, the female fluke, 10 to 25 millimetres (0.4 to 1 inch) long, releases 300 to 3,500 eggs daily into the blood. The eggs find their way into the intestine or bladder and are evacuated in the feces or urine. On contact with fresh water, the eggs hatch, releasing ciliated larvae that swim about until they find an appropriate snail host, in which they develop further. Fork-tailed larvae, the cercariae, subsequently emerge from the snail into the water and, upon contact with the skin of a mammal, drop their tails and penetrate the tissues, getting into the blood circulation, where they feed. The clinical course of schistosomiasis usually begins with an allergic reaction to the parasites and their by-products, and symptoms may include inflammation, cough, late-afternoon fever, skin eruption (giant urticaria), and swelling and tenderness of the liver. There may then be blood in the stools and urine in the more acute stage. The chronic stage of the disease is characterized by the gradual impactment of eggs into the walls of the body organs, leading to fibrous thickening and loss of elasticity. In the intestinal types there may be serious liver damage, and in urinary schistosomiasis the eggs in the bladder may become focuses of stone formation, fibrosis may extend to the pelvic organs, and there may be secondary bacterial infection of the urinary tract. Occasionally, eggs may also cause lesions of such organs as the brain and lungs. During autopsy, fluke eggs have been detected in almost all body structures. Unless exposure is overwhelming, however, early diagnosis and persistent treatment usually ensure recovery. Chemotherapy is aimed at killing the adult worms, which, if undisturbed, can live for 20 years and continue to cause damage. The drugs that are used are usually antimony compounds. Schistosomiasis is ordinarily contracted by working, bathing, or swimming in water populated by snails that carry the worms. The disease is more prevalent in rural communities in which standards of hygiene are low. Irrigation projects, aimed at improving the welfare of the people, have actually been known to introduce the carrier snails and the disease, as happened in Egypt. The World Health Organization has estimated the number of infected persons to number between 180 million and 200 million. Next to malaria, it is probably humanity's most serious parasitic infection. Snail control with such molluscicides as sodium pentachlorophenate, dinitro-o-cyclohexylphenol, and copper sulfate has been used extensively but with uneven success.