Schistosomiasis
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.
Resources
Books
Basch, P. F. Schistosomes: Development, Reproduction, and Host Relations. New York: Oxford University Press, 1991.
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.
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