Echinococcosis Hydatidosis

The larval stages of three tapeworms of the genus Echinococcus can cause severe disease in humans. All three normally become adults in the intestines of dogs or other canids. Eggs are passed in the feces and, if ingested by a herbivore, develop in the liver or other organs into a saclike container of larvae, the hydatid cyst. Carnivores become infected by eating cysts with the flesh of the herbivore. Echinococcus granulosus, which commonly has a sheep—dog cycle, but which may also infect goats, cattle, swine, and camels, is the most likely to infect human beings. Human echinococcosis occurs primarily in sheep-rearing areas. Dogs ingest cysts in the offal of dead sheep and pass eggs in their feces. Humans acquire the eggs from a dog's fur or from contaminated food or water. Cysts holding 2 or more liters of fluid and larvae can grow for years in the liver, lungs, brain, or other organs and exert enough mechanical pressure to cause grave or fatal consequences. Rupture of a cyst by trauma or surgery releases daughter cysts, which may grow elsewhere in the victim; the hydatid fluid can cause fatal anaphylactic shock.

Hydatid cysts in humans and animals have been known since Roman times, but, as was true for the other tapeworms, the relationship between the larval cyst and the adult worm was not suspected until the eighteenth century. E. granulosus was described as a separate species in 1850, and its life cycle was worked out with feeding experiments in 1863.

The parasite first became a serious danger to humans when animals were domesticated. The expansion of European settlement spread infection to the Americas and Australasia. Echinococcosis is also found in North Africa, Cyprus, the Middle East, northern and central Asia, and most of sub-Saharan Africa. Iceland was a major focus in the mid-nineteenth century, but education, dog control, and sanitary slaughtering have eliminated the disease. Similar methods, including the mass treatment of dogs, have greatly reduced its incidence in Australia and New Zealand. The highest known prevalence of echinococcosis is among the Turkana, a stock-raising people in northwestern Kenya. Colonial medical authorities in Kenya did not recognize the disease until the 1950s, and an incidence rate of 96 per 100,000 was estimated in 1976, more than seven times the previous record rate in Cyprus. The

Turkana problem is linked to a very close association between children and dogs.

Echinococcosis is often difficult to diagnose. Cysts can be detected by X-rays or surgery; several serologic tests are also used. Surgery was the only effective therapy until the early 1980s, when the drug mebendazole was employed with at least some success.

K. David Patterson

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