Hydatids – when a dog is not man's best friend
Box 1 | The life-cycle of E. granulosus
The hydatid tapeworm (Echinococcus granulosus) requires two hosts, usually dogs and sheep, to complete its life cycle. The adult tapeworms are 5-7 millimetres long and live in the small intestine of dogs (the definitive host). Each adult worm can produce – by sexual means – up to 1000 microscopic eggs every 10 days for up to 2 years. The eggs pass out in the faeces of the dog and stick to the animal's fur or to grass. These eggs can survive for at least a year in the outside world, during which time they disperse widely. Flies help to spread the eggs, as does the wind. The adult tapeworms do not make their hosts ill. Yet clearly, a single infected dog (with up to 300,000 of these worms) could contaminate a large area with eggs. Sheep (the intermediate host) may swallow tapeworm eggs when eating, and people may pick up eggs on their hands either from contaminated grass or when they play with infected dogs. The eggs are then easily transferred from the hands to the mouth. After an egg has been swallowed, it hatches into an embryo, which burrows through the wall of the intestine into the blood stream. The embryo finally lodges in an organ, usually the liver or the lung. The parasite then grows to form a cyst filled with fluid. Thousands of larval tapeworms, called protoscoleces, can be produced by asexual budding from the inner wall of the cyst. The life cycle is completed if a dog eats a cyst from an infected sheep. Each larval tapeworm can then develop into an adult tapeworm, which eventually will produce new eggs and thereby continue the cycle.
Related site
Echinococcosis (Centers for Disease Control and Prevention, USA)
Page updated August 2006.