The life cycle of this tapeworm is sustained between a definitive host (foxes, dogs, cats, coyotes, wolves and other wild canids) and intermediate herbivore hosts (sheep, cattle, voles, shrews, field mice, etc).
The life cycle involves extra-intestinal migration in the intermediate host (usually sheep or cattle) and formation of an hydatid cyst.
When the dog eats the intermediate host, ingests the hydatid cyst, the protoscolices attach to the small intestinal wall and the worms begin to form proglottids. Gravid proglottids, containing the eggs, detach from the end of the worm and spill their eggs into the lumen of the intestine. The eggs, with characteristic hooks visible on the oncosphere, pass out in the feces. The prepatent period is less than 40 days.
Humans get infected by accidental ingestion of eggs from petting, or via fecal contamination of food or utensils. Metacercariae develop into cystic lesions, principally in liver and lungs, after several years. Both canine echinococcosis and human hydatidosis are commonly found in rural farming communities, though there are some reports of human and dog infection in urban areas.
Species which have been reported in dogs include:
- Echinococcus granulosus
- Echinococcus multilocularis
- Echinococcus vogeli
- Echinococcus oligarthrus
- Echinococcus alveolaris
Most infected dogs are asymptomatic, however cystic hydatid disease does occur, with cyst found in the liver, spleen and sometimes lungs. These cysts become large irregular, fluid-filled masses which grow like a malignant tumor with metastases.
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