Toxocara canis life cycle.
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1. The life cycle normally involves dogs; humans are infected only accidentally. Unembryonated eggs are passed in feces of dogs (the definitive host).
2. In the environment, eggs embryonate and become infective.
3. After eggs are ingested by a dog, they hatch and release larvae, which penetrate the intestinal wall.
4. In younger dogs, the larvae migrate through the lungs and bronchial tree; larvae are coughed up, swallowed, and returned to the small intestine, where they mature. Adult female worms deposit eggs in the small intestine. Although older dogs may be infected in the same way, larvae are more likely to encyst in tissues.
5. Encysted stages are reactivated in female dogs during late pregnancy and infect the puppies via the transplacental or transmammary route.
6. As a result, adult worms become established in the small intestine of the puppies (a major source of environmental contamination).
7. Toxocara canis can also be transmitted through ingestion of transport hosts. Eggs ingested by small mammals (eg, rabbits) hatch into larvae, which penetrate the intestinal wall and migrate into various tissues where they encyst.
8. The life cycle is completed when dogs eat these hosts, and the larvae develop into egg-laying adult worms in the small intestine.
9–10. Humans are accidental hosts who become infected by ingesting infective eggs in contaminated soil or infected transport hosts. After ingestion, the eggs hatch into larvae.
11. The larvae penetrate the intestinal wall and are circulated to various tissues (eg, liver, heart, lungs, brain, muscle, eyes).
Image from the Centers for Disease Control and Prevention Image Library.