Biology

Causal Agents

Toxocariasis in humans is caused by infection with larvae of Toxocara spp., which are common ascarid roundworms of mammals. Confirmed zoonotic species include the dog roundworm T. canis (presumed most common) and the cat roundworm T. cati (frequency not known). It is not known whether other closely-related Toxocara species can infect humans (e.g. T. malaysiensis of cats).

Life Cycle

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Toxocara spp. can follow a direct (one host) or indirect (multiple host) life cycle. Unembryonated eggs are shed in the feces of the definitive host (canids: T. canis; felids: T. cati) . Eggs embryonate over a period of 1 to 4 weeks in the environment and become infective, containing third-stage (L3) larvae . Following ingestion by a definitive host , the infective eggs hatch and larvae penetrate the gut wall. In younger dogs (T. canis) and in cats (T. cati), the larvae migrate through the lungs, bronchial tree, and esophagus, where they are coughed up swallowed into the gastrointestinal tract; adult worms develop and oviposit in the small intestine . In older dogs, patent (egg-producing) infections can also occur, but larvae more commonly become arrested in tissues. Arrested larvae are reactivated in female dogs during late gestation and may infect pups by the transplacental (major) and transmammary (minor) routes in whose small intestine adult worms become established . In cats, T. cati larvae can be transmitted via the transmammary route to kittens if the dam is infected during gestation, but somatic larval arrest and reactivation does not appear to be important as in T. canis.

Toxocara spp. can also be transmitted indirectly through ingestion of paratenic hosts. Eggs ingested by suitable paratenic hosts hatch and larvae penetrate the gut wall and migrate into various tissues where they encyst . The life cycle is completed when definitive hosts consume larvae within paratenic host tissue , and the larvae develop into adult worms in the small intestine.

Humans are accidental hosts who become infected by ingesting infective eggs or undercooked meat/viscera of infected paratenic hosts . After ingestion, the eggs hatch and larvae penetrate the intestinal wall and are carried by the circulation to a variety of tissues (liver, heart, lungs, brain, muscle, eyes) . While the larvae do not undergo any further development in these sites, they can cause local reactions and mechanical damage that causes clinical toxocariasis.