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).
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.