Members of the genus Toxocara are zoonotic intestinal nematodes (roundworms) that mature in various mammals, including some domesticated species. Parasitized animals can shed large numbers of eggs in the feces, infecting people (particularly children) who ingest these eggs in contaminated soil, or on hands or objects. Although Toxocara eggs do not complete their maturation in humans, the developing larvae can migrate through the body for a time. In some cases, they cause symptoms ranging from mild, vague discomfort to ocular disturbances, blindness and neurological syndromes. Human toxocariasis is one of the most common helminth infections in the world, with children living in poverty at the highest risk of infection. In some areas, this disease may also be important in adults who eat undercooked animal tissues containing larvae.

Human toxocariasis is mainly attributed to Toxocara canis and T. cati, the major roundworm species found in dogs and cats, but other Toxocara may also be involved. In particular, the importance of T. malaysiensis, a recently recognized species in cats, and T. vitulorum, a parasite of cattle and water buffalo, remain to be clarified. In puppies and kittens, Toxocara infections can be associated with unthriftiness, diarrhea and poor growth, and in severe cases, may result in death. T. vitulorum in bovine or buffalo calves may, similarly, lead to illness, economic losses and increased mortality.