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.
Toxocara canis infects essentially all wild and domestic canids; patent infections are more prevalent among puppies than older dogs. T. cati is found in wild and domestic felids of all ages, but patent infections are slightly more common in kittens.
Paratenic host ranges for both species encompass numerous species of mammals and birds. Livestock are important paratenic hosts; some human cases have been linked to consumption of undercooked beef, lamb, chicken, and duck meat (particularly liver). Cockroaches and earthworms have been experimentally infected, and could possibly serve as paratenic or transport hosts.
Toxocara canis and T. cati are cosmopolitan parasites of domestic dogs and cats. While common globally, prevalence in both animals and people is highest in developing countries. In developed countries, more infections are detected among persons in lower socioeconomic strata.
The main clinical presentations of toxocariasis are visceral larva migrans (VLM) and ocular larva migrans (OLM), although most infections are asymptomatic. In VLM, which occurs mostly in preschool children, the larvae invade multiple tissues (commonly liver, lung, skeletal muscle, occasionally heart) and cause various nonspecific symptoms (e.g. fever, myalgia, weight loss, cough, rashes, hepatosplenomegaly) usually accompanied by hypereosinophilia.
Migration to the central nervous system (neurotoxocariasis or neural larva migrans (NLM)) is uncommon and can cause eosinophilic meningoencephalitis. Death can occur in instances of severe cardiac, pulmonary, or neurologic involvement.
In OLM, the larvae produce various ophthalmologic lesions, and may cause diffuse unilateral subacute neuroretinitis (DUSN). Involvement is typically unilateral (affecting one eye) and associated visual impairment usually presents with uveitis, retinitis, or endophthalmitis; permanent visual damage or blindness can occur. Associated larval granulomas have in some cases have been misdiagnosed as retinoblastoma. OLM most often occurs in older children or young adults, who uncommonly have visceral manifestations.