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

 

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) image . Eggs embryonate over a period of 1 to 4 weeks in the environment and become infective, containing third-stage (L3) larvae image . Following ingestion by a definitive host image , 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 image . 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 image in whose small intestine adult worms become established image . In cats, T. cati larvae can be transmitted via the transmammary route image 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 image . The life cycle is completed when definitive hosts consume larvae within paratenic host tissue image , and the larvae develop into adult worms in the small intestine.

Humans are accidental hosts who become infected by ingesting infective eggs image or undercooked meat/viscera of infected paratenic hosts image . 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) image . While the larvae do not undergo any further development in these sites, they can cause local reactions and mechanical damage that causes clinical toxocariasis.

Hosts

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.

Geographic Distribution

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.

Clinical Presentation

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.

Page last reviewed: July 9, 2019