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Toxocariasis or toxocarosis is a helminthic infection of humans caused by the dog or cat roundworm, Toxocara canis or Toxocara cati, respectively. Humans can become infected by ingestion of embryonated eggs (containing fully developed larva, L3) from contaminated sources. There are two main syndromes: visceral larva migrans (VLM), which encompasses diseases associated with major organs; and ocular larva migrans (OLM), in which toxocariasis pathological effects on the host are restricted to the eye and the optic nerve.[1]
Toxocariasis occurs around the world. Epidemiologic surveys show a 2-5% positive rate in healthy adults from urban Western countries and 14.2-37% in rural areas.[2] In tropical countries, surveys show a positive rate of 63.2% in Bali, 86% in Saint Lucia, and 92.8% in Réunion.[2] Toxocariasis is most commonly a disease of children, typically children aged 2-7 years.
Risk factors include exposure to contaminated soil, and contact with unwormed pups. In young children, geophagia (a form of pica) is also a factor (2-10% of children between one and three years of age indulge in habitual geophagia).
Adult worms of the Toxocara family often live in the small intestine of dogs and cats. They range from 4-12 cm in length. Almost all puppies are infected at or soon after birth. During the summer, Toxocara infective eggs are shed. They survive for years in the environment, and humans typically ingest the eggs orally by eating with contaminated hands (Most commonly from handling infected feces with bare hands). Once introduced into the human intestine, the eggs develop into larvae. The larval form is less than 0.5 mm in length and 0.02 mm wide. The larvae penetrate the bowel wall and migrate through blood vessels to reach the liver, muscles, and lungs. Sometimes the parasite penetrates into the eye and brain.
Disease severity is affected by the number of eggs ingested, duration of infection, tissue location of larvae, and the immune response to the infection.
In suspected cases, diagnosis is confirmed by: an increase in the anti-Toxocara excretory-secretory antigen IgE level; a recent history of exposure to susceptible dogs and cats; sustained eosinophilia, hyperglobulinemia, and hepatomegaly; and liver biopsy demonstrating degenerated larvae at the centre of eosinophilic granulomas.
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