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Toxocariasis: Visceral and ocular larva migrans

Peter F Weller, MD, MACP
Karin Leder, MBBS, FRACP, PhD, MPH, DTMH
Section Editor
Edward T Ryan, MD, DTMH
Deputy Editor
Elinor L Baron, MD, DTMH


Toxocariasis (also called visceral larva migrans [VLM]) refers to human infection caused by roundworms that are not natural human parasites. Toxocariasis occurs as a result of human infection with the larvae of the dog ascarid, Toxocara canis, or, less commonly, the cat ascarid, Toxocara cati. Another form of VLM is caused by human ingestion of eggs of the pig ascarid, Ascaris suum. Clinical presentations consist of VLM and ocular larva migrans (OLM); infection may also be subclinical.


Toxocariasis occurs worldwide [1]. Infection tends to occur more frequently in tropical regions than in temperate regions and more frequently among rural populations than urban populations [2]. Toxocara larvae can develop at temperatures <50°F, although efficiency decreases as the temperature decreases [2].

In the United States, the seroprevalence of Toxocara has been estimated at 13.9 percent [1]; rates are increased among individuals living in poverty and among certain minority groups (especially African Americans) [3]. Prevalence rates of 40 percent or more have been reported in Indonesia and Brazil [4].

Life cycle — The life cycle of Toxocara canis occurs in dogs and the life cycle of Toxocara cati occurs in cats; the prevalence of Toxocara infection is highest among puppies and kittens. In North America, it is estimated that about 5 percent of dogs and puppies are infected [5].

Humans acquire the infection as accidental hosts (figure 1). Eggs are shed in the stool of the definitive host. In the environment, eggs embryonate and become infective after about three weeks. Soil contamination with infectious eggs occurs most readily in relatively warm climates, and these embryonated eggs can remain infective in the environment for several years [6]. Following ingestion of infective eggs by dogs or cats, the eggs hatch and larvae (0.5 mm in length) penetrate the gut wall. The larvae then migrate through the lungs, bronchial tree, and enter the esophagus; adult worms develop in the small intestine, where they lay eggs that are shed in the stool.

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Literature review current through: Nov 2017. | This topic last updated: Oct 28, 2016.
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