Toxocariasis is a zoonotic parasitic disease caused by the nematode, Toxocara, of which there are 2 species: Toxocara canis and Toxocara cati. Toxocara eggs are shed in the feces of infected animals and, once in the environment, become infectious within 2 to 4 weeks. Humans are accidental hosts and become infected through ingestion of dirt or contaminated material containing Toxocara eggs. Although uncommon, individuals can also get toxocariasis by eating undercooked or raw meat from infected animals. Upon ingestion, Toxocara eggs hatch and larvae are released, which can penetrate the intestinal wall, travel through the bloodstream, and migrate to a variety of tissues (eg, liver, heart, lungs, brain, muscles, eyes). Although Toxocara larvae do not undergo any further development at these sites, they can cause severe local inflammatory reactions, which are the basis of toxocariasis.
While the majority of infected people do not have any symptoms, the 2 primary clinical presentations of toxocariasis are visceral larva migrans (visceral toxocariasis) and ocular larva migrans (ocular toxocariasis). Manifestations of toxocariasis reflect parasitic burden, immune response, and resulting inflammation. Symptoms of larva migrans may be characterized by Loffler syndrome (eg, fever, coughing, wheezing, abdominal pain), hepatomegaly, eosinophilia, or irreversible eye problems. Rarely, larvae migrate to the central nervous system, causing eosinophilic meningoencephalitis or granuloma formation. Larvae can also migrate to and penetrate the eye, resulting in ocular toxocariasis, which may lead to retinal scarring, decreased vision, and leukocoria.
A Toxocara seroprevalence study in the United States showed that approximately 5% of the US population is infected with Toxocara. Globally, toxocariasis is found in many countries, and rates of prevalence can be as high as 40%, particularly in tropical regions where eggs remain viable in the soil. Children and adolescents under the age of 20, as well as dog owners, are at higher risk of infection.
Diagnosis of Toxocara infections involves obtaining relevant clinical and exposure history and relies on antibody detection to Toxocara species. Eosinophilia may also be present, more commonly in visceral toxocariasis. Stool examination for ova and parasites is not useful since eggs are not excreted by humans, only by domestic animals. Currently, antibody testing is the only means of confirming a clinical diagnosis. The recommended serologic test for toxocariasis is an enzyme-linked immunosorbent assay using larval-stage antigens. However, a measurable titer does not distinguish between current and past Toxocara infection. Laboratory findings should be correlated with clinical history.