Toxocariasis (Visceral and Ocular Larva Migrans)

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Chapter 290 Toxocariasis (Visceral and Ocular Larva Migrans)

Arlene E. Dent, James W. Kazura

Etiology

Most cases of human toxocariasis are caused by the dog roundworm, Toxocara canis. Adult female T. canis worms live in the intestinal tracts of young puppies and their lactating mothers. Large numbers of eggs are passed in the feces of dogs and embryonate under optimal soil conditions. Toxocara eggs can survive relatively harsh environmental conditions and are resistant to freezing and extremes of moisture and pH. Humans ingest embryonated eggs contaminating soil, hands, or fomites. The larvae hatch and penetrate the intestinal wall and travel via the circulation to the liver, lung, and other tissues. Humans do not excrete T. canis eggs because the larvae are unable to complete their maturation to adult worms in the intestine. The cat roundworm, Toxocara cati, is responsible for far fewer cases of visceral larva migrans (VLM) than T. canis. Ingestion of infective larvae of the raccoon ascarid Baylisascaris procyonis rarely leads to VLM, but can cause neural larva migrans resulting in fatal eosinophilic meningitis. Ingestion of larvae from the opossum ascarid Lagochilascaris minor leads to VLM rarely.

Epidemiology

Human T. canis infections have been reported in nearly all parts of the world, primarily in temperate and tropical areas where dogs are popular household pets. Young children are at highest risk because of their unsanitary play habits and tendency to place fingers in the mouth. Other behavioral risk factors include pica, contact with puppy litters, and institutionalization. In North America, the highest prevalences of infection are in the southeastern USA and Puerto Rico, particularly among socially disadvantaged African-American and Hispanic children. In the USA, serosurveys show that 4.6-7.3% of children are infected. Assuming an unrestrained and untreated dog population, toxocariasis is prevalent in settings where other geohelminth infections such as ascariasis, trichuriasis, and hookworm infections are common.

Pathogenesis

T. canis larvae secrete large amounts of immunogenic glycosylated proteins. These antigens induce immune responses that lead to eosinophilia and polyclonal and antigen-specific immunoglobulin E (IgE) production. The characteristic histopathologic lesions are granulomas containing eosinophils, multinucleated giant cells (histiocytes), and collagen. Granulomas are typically found in the liver but may also occur in the lungs, central nervous system (CNS), and ocular tissues. Clinical manifestations reflect the intensity and chronicity of infection, anatomic localization of larvae, and host granulomatous responses.

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