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Microbiology · Nematodes

Toxocara canis / Toxocara cati !

  • Parasite
  • Exam priority: medium

A roundworm of dogs and cats. A child who swallows its eggs from soil can have larvae wandering through liver, lung or eye, with no adult worm.

Updated 2026-11-01

Also called: dog roundworm, cat roundworm

At a glance

Shape
Microscopic larva in human tissues
Family
Toxocaridae
Type
Dog and cat nematode; humans harbour only migrating larvae
Size
The adult, in the dog or cat gut, is several centimetres long

Morphology

  • Humans harbour only larvae in tissue, surrounded by eosinophilic granulomas.
  • Eggs (in dog and cat faeces) are subspherical, thick-walled and pitted.
  • There are no Toxocara eggs in human stool.

Transmission

  • Swallowing embryonated eggs from soil in parks, sandboxes and yards with dog or cat faeces.
  • Geophagia and pica in young children.
  • Rarely, raw meat or offal from paratenic hosts (chicken, cattle, rabbit).

Reservoir: Dogs, especially puppies (T. canis); Cats (T. cati); Contaminated soil

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Life cycle

  1. The dog (T. canis) or cat (T. cati) passes unembryonated eggs in faeces.
  2. In soil they embryonate in 1–4 weeks and become infective.
  3. Puppies are infected mostly across the placenta and through milk; adult animals by eggs or prey.
  4. A child swallows eggs from soil; the larva hatches, crosses the gut and migrates in the blood.
  5. In humans the larva never matures: it wanders through liver, lung, eye or brain and leaves granulomas.

Virulence factors

  • Migrating larvae release antigens that drive eosinophilic inflammation and granulomas.
  • In the eye, the granuloma can mimic retinoblastoma (leukocoria).
  • The number of eggs swallowed determines severity.

Diseases it causes

Clinical picture

  • Visceral larva migrans mainly in preschool children with geophagia.
  • Ocular larva migrans in older children or young adults: one-sided vision loss, strabismus, leukocoria.
  • Covert toxocariasis: abdominal pain, cough and mild eosinophilia.
  • Many infections are asymptomatic.

Diagnosis

  • EIA serology with larval excretory-secretory antigens; a positive titre does not prove active infection.
  • Marked eosinophilia and high IgE in the visceral form.
  • Ocular form: fundoscopy and antibodies in vitreous or aqueous fluid.
  • Stool exams are useless: humans pass no eggs.

Treatment

Benzimidazole antiparasitics (albendazole) for the visceral form; the ocular form is managed with corticosteroids and sometimes vitreoretinal surgery.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Deworm puppies, kittens and adult pets under veterinary care.
  • Pick up pet faeces in parks and yards.
  • Handwashing after playing in soil and before eating.
  • Prevent geophagia and cover sandboxes.

Test yourself

Think of the answer, then tap the question to see it.

Why is a stool exam useless for human toxocariasis?

Because in humans the larva never becomes an adult and no eggs are produced.

Which childhood tumour can ocular larva migrans mimic?

Retinoblastoma, since both can cause leukocoria.

What is the main route of Toxocara infection in children?

Swallowing embryonated eggs from soil contaminated with dog or cat faeces, often through geophagia.

How are puppies mainly infected with T. canis?

Transplacentally and, to a lesser degree, through the mother's milk.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Toxocariasis (CDC, 2019); About Toxocariasis | Toxocariasis | CDC (CDC, 2024).

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Representative model, for educational purposes only.