PARASITES AND PARASITIC DISEASES OF DOMESTIC ANIMALS


Toxocara canis


Life Cycle

Toxocara canis has a complex ascarid life cycle.   Like most other nematodes, T. canis is not immediately infectious when it leaves the definitive host.  It must grow and develop into the infective stage, ensheathed L2, in order for it to infect the definitive host.

Dogs may become  infected by four routes:

  1. Direct transmission, by ingesting infective eggs.
  2. Paratenic host transmission, by ingesting infected mice.
  3. Transmammary transmission in which nursing pups ingest L3s in their mother's milk.
  4. Prenatal transmission where pups are born infected as a result of L2s migrating from tissue reservoirs in the pregnant bitch - across the placenta and through the umbilical vein to the fetal liver, where they remain until birth.   They then resume migration to the lungs of the newborn pups.

The life cycle patterns of T. canis in puppies and in dogs over six months of age are different.

Nursing pups are mainly infected by prenatal transmission of larvae from their mothers (A).  Larvae reach the small intestine after migrating from the lungs of newborns and move up the bronchial tree and trachea to the pharynx, where they are swallowed and develop to maturity in the small intestine.  If pups are infected by direct ingestion of infective eggs (B), hatched larvae will also follow a tracheal migration.  The third, and least common method of transmission in nursing pups - transmammary transmission of L3s (C) - does not involve tracheal migration. Instead, ingested larvae develop directly to adults in the small intestine. 

In dogs over six months old, infections occur either by direct ingestion of infective eggs (D) or by ingesting infected mouse paratenic hosts (E).  In the case of paratenic host transmission, no further migration takes place in dogs since the requirement for lifecycle migration is satisfied in the mouse hosts.  In direct transmission, only a small proportion of larvae undergo tracheal migration, while the majority continue migrating through the lungs and the pulmonary veins to the heart, where they are distributed to somatic tissues via the peripheral circulation.  This somatic migration sets up the conditions for prenatal and transmammary transmission to pups since latent somatic larvae are reactivated during each pregnancy and migrate either across the placenta to the fetal liver after the 42nd day of gestation or to the mammary gland.

 

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Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000