Dogs become infected with Toxocara canis after ingesting embryonated eggs. Following ingestion, the larva are released in the small intestine. They then enter the circulatory system, and travel to the lungs, to the trachea, and then to the pharynx, where they are swallowed and sent to the lumen of the intestine. Eggs are passed into the external environment through the feces, and enter the infective stage.
While T. canis is found most commonly in dogs, other hosts can become infected following the ingestion of T. canis eggs.
The pathology of Toxocara canis results from migrating larva. In dogs, although this infection is typically asymptomatic, heavy infections can be fatal to puppies.
In humans, this can lead to two syndromes, including visceral larva migrans, in which the larva travel throughout the body, producing varied symptoms such as fever, coughing, enlarged liver, pneumonia, or anorexia, and ocular larva migrans, in which the larva reach the posterior region of the eye, which can result in blindness.