PARASITES AND PARASITIC DISEASES OF DOMESTIC ANIMALS
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:
- Direct transmission, by ingesting infective eggs.
- Paratenic host transmission, by ingesting infected
- Transmammary transmission in which nursing pups
ingest L3s in their mother's milk.
- 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.