VPA 211: General Veterinary Parasitology and Helminthology (3+1)

Life cycle of toxocara canis

LIFE CYCLE OF T. canis

• Very complex. It depends on the age of the host.

Types of life cycle

• Direct
• Prenatal
• Transmammary
• Paratenic host involvement

Direct

• Eggs developed to infective stage within 10 to 15 days.
• Pups upto 3 months of age:
  • Following ingestion of embryonated eggs hatch in the duodenum, the liberated L₂ penetrate intestinal wall and follow the tracheal route of migration.
  • The L₂ moult to become L₃ in lungs about 10 days after infection.
  • After reaching the intestine L₃ moult to L₄ in 2 weeks of infection. The L₄ moult to L₅ in about 5 weeks of infection.
  • Tracheal route of migration occur only upto 3 months pup. If the pup is above 3 months, tracheal route is less frequent but somatic migration is common.

Prenatal infection

• In pups above 3 months of age, following ingestion of embryonated egg hatch in the intestine, the liberated L₂ are distributed to various tissues and organs like liver, kidney, brain and lungs, where they remain dormant.
• The dormant larvae begins to migrate in the pregnant bitches about 3 weeks prior to whelping (after 42^nd day of pregrancy) and enter into the foetus. When the larvae reaches the liver of the foetus, L₂ moult to become L₃.
• At birth L₃ present in the lungs of puppy. During 1^st week of birth L₃ moult to L₄ in the lungs, by the 2^nd week of birth L₄ moult to L₅ and pass to the intestine.
• Eggs appear in the faeces 23 to 40 days after birth of the pup.

Transmammary route

• Larvae passed to suckling pups via colostrum. These larvae reach the intestine and directly develop to adult without migration.

Paratenic host

• Rodents may act as a paratenic host. Rodent may ingest the embryonated eggs. In which the liberated L2 distributed to various organs and remain dormant. Infection of dogs is by ingestion of paratenic host. In the intestine of dog larvae develop to adult directly without migration.