Changes

Typically of an ascarid ''T. canis'' has larvae have a migratory life cycle that is significance in the pathogenesis of infection. This species also has the most complex life cycle in the Ascaridoidea superfamily. There are four different life cycles that can occur dependant on the circumstances that the larvae or adult encounter.       

Typically of an ascarid ''T. canis'' has larvae have a migratory life cycle that is significance in the pathogenesis of infection. This species also has the most complex life cycle in the Ascaridoidea superfamily. There are four different life cycles that can occur dependant on the circumstances that the larvae or adult encounter.       

====Cycle 1====

====Cycle 1====

This is mostly a typical ascarid life cycle and commonly occurs in dogs that are infected between 2 and 3 months old. The infective eggs contain L3 larvae which hatch in the small intestine of the host dog after being ingested. The larvae then enter the hepatic portal vein and travel through the liver and further to the lungs where they moult to L4. The larvae then migrate to the trachea where they are coughed up and swallows again by the host. This is known as hepato-tracheal migration. On returning to the small intestine they undergo two further moults before becoming adults.

This is mostly a typical ascarid life cycle and commonly occurs in dogs that are infected between 2 and 3 months old. The infective eggs contain L2 larvae which hatch in the small intestine of the host dog after being ingested. The larvae then enter the hepatic portal vein and travel through the liver and further to the lungs where they moult to L3. The larvae then migrate to the trachea where they are coughed up and swallowed again by the host. This is known as hepato-tracheal migration. On returning to the small intestine they undergo the final moults (L3-->L4-->L5) before becoming adults.

 

====Cycle 2====

====Cycle 2====

In older dogs (above 3 months) the migration changes and the hepato-tracheal route occurs far less often, though can still occur. In these animals the L2 larvae hatch in the small intestine and travel to a wide variety of tissues throughout the body. Once the larvae have reached a tissue they will begin hypobiosis and encyst in the tissue until reactivated. In some animals the hypobiotic larvae will not reactivate and this will be the end of their life cycle. Hypobiotic larvae in the tissues of the dog are known as '''somatic larvae''', although these do not grow or develop they are highly metabolically active. The produce large quantities of excretory/secretory antigens which are spread over the cuticle of the worm. These antigens are important in immune evasion by way of having a rapid turnover and sloughing off host antibodies and immune cells.

In older dogs (above 3 months) the migration changes and the hepato-tracheal route occurs far less often, though can still occur. In these animals the L2 larvae hatch in the small intestine and travel to a wide variety of tissues throughout the body. Once the larvae have reached a tissue they will begin hypobiosis and encyst in the tissue until reactivated. In some animals the hypobiotic larvae will not reactivate and this will be the end of their life cycle. Hypobiotic larvae in the tissues of the dog are known as '''somatic larvae''', although these do not grow or develop they are highly metabolically active. The produce large quantities of excretory/secretory antigens which are spread over the cuticle of the worm. These antigens are important in immune evasion by way of having a rapid turnover and sloughing off host antibodies and immune cells.