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{{Taxobox
{{Taxobox
|kingdom =Animalia
|kingdom =Animalia
|phylum =Nematoda
|phylum =Nematoda
|class =Secernentea
|class =Secernentea
|sub-class =
|sub-class =
|order =Ascaridida
|order =Ascaridida
|super-family =
|super-family =
|family =Toxocaridae
|family =Toxocaridae
|sub-family =
|sub-family =
|genus =Toxocara
|genus =Toxocara
|species =''T. canis''
|species =''T. canis''
}}
}}
==Life cycle==
==Life cycle==
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 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.
====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 L3 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.
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 L3 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.
====Cycle 3====
====Cycle 3====
In the pregnant bitch larvae that have become hypobiotic as described in cycle 2 above are reactivated by hormonal changes. These larvae become mobile about three weeks before parturition and migrate across the placenta to the lungs of the fetus. Within the fetal lungs the larvae moult just prior to birth. From the lungs the larvae complete their life cycle in the same way as in the young animal, by being coughed and swallowed to enter the small intestine. The adults will then produce eggs which are released in the faeces as normal.
In the pregnant bitch larvae that have become hypobiotic as described in cycle 2 above are reactivated by hormonal changes. These larvae become mobile about three weeks before parturition and migrate across the placenta to the lungs of the fetus. Within the fetal lungs the larvae moult just prior to birth. From the lungs the larvae complete their life cycle in the same way as in the young animal, by being coughed and swallowed to enter the small intestine. The adults will then produce eggs which are released in the faeces as normal.
The final life cycle involves transmission of L3 larvae to pups through the milk. Hypobiotic L3 larvae are reactivated and are either already present in the mammary glands or travel to them and are capable of passing in the milk during the first 3 weeks of lactation. There is no further migration in the pup when the larvae are ingested in this way and the remaining life cycle of the worm is completed in the small intestine of the pup.
The final life cycle involves transmission of L3 larvae to pups through the milk. Hypobiotic L3 larvae are reactivated and are either already present in the mammary glands or travel to them and are capable of passing in the milk during the first 3 weeks of lactation. There is no further migration in the pup when the larvae are ingested in this way and the remaining life cycle of the worm is completed in the small intestine of the pup.
As well as the above life cycles ''T. canis'' can infect paratenic hosts such as mice, rats and some birds. In these animals the L3 larvae migrate to tissues hypobiose until the hosts tissues are consumed by a dog in which the worm can complete its life cycle. The prepatent period of ''T. canis'' is 4 - 5 weeks.
As well as the above life cycles ''T. canis'' can infect paratenic hosts such as mice, rats and some birds. Events occur just as in the older dog, i.e. larvae migrate → liver → lungs → heart → somatic tissues → granulomatous reactions → 'waiting phase'; but in this case, the somatic larvae are waiting for the animal that they are in (which is acting as a '''paratenic host''') to be eaten by a dog, fox, wolf or other canid, where they will establish as adults or somatic larvae (depending on the age of the predator). This explains how humans (as warm-blooded non-canid animals) enter into the epidemiological picture. The prepatent period of ''T. canis'' is 4 - 5 weeks in the canid host.
==Epidemiology==
==Epidemiology==
* Causes eosinophilic enteritis in the dog
* Causes eosinophilic enteritis in the dog
'''In warm-blooded non-canid animals''':
'''In warm-blooded non-canid animals''':
Panacur
Panacur
Advocate
Advocate
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