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=Overview=

==Overview==

''Toxacara canis'' is a typical ascarid nematode that infects dogs where its predilection site is the small intestines. These worms can be found throughout the world with varying prevalence. Control of this ascarid is typically difficult due to its extended persistence in the environment. ''T. canis'' is also important in human medicine as the species most responsible for '''visceral larval migrans''' (VLM). The human is a non-permissive host of ''T. canis'' meaning it cannot complete its life cycle and reproduce, however the larval stages do migrate through the human body causing pathology.

''Toxacara canis'' is a typical ascarid nematode that infects dogs where its predilection site is the small intestines. These worms can be found throughout the world with varying prevalence. Control of this ascarid is typically difficult due to its extended persistence in the environment. ''T. canis'' is also important in human medicine as the species most responsible for '''visceral larval migrans''' (VLM). The human is a non-permissive host of ''T. canis'' meaning it cannot complete its life cycle and reproduce, however the larval stages do migrate through the human body causing pathology.

=Identification=

==Identification==

''Toxacara canis'' has the typical gross morphology of an ascarid, it is a large, fleshy white worm and can be up to 18cm long. The females are longer than the males who can normally reach 10cm in length. Microscopically ''T. canis'' has a fairly standard [[:Category: Ascaridoidea|ascaridoid]] appearance, though the adult head is given an elliptical shape by large alae or 'wings'. The eggs of ''T. canis'' are dark brown with a thick, pitted shell, the thick shell makes them very resistant in the environment.

''Toxacara canis'' has the typical gross morphology of an ascarid, it is a large, fleshy white worm and can be up to 18cm long. The females are longer than the males who can normally reach 10cm in length. Microscopically ''T. canis'' has a fairly standard [[:Category: Ascaridoidea|ascaridoid]] appearance, though the adult head is given an elliptical shape by large alae or 'wings'. The eggs of ''T. canis'' are dark brown with a thick, pitted shell, the thick shell makes them very resistant in the environment.

=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.

===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.

===Cycle 4===

====Cycle 4====

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. 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.

=Epidemiology=

==Epidemiology==

''T. canis'' is present worldwide with a wide range of prevalances in different areas from 5 - 80%. Adult animals carry the fewest worms since initial infection causes immunity which leads to the shedding of adult worms from the intestines. The low parasite burden in adult animals can often lead to asymptomatic infection though the parasites wil still shed eggs in the faeces. The largest numbers of worms are found in dogs less than 6 months old who have not yet gained immunity to the worms. The high levels of prevalence of this species worldwide is largely due to the difficulty in controlling its spread. The eggs are extremely resistant in the environment and so can persist for several years. The females lay very large numbers of eggs, up to 700 per gram of faeces, making the removal of such a large number difficult. This final reason for such a large spread is the long lasting reservoir of hypobiotic larvae that can be reactivated in pregnancy in the bitch, these are not susceptible to anthelmintics and so are only eliminated by preventing pregnancy or the death of the host. As a result of the large number of infected bitches almost all puppies are born with ''T. canis'' infections which increases the spread of the eggs as they pass faeces in new environments once the litter is split up.

''T. canis'' is present worldwide with a wide range of prevalances in different areas from 5 - 80%. Adult animals carry the fewest worms since initial infection causes immunity which leads to the shedding of adult worms from the intestines. The low parasite burden in adult animals can often lead to asymptomatic infection though the parasites wil still shed eggs in the faeces. The largest numbers of worms are found in dogs less than 6 months old who have not yet gained immunity to the worms. The high levels of prevalence of this species worldwide is largely due to the difficulty in controlling its spread. The eggs are extremely resistant in the environment and so can persist for several years. The females lay very large numbers of eggs, up to 700 per gram of faeces, making the removal of such a large number difficult. This final reason for such a large spread is the long lasting reservoir of hypobiotic larvae that can be reactivated in pregnancy in the bitch, these are not susceptible to anthelmintics and so are only eliminated by preventing pregnancy or the death of the host. As a result of the large number of infected bitches almost all puppies are born with ''T. canis'' infections which increases the spread of the eggs as they pass faeces in new environments once the litter is split up.

=Control=

Control of ''T. canis'' relies on effective clearing of the eggs form the environment as these can be infective in the environment for several years. This will prevent new infections of animals that have no been exposed previously as pups or as young dogs. However there are a number of endemic regions of the world where most animals have been exposed as pups and therefore can harbour hypobiotic larvae. These are difficult to eliminate and there are likely to be constantly be a small number of worms present, therefore regular treatment of dogs with anthelmintics is recommended.  

==Pathogenesis==

In puppies heavy infections can cause weight loss and poor growth as well as diarrhoea and vomiting. Pot belly may also be seen in pupies in some cases. With extremely heavy infections a plug may form that can cause intestinal impaction and prevent gastric movements. In adults, once immunity has developed, there are few clinical signs as most infections are too small for pathology to develop. In humans there is a zoonotic risk, as ''T. canis'' is the major agent of visceral larval migrans in children primarily with occular migration.

==== Life-Cycle ====

==== Life-Cycle ====

*''T. canis'' has a complicated life-cycle.

*''T. canis'' has a complicated life-cycle.