Centretown Veterinary Hospital

[Centretown Veterinary Hospital]

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Parasites: Heartworm

[Photo of heartworms in the chest cavity of a pet]
Heartworms By permission from Darke P, Kelly DF, Bonagura JD, Color Atlas of Veterinary Cardiology, Mosby, 1995

Heartworm (Dirofilaria immitis) is transmitted to dogs from mosquitoes. The worm infects dogs as a microscopic little worm called a microfilaria and then grows to large spagettini type worms that can completely block the heart causing death. Along the course of this lifecycle this worm can cause a number of other serious problems.

This parasite was principally found further South, however it is now being found north of the border, most likely due to the marginal increase in the length of our summers which allow for the heartworm to complete an extra lifestage in the mosquitoes before they are killed by frost.

Heartworm preventatives kill the heartworm before they reach the heart. There are a number of heart worm medications which each have their pros and cons. But before you give ANY heartworm medication your pets should be tested to see if they already have heartworm. If medication is given to an infected animal it can kill adult worms which then die and act as bullets hitting the lungs. For this reason we recommend testing for heart worm if your dog has been untreated during mosquito season the previous year.