Heartworm Treatment

By Dr Jennifer Merry BSc(Agr) DVM

Spring is here – which means soon we will begin testing Ontario dogs for heartworm. The blood test is taken 6 months after the last possible date of exposure the year before. In this area, that means April 15.

Due to a large selection of easy to administer monthly heartworm prevention, we do not see many heartworm positive dogs, and that is lucky because heartworm is difficult and expensive to treat.

If a dog has a positive heartworm test, the first step is to repeat the test. We don’t want to start this process until we are sure – and false positives do happen occasionally. Then the dog’s blood is looked at under a microscope to see if any microfilaria (baby heartworm) can be seen. The more there are the more dangerous the treatment. And this may change how quickly we want the treatment to occur. Dogs will then have complete bloodwork done to ensure there is no preexisting damage to internal organs that may affect the drugs chosen. Then, chest x-rays are taken to check for changes to the heart and lungs.

Medication that kills adult heartworms is not effective in worms under 4 months old so a strict schedule has to be created to maximize effectiveness while minimizing risk.  Many heartworms have bacteria themselves which add to the heartworm disease so dogs are treated for twice daily for 4 weeks with an antibiotic to decrease this risk.  Heartworm medication is started at the same time to reduce new infections and eliminate existing susceptible life-stages (larvae). At this time steroids are started and exercise restriction enforced to protect against a violent immune reaction to the dead and dying worms.  This is the part that many owners find the most difficult: no exercise – at all – for the next 3 months. Leash walks and cage rest only.

The medication used to kill the adult worms is given in 3 separate doses 1 month apart. It is given deep into the back muscles and so the dog is sedated each time  to prevent motion or discomfort.  Three months after beginning, another heartworm test is done to test for effectiveness of the treatment.

Heartworm is a serious disease affecting dogs. It is easy to prevent and really difficult to treat, so see your veterinarian to start your dog on seasonal prevention that is right for your lifestyle.