So what is Heartworm?
Thanks to increased media attention, most people have heard of heartworm but may not understand how it can affect their pets. Heartworm (Dirofilaria immitis) is a parasite that is spread by mosquitoes and affects domestic dogs and wild canids (ex. foxes and coyotes). Cats can very occasionally have a low-level, transient infection and rarely develop symptoms. Unlike in the southern States where warm temperatures make heartworm a year round problem, in Ontario we only have to worry about it in warmer months. Usually the season is considered June 1-November 1 as frost normally occurs in early October.
Each year heartworm testing is done 5-6 months after the last possible infection date of the previous year. So after April 15 a blood test is taken from your dog to check for the presence of heartworm antigens. That is your dog’s response to the presence of female heartworms in it’s blood. Yup sexist as it may be – we can only test for the presence of female heartworms. So it is possible to be infected with only male worms and give a negative test. Luckily we don’t think that happens much.
If your dog’s heartworm test is negative you can start giving heartworm prevention on June 1. Many people use a combination product that also helps kill fleas. And there are new products on the market that help kill ticks, too. Products are mostly given monthly, and come in either topical (liquid applied to the pet’s skin) or oral formulations. Talk to your veterinary team about the product combination that suits your pet’s lifestyle the best.
How Clappison Vet Tackles Parasite Season
Spring is here – which means we have begun 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. 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 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 can be very painful 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 Waterdown veterinarian to start your dog on seasonal prevention that is right for your lifestyle.
Dr. Jennifer Merry BSc(Agr)
