Heartworm & Parasite Season by Dr. Tiffany Rennick

As spring approaches, so does Ontario’s heartworm season.  June to November in our province brings mosquitoes, and mosquitoes bring heartworm. Heartworm disease is caused by a parasite called Dirofilaria immitis. Any dog can become infected with heartworm. Heartworm infections can occur in cats but are less common.

Heartworm in Ontario has been on the rise. The most recent Canada wide study was done in 2010 and showed a 60% increase in the number of heartworm cases since 2002. Over 75% of the dogs that tested positive in Canada were located in Ontario. The majority of the positive cases in Ontario occurred in Southwestern Ontario and in dogs that had not been on preventative medication.

In an infected animal, adult heartworms produce immature heartworms called microfilariae. When a mosquito bites an infected pet, it becomes infected with heartworm microfilariae. In warm weather, the microfilariae mature to larvae within the mosquito and are transferred to another pet when the mosquito bites/feeds. The larvae mature into adult worms and travel to the heart and lungs. They can grow to be up to 14 inches long. The adult worms cause serious damage to the arteries of the lungs and lead to heart failure. If left untreated, a severe heartworm infection can be fatal.

Clinical signs of heartworm infection usually do not appear until later in the disease process when there are a number of adult worms present and considerable damage has occurred. A persistent cough, decreased tolerance for exercise, decreased appetite and weight loss are commonly seen in dogs. In cats, smaller numbers of worms can cause heart failure and be fatal much earlier in the infection.

Treating heartworm involves a series of injections that kill the adult worms. The treatment and recovery can take one to two months and is very costly. In addition to the cost, there is the risk of serious and life threatening complications related to the dead worms blocking arteries in the lungs.

Preventing and controlling heartworm is very easy. A simple blood test each year can detect if your pet has been exposed to heartworm. This allows prompt treatment before the heartworms do irreversible damage to organs like the heart and lungs. In addition to testing, monthly preventative medications are given during heartworm season to prevent infections. The medication disrupts the lifecycle of the heartworm to prevent adult worms from developing.

So what can you do to keep your dog healthy? Annual veterinary examinations can help detect problems early. The best time to test your dog for heartworm is in the spring before starting heartworm prevention medication. For cats, testing has been less reliable because of the small number of worms present. Preventative medication is still recommended for cats. At North Town Veterinary Hospital, our doctors and technicians will perform the heartworm blood test for you and help you select the best preventative medication for your pet. We look forward to partnering with you in keeping your dogs and cats as healthy as possible!