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Heartworm Disease in Ontario

With the warmer weather already here, it is important that we discuss some of the parasites and health-risks that are associated with the spring and summer months. Heartworm represents an important part of this discussion.


Heartworm disease is a serious and potentially life-threatening condition caused by parasitic worms that find refuge in the heart and surrounding blood vessels. Our pets contract heartworm disease from mosquitoes that are infected with heartworm larvae. When an infected mosquito bites a dog, the heartworm larvae are injected into the skin, where their eventually find their way to the heart and lungs where they mature into adults, and begin to reproduce. As their numbers increase, they begin to occlude the pulmonary artery and eventually the chambers of the heart. This decreases the ability for the heart to pump blood and deliver oxygen to the vital organs. The presence of the worms also creates a significant inflammatory response, causing permanent scarring of the heart and surrounding tissues. Left untreated, heartworm disease is fatal. The worms can grow to a length of 15-30 centimetres, and in severe cases a dog may be infested with hundreds of worms. Mature worms produce thousands of larvae that can then be spread to other dogs or cats by mosquitoes. Dogs are extremely susceptible to heartworm disease, with nearly 100% who are exposed to larvae becoming infected.

Heartworm is a disease that poses a serious threat to pets in Canada and the United States, and especially to dogs. A study on “Heartworm in dogs in Canada in 2010” published by the Ontario Veterinary College’s Department of Pathobiology at the University of Guelph has revealed a 60 per cent increase in the number of dogs in Ontario with heartworm since the last study of its kind was conducted in 2002. The Canada-wide study showed that 564 dogs tested positive for heartworm in Canada in 2010; 431 of those dogs were located in Ontario (that’s over 75 per cent of the total!). Also of concern is that eighty per cent of animals that were found to have heartworm had not been on a heartworm preventive medication. In a few cases, pets that had been receiving heartworm preventive medication also tested positive for the disease; the most common reason for this was that the pet owners indicated they had forgotten to give their pet the preventive medication at the prescribed intervals.

Fortunately, heartworm can be easily prevented. Regular testing and the use of an easy, once-a-month preventive medication is the first step. Heartworm testing is of great importance, as it provides a fast, efficient and reliable method of detecting heartworm disease early, before any permanent damage occurs to the heart, lungs and other organs such as the kidneys and liver. Without regular testing, heartworm disease is often not detected until a patient is showing clinical signs of infection, which includes coughing and exercise intolerance. Unfortunately, by the time clinical signs are present, irreversible damage has likely occurred to the heart and major blood vessels.

We have started heartworm testing at our clinic, and preventative treatments are started on June 1st, 2012. Call us at 905.285.0002 to book an appointment today!



Dog thinking about ticks and fleas

Year-round protection means more peace of mind!

“Tick season” used to mean spring/summer/fall, with a break over the winter – the colder weather meant that we could take a break from worrying about these pesky bugs and the diseases they can transmit. But in the last few years, we’ve seen a change creeping up on us, with the weather staying warm later into the season, and spring arriving earlier each year – and the bugs are loving it!

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