Heartworms are parasites that live in the heart and lungs of dogs and, more rarely, cats. They are transmitted from animal to animal by mosquitoes. When a mosquito bites a dog infected with heartworms, the microscopic immature heartworms (microfilaria) in the blood get sucked up by the mosquito, where they develop into larvae. When the infected mosquito bites another dog, these larvae get transferred to him or her. The larvae then travel throughout the dogs body and settle in the right side of the heart and adjacent major vessels. It takes about 6 months for the larvae to fully develop into adult worms. Accumulation of adult worms in the heart impairs circulation and can cause congestive heart failure in time. Heartworms are treatable, but the damage they do to the heart is permanent, thus early detection is very important.
There are basically two ways to detect heartworms in a dog: by looking for microfilaria in the blood under a microscope, or by doing a test which detects proteins on the surface of the worms. The test for proteins (Antigen Test) is more reliable, as you can have adult worms that have not yet produced offspring (microfilaria), and thus would not be detectable by looking at the blood under a microscope. Dogs that have not been on heartworm preventative for over a year and have been infected for over 6 months should test positive by either testing method.
Treatment for heartworm disease can be very dangerous for the dog. As adult heartworms die, pieces of the worms break off and can lodge in the vessels in the lungs and other organs, cutting off blood supplies to these areas. The prevention is still far better than treatment.
Heartworms are very easy to prevent. Heartworm preventative kill the larvae that are passed along when an infected mosquito bites your dog. All it takes is once-a-month pills, some of which are even chewable and taste like a treat. In Ontario, “Heartworm Season” is June to November. Dogs that have not been on a preventative year-round must be tested each spring before starting back on preventative, to make sure they have not been infected by particularly hearty mosquito over the winter. Giving a preventative to an infected dog can be dangerous. Used properly, heartworm preventative is very safe and easy to use. The product we use at our practice also prevents infection by several types of intestinal worms!