My-PetDoctor News Letter

Heartworm Disease in the Greater Toronto Area

Introduction:

-Heartworm disease is a parasite disease of dogs caused by Dirofilaria immitis, which is transmitted between animals by mosquitoes.
-Once found only in the southeastern United States, heartworm disease has been diagnosed in nine Canadian provinces and in every state except Hawaii.

Heartworm Infection in the greater Toronto Area:

Ontario continues to be one of the principal foci for heartworm infection in Canadian dogs. The vast majority of positive diagnoses are made in southwestern part of the province, and in the Greater Toronto area.

Tracking the Spread of Heartworm Disease:

For more than 2 decades, veterinary pathologist Dr. Owen Slocombe of the University of Guelph has been studying the incidence of heartworm disease in Canadian dogs. According to Dr. Slocombe's 1998 preliminary poll of Canadian veterinarians, there were 605 diagnosed cases of canine heartworm disease last year. Of those, nearly 85 percent were diagnosed in Ontario.
In considering Dr. Slocombe's results, it should be noted that it has been estimated that only about a quarter of the canine population of Ontario is tested for heartworm infection each year. That means that the vast majority of dogs are of unknown infective status. It is possible, even likely, that regional rates of heartworm infection are higher than Dr. Slocombe's survey indicates. Any undiagnosed, untreated dogs represent a reservoir of infection.
Heartworm disease is able to spread widely because infection can be introduced into a small area by a visiting dog. If a simple diagnostic blood test is not performed, it may take months before symptoms develop and an active heartworm infection is identified. In that time, many mosquitoes may feed on the infected animal, each one taking away heartworm microfilariae (immature heartworms) with which it may infect other dogs and susceptible wild animals when it feeds again.
Heartworm disease is transmitted during mosquito season - spring and summer- when more people hit the roads to visit friends, see the sights or just travel for travel's sake. A recent pet owner study has shown that 69% of dog owners bring their pets with them when they travel. Ontario has a proud tradition as a summer tourist destination, which also makes it susceptible to the importation of heartworm disease.
In addition to local and visiting dogs, other species may also constitute a reservoir for infection. Heartworm disease has been diagnosed in foxes, wolves, coyotes, and racoons. Once heartworm disease infects local wildlife, a permanent source of the infection becomes established, since no preventive medication or treatment for the disease is given to these animals. Although rare, the disease has even been reported in the domestic cat.
What can a conscientious dog owner do? The recommendation of veterinarians across Canada is to have dogs tested annually for heartworm disease. If the result is negative, the dog should be placed on preventative medication.

The Life Cycle of the Canine Heartworm:

1. The cycle begins when a mosquito bites a heartworm-infected dog and picks up the microfilariae (immature heartworms found in the infected dog's bloodstream).
2. During the next two to three weeks, the microfilariae develop into infective larvae with in the mosquito.
3. When the mosquito feeds again, it can transmit the infective larvae into a healthy dog. The larvae penetrate the dog's skin and migrate through the tissues, developing over the next few months. Eventually they reach the dog's heart.
4. Once inside the dog's heart, the worms can grow up to 35 cm long and cab\n cause significant damage to the heart and lungs. Literally hundreds of worms may be found in an infected dog's heart and surrounding blood vessels. If left untreated, heartworm disease can result in death.

Heartworm Disease:

The symptoms of heartworm disease develop gradually over many months and resemble those of congestive heart failure. In advanced cases, the dog has difficulty breathing, can tolerate only very light exercise and loses its appetite. Untreated dogs eventually die.
While treatment is possible, the drug required to kill the adult worms is very toxic and must be administered under close veterinary supervision. The dog must be restricted to cage rest until all danger from treatment is past.

Heartworm Disease Prevention:

Protecting your dog from heartworm disease is as easy as giving it a preventative once a month from spring to fall.
First the dog must be tested to ensure it is not already infected. A simple blood test is all that is required.
Once your dog has been declared free of heartworm infection, preventative medication can be started. The dosage is based on your dog's weight. In Canada, preventatives are usually administered once a month from May or June until October or November. The important thing is to start giving the medication when mosquitoes appear and to keep giving it until about a month after they are killed off by a hard frost. The medication works by killing any microfilariae a mosquito might have injected into your dog up to 30 days before treatment.
*All articles are for general information only. Please consult a veterinarian for specific health related information related to your pet. © 2001 The Practice Marketers Inc. 905-936-6884 VESU-01