Heartworm Disease in Manitoba

Introduction

Heartworm disease is a parasitic 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 Manitoba

Southern Manitoba continues to be one of the principal foci for heartworm infection in Canadian dogs. The vast majority of positive diagnoses are made in the Winnipeg area. In 1998, there were 23 positive cases of heartworm reported in Manitoba.

Transmission of Heartworm Disease

Heartworm disease is able to spread widely because a visiting dog, carrying heartworms, can bring the disease into an area. Without a simple blood test for early diagnosis, it could be months before symptoms of heartworm disease develop in the carrier. In that time, many mosquitoes could feed on the infected animal, each one taking away heartworm microfilariae (immature heartworms) with which other dogs and susceptible wild animals could be infected when the mosquito 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 68% of dog owners bring their pets with them when they travel. Summertime visitors with infected dogs can bring heartworm disease with them.

Other species may also constitute and important reservoir for infection. Heartworm disease has been diagnosed in foxes, wolves, coyotes and raccoons. Once heartworm disease infects local wildlife, a permanent source of infection becomes established, since no preventative medication or treatment for the disease is given to these animals. Although rare, the disease has been reported in the domestic cat.

Tracking the Spread of Heartworm Disease

Dr. Owen Slocombe is a veterinary pathologist and researcher at the University of Guelph. For more than two decades, he has been surveying veterinarians across Canada in an effort to determine the incidence of heartworm disease, regionally and nationally.

Relatively low numbers of dogs are tested for heartworm disease each year in Manitoba. In 1998, approximately 13,198 dogs were tested here as opposed to approximately 260,000 in Ontario. Although the figure for Ontario may seem high, it is estimated to represent only about one quarter of the dogs actually living in that province.

That indicates that the vast majority of Canadian 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.

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 preventive medication.

The Life Cycle of the Canine Heartworm

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).
During the next two to three weeks, the microfilariae develop into infective larvae within the mosquito.
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.
Once inside the dog's heart, the worms can grow up to 35 cm long and can cause significant damage to the heart and lungs. Literally hundreds of worms may be found in an infected dog's heart and the surrounding 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.

Protecting your dog from heartworm disease is as easy as giving it preventive medication once a month from May to October.

Heartworm Disease Prevention

Protecting your dog from heartworm disease is as easy as giving it a preventive once a month from May to October.

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, preventive medication can be started. The dosage is based on your dog's weight.

In Canada, preventives are usually administered once a month from May until October. The important thing is to start giving the medication when mosquitoes appear and keep on giving it until about a month after they are killed off by hard frost. The medication works by killing any microfilariae a mosquito might have injected into your dog up to 30 days before treatment.