Canine Heartworm Disease

It’s heartworm season at the Markham Veterinary Clinic!

What Is Heartworm?
Heartworm disease is a serious and potentially fatal disease caused by a blood-borne parasite calledDirofilaria immitis. Heartworms may infect more then 30 species of animal including, coyotes, foxes, wolves and other wild canids, domestic cats, wild felids, ferrets, sea lions etc. Dogs are considered the definitive host for heartworms.

How Does Infection Occur?
Infection occurs when a mosquito carrying infective heartworm larvae bites a dog and transmits the infection. The mosquito is required for the heartworm life cycle; therefore heartworm disease cannot be transmitted directly from dog to dog without the mosquito. When a mosquito carrying infective heartworm larvae bites a dog it transmits the infection. The larvae grow, develop and spread through the body over a period of six to seven months to become mature male and female worms. These reside in the heart, lungs and associated blood vessels of the now infected dog. As adults, the females release their offspring (microfilariae) into the bloodstream and the dog now becomes a source of infection for other dogs to contract heartworm disease.

How Does Heartworm Infection Cause Disease?
The onset and severity of disease in the dog depends on the number of adult heartworms present, the age and the activity level of the dog. The higher number of mature worms present is generally found to have more severe heart and lung disease. Adult heartworms cause disease by clogging the heart and major blood vessels, interfering with the action of the heart as well as the blood supply to other organs in the body, particularly the lungs, liver and kidneys, leading to malfunction in these organs. A very active dog is more likely to develop severe disease with a relatively small number of heartworms then an inactive one, since the heart may enlarge and become weakened with the increased workload. The circulating microfilariae remain primarily in the small blood vessels, and may block the blood flow in these vessels, depriving the body of nutrients and oxygen normally supplied by the blood.

What Are the Signs of Heartworm Infection
Most dogs infected with heartworm do not show signs of the disease for as long as 2 years. Unfortunately, by the time clinical signs are seen, the disease is well advanced.

Clinical Signs Associated with Canine Heartworm Disease

Early Infection No abnormal clinical signs observed
Mild Disease Cough
Moderate Disease Cough, shortness of breath, weakness, nervousness, listlessness, loss of stamina
Severe Disease Cough, shortness of breath, weakness, nervousness, listlessness, loss of stamina, difficulty breathing, fainting, abnormal heart sounds, enlarged liver, congestive heart failure, fluid accumulation in abdomen, death

Chart from Canine Heartworm Disease Article – Michigan State University

How is Heartworm Disease Diagnosed?
Blood Antigen Testing – will accurately detect infections with one or more mature female heartworms that are at least 7 to 8 months old. These tests generally do not detect infections less then 5 months duration.

A heartworm infected dog with mild disease may appear to be perfectly normal on physical exam. Severely affected dogs, however, may reveal a history of coughing, exercise intolerance, blood in sputum, loss of appetite and fainting. A physical exam may reveal;

  • Noisy lung sounds
  • Rapid heartbeat
  • Fluid in the abdomen
  • Weight loss
  • Jaundice

How is Heartworm Disease Treated
Most dogs infected with heartworm can be successfully treated. The goal of treatment is to kill the adult worms and all the circulating microfilaria with minimal harmful effect from the drug used for treatment, and the complications created by the dying heartworms. Patients with more severe heartworm infections can be treated successfully, but the possibility of complications and death is greater. The most common post treatment complication results from the obstruction of blood flow due to the presence of dead heartworms in the capillaries of the lungs (pulmonary thromboembolism). If treatment is effective, some degree of pulmonary thromboembolism will occur. When dead worms are numerous widespread blocking of arteries can occur.

Prevention of Heartworm Disease
Prevention of heartworm disease is much safer and more economical. At Markham Veterinary Clinic we have a variety of different options for preventing heartworm infection which are extremely effective when administered properly and on a timely schedule. The American Heartworm Society is now recommending year-round prevention, even in seasonal areas. One reason for this is compliance – to make sure the medicine has been given properly by the pet owner. In addition, most monthly heartworm preventives have activity against intestinal parasites. With estimated infections occurring in three to six million people every year, the added benefit of monthly deworming makes great sense.

Before starting a preventive program, all dogs that could possibly be infected with mature heartworms should be tested!

How Prevalent is Heartworm in Ontario
The number of heartworm positive pets in Ontario varies from year to year. There is currently no system in place to track every case of heartworm in the province. A survey performed in 2010 found that the number of dogs with heartworm in Ontario increased by 60% between 2002 and 2010. Canadian statistics show that nine per cent of the heartworm cases were imported from the Southern United States (Katrina dogs) and 12 per cent had been imported from other parts of the United States or other countries. Fifty-one per cent had never left their local area. The take home message is that Ontario pets are vulnerable from a variety of sources, and prevention is the best approach.

Markham Veterinary Clinic services the Markham, Stouffville and Unionville area and answers some common heartworm questions.

Information for this article gathered from the American Heartworm Society, Ontario Veterinary Medical Association and Michigan State University.

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