What is heartworm?

Heartworm disease occurs primarily in dogs, but can occur in cats and other animals. Heartworm disease is caused by a parasite called Dirofilaria immitis, which, in its adult and reproductive forms, lives in the right side of the heart and the adjacent blood vessels. Its presence in these blood vessels causes cardiovascular weakness, compromised lung capacity and, on occasion, death. When a mosquito draws blood from an infected dog or cat, it consumes microscopic forms of the parasite called microfilariae. Once inside the mosquito, the microfilariae evolve to infective larvae. Later, when the mosquito bites a new victim, the infective larvae are injected and that dog or cat becomes infected.

What are the signs of a heartworm infection?

Once infected, a pet becomes a “carrier” or reservoir of infection. It takes about six and a half to seven months for the infective larvae to migrate to the heart, mature into adult worms and begin producing new microfilariae inside the circulatory system. Depending on the parasite burden, the adult worms may end up occupying the right chamber of the heart and the pulmonary arteries, while the microscopic microfilariae circulate throughout the bloodstream. A dog or cat may be infected with heartworms and show no clinical signs. If clinical signs occur, the disease is well advanced and more difficult to treat. At first, pets may exhibit a chronic cough and reduced exercise tolerance and, in rare situations in Canada, death.

How often should my pet be tested for heartworm?

In Canada, parasite prevention is dealt with on a case-by-case basis, as your pet’s risk of parasitic disease is taken into consideration. Factors that may influence your veterinarian’s decision to test for heartworm may include your pet’s lifestyle, health status, your geographic location, any household considerations that may be relevant and the proposed preventive therapy. Your veterinarian will advise you on the recommended frequency of testing for your dog.

How can I protect my pet from heartworm?

Prevention is preferred to treatment. While there is a treatment available, it can be associated with health risks and be costly. Oral and topical medications that are given monthly have been shown to be highly effective in preventing heartworm disease and are available from your veterinarian.

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