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Heartworm Fact Sheet

Foothills Humane SocietyHeartworm Fact Sheet

What is Heartworm Disease?

Canine heartworm disease develops when a dog is bitten by a mosquito carrying microscopic larvae of a parasite called Dirofilaria immitis.  As a mosquito feeds, these microscopic larvae infect and begin their migration into the dog’s bloodstream, where they grow into adult worms. Adult female heartworms are larger than male heartworms and can grow 10 to 12 inches in length . They make their home in the right side of the heart and vessels of the lungs (pulmonary arteries), often causing lung disease and heart failure.

Although easy to prevent, heartworm disease continues to be a major health problem for dogs living in the United States and wherever mosquitoes live. If you ever see or get bitten by mosquitoes, your dog is at risk!



What is the Heartworm Life Cycle?

When a dog has a mature heartworm infection, female worms release their voung (microfilariae) directly into the dog’s bloodstream. When a mosquito bites a dog with microfilariae in the blood. it ingests the microfilariae along with the blood. Over the following 10 to 14 days, these microfilariae develop and mature into infective larvae inside the mosquito. When the mosquito bites another dog, the larvae are left behind to enter the fresh wound. In 6 to 7 months, these infective larvae migrate inside the dog, eventually reaching the heart and vessels of the lungs, where they continue to grow to full maturity. The mature adult worms produce microfilariae of their own, which are available in the dog’s blood to infect other mosquitoes. Because heartworms may live for 5 to 7 years in the dog, each mosquito season can lead to increasing numbers of worms as they accumulate in unprotected clogs.

Heartworm Lifecycle

Where is Heartworm Found?

While the risk of infection in dogs varies from one region of the country to another and even from one community to another, one fact remains: heartworm disease is a threat to unprotected dogs in every state, even some parts of Alaska.  Unprotected dogs, foxes, coyotes, and wolves act as reservoirs, or sources, for the spread of this serious disease. The relocation of dogs, as with humanitarian  efforts following natural disasters such as Hurricane Katrina, can introduce heartworm disease into parts of the country where it is not normally found.
Furthermore, unprotected dogs traveling with their owners to areas where heartworms exist will be at risk for heartworm exposure.  Heartworm disease is a complicated and deadly illness – the best approach is prevention.

Where can I Get More Information on Heartworm Disease?

The information provided here only highlight general aspects of heartworm disease.  Comprehensive guidelines providing the most up-to-date heartworm information can be found at the American Heartworm Society.  Visit  www.heartwormsociety.org to obtain more in-depth information regarding prevention, diagnosis and management of heartworm disease.