Canine Heartworm Disease

Heartworm disease is a serious and potentially fatal disease caused by a worm called Dirofilaria immitis. Heartworms are found in the heart and blood vessels of infected dogs. Adult female worms are 6 to 14 inches long and 1/8 inch wide. Male worms are about half that size. One dog can have up to 300 adult worms.

Heartworms can only be transmitted by mosquitoes. A mosquito is required for some stages of the heartworm life cycle. As a result, heartworm disease is more common in areas with heavy mosquito infestation, such as the tropics.

A mosquito must first bite an infected dog and ingest microfilaria, which are microscopic offspring. The microfilaria undergo several larval stages in the mosquito. This takes between 10 and 30 days. After these transformations, the mosquito is able to transmit the advanced larval stages to a new host.

Once the new host is infected, the advanced larval stage takes six to seven months to reach adulthood. At this stage they can reproduce and create new microfilaria to start the cycle over again.

Adult heartworms cause disease by obstructing the major blood vessels in the body, resulting in reduced blood flow to vital organs and eventual organ damage. Most infected dogs will not show any signs of the disease until several years have passed. The most common signs of heartworm disease are coughing, weakness, lethargy and exercise intolerance. Most heartworm positive dogs are four to eight years of age when they become clinical.

Heartworm is usually diagnosed by a blood test that tests for antigens (proteins) specific to adult female heartworms. Dogs less than seven months of age need not be heartworm tested because there is no way for adult heartworms to be present yet. Dogs infected with less than five female heartworms will test negative, as will dogs with all male heartworm infections.

Heartworm positive dogs are worked up further to guage the disease and help veterinarians make a more definitive treatment plan. About 95% of heartworm positive dogs can be treated successfully.

Heartworm disease is easily prevented with a number of commonly used preventatives including Milbemycin oxime (Interceptor� and Sentinel�), Ivermectin (Heartguard�) and Selamectin (Revolution�). Puppies over six weeks of age can be started on heartworm preventative. Any dog over seven months of age should be heartworm tested before starting heartworm medications.

All dogs living or traveling to endemic regions (mosquito areas) should be on a heartworm preventative. We do have heartworm disease in Colorado but it is uncommon. In our busy general practice we diagnose one or two cases per year. In comparison, when we took in 25 Katrina evacuees, eight of them were heartworm positive.

For dogs that remain in the Denver area, we recommend heartworm testing every other year and a preventative is optional. For dogs that travel to endemic regions, we recommend heartworm testing yearly and keeping those dogs on preventatives. Yearly testing catches breaks in prevention and ensures that infected dogs are diagnosed with plenty of time to effectively manage the disease.

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