Heartworm Disease in Colorado

Posted May 31st, 2011 by Lisa Cass, DVM

I’ve recently had many clients asking questions about heartworm disease, so I wanted to clarify a few facts and debunk some myths about the disease.  Heartworm disease is caused by a parasite, Dirofilaria immitis, which is transmitted by mosquitoes.  After a mosquito bites an infected dog, the mosquito becomes infected with the micofilaria (or larval stage) of the parasite. The parasite then undergoes a maturation in the mosquito, and the larvae of the worm are transmitted when the mosquito bites another dog.  The larvae migrate through the dog’s body to the right side of the heart and pulmonary arteries, where each worm can grow to the size of a spaghetti strand. Yes, these are real worms in the heart.

Life Cycle of Heartworm

Historically, heartworm disease was not seen in Colorado or was very rare, due to the fact that we have a dry climate, cool temperatures, and a short mosquito season at higher altitudes.  I don’t know if climate change has been partially responsible for the emergence of heartworm disease in Colorado, but I do know that the increased travel of dogs from other areas of the country, especially the southeastern U.S., has contributed to its presence here.  The relocation of dogs after Hurricane Katrina is thought to have played a large part, with approximately 85% of dogs from the hurricane area being infected with heartworms.

In the past, most dogs I saw that were infected with this disease had a history of travel somewhere outside of Colorado, but unfortunately now I am seeing the disease in dogs that have never left Colorado, and in one case, a dog that has never left Boulder County.

The following is a map showing the incidence of heartworm disease across the U.S. Please note it is a map from 2007, but clearly shows heartworm disease in Colorado. I suspect that the map is much darker four years later.

Incidence of Heartworm Disease


Heartworm disease can be diagnosed with a simple and inexpensive blood test.  This blood test should be done yearly unless dogs have been on heartworm preventative year round.  The blood test is positive if a protein found on the adult female worm is present.  It takes six months for worms to mature in to an adult once a dog is infected, so it is possible to have a negative test even if the dog has been infected. For this reason, the American Heartworm Society recommends yearly testing.

The preventative medication consists of a chewable treat that is given once monthly during mosquito season. In Colorado, I recommend dogs be given the preventative May through October.  If dogs travel outside of Colorado, especially to the southeastern United States or California, where mosquitos are present all year, they should be given preventative year round.

Fortunately heartworm disease can be inexpensively and safely prevented. For dogs infected with heartworms, the treatment is toxic, painful, and expensive.  It involves a series of injections of an arsenic-based drug, followed by 6-8 weeks of kennel rest.

I urge you to have your dog tested for heartworms and to consider starting heartworm preventative, even if you haven’t in the past. For more information, please visit the following website  http://www.heartwormsociety.org/


copyright 2011 Lisa Cass DVM

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