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It’s Real. It’s Dangerous.

With Heartworm, ‘Tis Far Better To Prevent Than To Treat

“My old veterinarian never made me get a test or have my pet take heartworm preventative.”

“My old doc says that there’s not heartworm in Colorado.”

Those are comments that we frequently hear from some of our owners. And, I can tell you it’s frustrating to hear that some veterinarians still say that in 2018 in Colorado about heartworm in our canine patients. Every year since we opened in 2011, our doctors have treated several heartworm-positive dogs. Between 25 to 35 percent were local with no travel history outside the state.

Mosquitos are well established, including all four species that can carry heartworm, all along the Front Range and Western Slope. Up at mountain altitudes, the population of mosquitos is low, but we can’t say that here in Park Hill or Denver. As each year seems to be warmer and wetter, it’s good practice to police your yard to make sure you’re not allowing mosquito-breeding habitat. That is, anything that can hold water.

Those buzzy bugs are a huge source of disease worldwide and they represent a perfect vector for transmission between a variety of mammalian species, including dogs and humans. Some of the diseases mosquitos carry include Malaria, Chikungunya, Canine Heartworm, Dengue, Yellow Fever, Eastern Equine Encephalitis, St. Louis Encephalitis, Western Equine Encephalitis and West Nile Virus.

Heartworm, or Dirofilaria immitus, is a roundworm that is not an intestinal parasite. Its life cycle is dependent on mosquitos to move a larval stage called microfilaria from dog to dog.

Canids like dogs, foxes and wolves are the natural host for heartworm, but cats, raccoons and other species can carry Dirofilaria as wrong-ended hosts. Then over the following seven to eight months, the heartworm larva go through several stages and ultimately end up as adult worms living in the blood vessels, leaving the heart to go to the lungs.

These worms can grow to up to a foot long. In the early stages of infection, many dogs show few to no signs at all, but the heartworms are already causing significant irritation and inflammation along the pulmonary arteries. Typical clinical signs include mild persistent cough, reluctance to exercise and weight loss. Unchecked, the burden of the heartworm load will ultimately lead to obstructive pressure on the heart, causing it to go into failure.

Heartworms can live up to seven years, but most die much earlier – however, the dead worms can become a thrombus and leading to partial or fully-obstructive clots in downstream blood vessels.

Treatment of heartworm disease when in the early stages is expensive and treatment carries its own risks. The cost of treatment can run $800 to more than $1,000 – that’s the equivalent of more than eight years of heartworm preventative for a 75-pound dog.

By contrast, prevention is easy! Test annually and give a small preventative chew or pill once monthly. And, heartworm preventatives have an added bonus as all are compounded with broad-spectrum intestinal parasite deworm. With Denver’s increased population in the past several years, we have documented a marked increase in roundworms and hookworms – intestinal parasites with zoonotic risk to humans.

For the best and most accurate information on the web, visit

Dr. Margot can be reached at

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