Heartworm Disease in Colorado

Is it important to provide heartworm disease protection for your dog in Colorado?  Yes!

The American Heartworm Society (an organization founded in 1974 to promote understanding of heartworm disease for both the public and veterinary professionals) collects data every three years on the incidence of heartworm positive dogs in the U.S.  These are reported cases from veterinary clinics.  There are many more dogs out there who do not receive veterinary care and die from heartworm disease without a diagnosis.   Heartworm positive dogs serve as a reservoir for the parasite, as do the many coyotes and foxes that are prevalent in the areas surrounding our homes.  Here is the latest incidence map from the American Heartworm Society:

You can see that heartworm transmission occurs in Colorado.  In 2010,  Colorado veterinary clinics reported diagnosing 1-5 heartworm positive dogs per clinic for the year.

Heartworm transmission occurs when a mosquito takes a blood meal from an infected mammal (e.g. dog, coyote, or fox), picking up immature heartworms (microfilariae).  The microfilariae develop through a life stage in the mosquito’s body over 10-14 days.  When it takes another blood meal, the mosquito injects a small amount of anti-coagulant to facilitate ingestion of  blood and at the same time will inject an infective stage of the immature heartworm.  This juvenile stage enters the subcutaneous tissues, then the vasculature and ultimately the heart as well as the arteries of the lungs.  The life cycle of the parasite takes about 7-9 months.  Be aware that cats are susceptible to heartworm infection too, but tend to mount a drastic immune system response to the young heartworms with the lungs being the battlefield and with gagging, vomiting, and respiratory symptoms as the primary findings.  Dogs are more of a natural or friendlier host and do not mount much of an immune response, allowing the immature heartworms to fully develop to their adult size.  Eventual symptoms in dogs are those of heart failure: coughing, fatigue, decreased appetite, and weight loss.

A dog or cat of any age, size, or breed is susceptible to heartworms because they are susceptible to mosquito bites.  It takes just one bite from an infected mosquito.

Heartworm treatment is a prolonged course that is hard on dogs and their owners, involving expensive hospitalized treatments and strict confinement.  Not all dogs will survive heartworm treatment.

Heartworm prevention is simple: a once monthly flavored chewable tablet or chunk.  Some heartworm preventives are also formulated to protect against common intestinal parasites such as roundworms, hookworms and whipworms.  A quick blood test for heartworms is needed before placing your dog on heartworm preventive.

The beginning of March before the weather warms up is a great time to talk to your veterinarian about doing your part to protect your four-legged friend and at the same time help control heartworm disease in Colorado.


American Heartworm Society

American Animal Hospital Association



About Dr. Lowery

Over 20 years experience in large and small animal medicine after graduating from the Ohio State University College of Veterinary Medicine in 1992. Owner of ComforVet, LLC, a mobile veterinary house call service for cats and dogs in south metro Denver.
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