Written by Dr Jennifer Kelley

At Alpine Animal Hospital we pride ourselves on providing you, our clients, with the most up to date and relevant medical information for your feline and canine family members. With that goal in mind, new studies on the increasing prevalence of heartworm in Colorado, including the Western Slope, has lead us to adjust our recommendations for testing and prevention of this potentially fatal and easily preventable disease. In the following article we will discuss why heartworm is increasing in Colorado, how we can best test for and prevent it, and additional benefits of monthly deworming treatment.

Information compiled from the Companion Animal Parasite Council and the Colorado Department of Agriculture Pet Animal Care Facilities Program shows that from 2013-2017 the number of heartworm cases in Colorado dogs increased 69%. The national change in heartworm cases was just 21% (See figure below). So why is heartworm increasing so much in Colorado, a state that traditionally has not seen as much heartworm as the national average?

Colorado is a dog loving state and our adoption rates are so high that dogs from other states are brought here to find their forever homes. One of the consequences of these philanthropic efforts is that dogs from states containing much higher rates of heartworm are being brought into Colorado in greater numbers each year. The studies we reviewed found that only about 35% of animal shelters and rescue organizations test for or provide heartworm prevention before moving dogs into Colorado. This may in part be due to the fact that heartworm takes six months to replicate in dogs and show up on tests. So even if a dog from a heartworm highly prevalent area tested negative once, he may have been harboring a heartworm infection and test positive when he gets to Colorado.


So how is this information relevant to us here in the Roaring Fork Valley? Even if you and your dog are not leaving the valley there are plenty of people and organizations bringing dogs here from all over the country, and unfortunately may be bringing their parasites with them. Garfield and Pitkin counties do have heartworm cases reported, and we are also home to mosquito species that serve as heartworm carriers. The study we have been referencing in this article recommends that “veterinarians in Colorado should no longer base heartworm testing and prevention recommendations on only historic heartworm risks and prevalence (Drake 2019).” The authors encourage veterinarians to follow the recommendations laid out by the American Heartworm Society.

The American Heartworm Society recommends that dogs be tested once a year for heartworm and be provided with a monthly heartworm preventative year round. Since heartworm is increasing in Colorado we as a team think that testing your dog and providing year round prevention regardless of whether they leave the valley or not is the best way to ensure they do not contract heartworm and are protected from a variety of intestinal parasites.

Given this new recommendation we want to address common questions we receive about heartworm and its prevention. Such as, we only have mosquitos a few months a year- why does my dog need to be on prevention year round? This a great question! As previously mentioned, heartworm takes six months to replicate in the body and show up on tests. This means that if a dog receives his last dose for the year before the mosquitos have all died they could contract an infection that may not show up on a test for at least six months that could have otherwise been prevented. Additionally, urban centers that contain large heated building and parking lots create microenvironments that provide warm places for mosquitos to continue to reproduce leading to an increased transmission season length. Due to these factors the risk of contracting heartworm never reaches zero so year round prevention is the surest way to prevent infection.

So when and why does my dog need to be tested? The American Heartworm Society (AHS) recommends that puppies be started on heartworm prevention ideally by 8 weeks of age and tested six months after the first dose is given. Dogs older than 7 months who have not been on a monthly preventative should be tested prior to starting preventative therapy. They should be tested first to avoid delays in detecting an infection that may not be yet showing clinical signs and because the treatment of the adult worms requires different drugs than the preventative which kills certain heartworm larval stages. The AHS recommends that dogs on year round prevention be tested every year since any delays in giving the monthly preventative can result in a period of susceptibility to infection. And fortunately there are additional benefits to giving a monthly heartworm preventative than just preventing heartworm disease!

The heartworm prevention medication we typically prescribed contains other deworming medications that treat a wide range of intestinal parasites. This means that if heartworm prevention is given year round that your dog will be protected from roundworms, hookworms, whipworms and tapeworms year round as well. The hookworms, roundworms and tapeworms that our dogs get can infect us as well. These parasites are transmitted by fecal oral contact. This is especially important for households with young children who put their hands in their mouths after playing outside. The most effective way to prevent the transmission of these zoonotic (animal to people) parasites is to provide monthly prevention.

In conclusion, heartworm is increasing and changing in Colorado and so we want to increase and change our testing and prevention efforts to best protect the dogs in our community. Giving a monthly heartworm preventative year round protects against heartworm and intestinal parasites that can infect humans. We hope that this information helps you make the most informed choice about heartworm and parasite prevention for your furry family member so you can hit the parks and trails with no worries about worms!