Heartworm is not common in Colorado compared to states along the Gulf Coast or the Mississippi River valley, but it’s far from nonexistent. In 2017, Colorado reported 1,438 positive heartworm tests out of roughly 171,000 dogs tested, a prevalence rate of about 0.84%. That’s low by national standards, yet it still means more than a thousand dogs in the state tested positive in a single year. The combination of local mosquito activity, rising temperatures, and a steady flow of rescued dogs from high-prevalence states keeps the risk real enough to take seriously.
How Colorado Compares to the Rest of the U.S.
Heartworm prevalence in the southeastern United States regularly exceeds 5% in some counties, and parts of the lower Mississippi Delta see even higher numbers. Colorado’s 0.84% rate is a fraction of that. Still, that percentage has been climbing. Earlier surveys found even lower numbers: a 1990 statewide survey detected heartworm larvae in the blood of 0.77% of nearly 8,000 dogs tested, and by 2008 that figure had dropped to 0.4%. The jump to 0.84% by 2017 suggests the risk is growing rather than shrinking.
Only about 14% of Colorado’s estimated 1.2 million dogs were tested for heartworm in 2017, so the true number of infected animals is likely higher than reported. Dogs that never see a vet or never get tested won’t show up in the data.
Why Colorado Isn’t Immune
Heartworm spreads through mosquito bites. An infected mosquito deposits microscopic larvae into a dog’s skin, and over the next six to seven months those larvae migrate to the heart and lungs, where they grow into foot-long worms. Colorado’s dry climate, cool mountain nights, and high altitude have traditionally limited mosquito activity, which is why many dog owners assume the state is safe. That assumption is increasingly outdated.
Heartworm larvae need sustained warmth to develop inside a mosquito to the stage where they can infect a new host. Scientists measure this using “heartworm development units,” a calculation based on daily temperatures above roughly 57°F (14°C). Once temperatures accumulate enough warm hours, about 130 degree-days above that threshold, the larvae are ready. Colorado’s Front Range cities like Denver, Fort Collins, and Colorado Springs reliably hit those numbers during summer. The larvae can also pause their development during cooler stretches and resume when temperatures climb again, making intermittent warm spells enough to complete the cycle.
At least three mosquito species capable of transmitting heartworm live in Colorado. The most abundant is a floodwater mosquito that thrives after irrigation and rainstorms and can travel long distances toward urban areas. These mosquitoes are active from late spring through early fall along the Front Range and in the eastern plains, giving heartworm a viable transmission window of several months each year.
Imported Dogs Are a Major Factor
One of the biggest drivers of heartworm in Colorado has nothing to do with local mosquitoes. Thousands of dogs are transported into the state each year from shelters and rescue organizations in the South, where heartworm infection rates are dramatically higher. A dog adopted from Texas, Louisiana, or Mississippi may already be carrying heartworm when it arrives in Colorado. If that dog is bitten by a local mosquito, the mosquito can pick up the larvae and pass them to the next dog it bites, seeding local transmission cycles that wouldn’t exist otherwise.
This is a well-documented pattern. Research published in Parasites & Vectors specifically linked dog importation to rising heartworm prevalence in Colorado counties. The trend has accelerated as interstate rescue transport has become more common, and it means that even communities with minimal local mosquito pressure can see cases tied to relocated animals.
What Heartworm Looks Like in Dogs
Early heartworm infection produces no visible symptoms. Dogs can carry the parasites for months before anything seems wrong, which is why routine testing matters more than watching for signs. As the worms mature and multiply in the heart and pulmonary arteries, dogs develop a persistent cough, tire easily during exercise, and may lose weight. In advanced cases, fluid builds up in the abdomen, and the heart struggles to pump effectively. Left untreated, heartworm is fatal.
Cats and ferrets can also become infected, though they’re less common hosts. Cats tend to carry fewer worms but can still develop serious lung inflammation and sudden death from even a small number of parasites.
Testing and Treatment
A simple blood test at your vet’s office can detect heartworm proteins in about 10 minutes. Because it takes roughly six months after a mosquito bite for the infection to become detectable, annual testing is the standard recommendation. If your dog tests positive early, treatment is much more manageable than if the disease has progressed.
Treatment for an infected dog involves a series of injections that kill the adult worms over several weeks. During that time, exercise must be strictly limited because dying worms can break apart and block blood vessels in the lungs, a potentially life-threatening complication. The full treatment and recovery process typically takes several months and costs significantly more than a year’s worth of preventive medication.
Year-Round Prevention in Colorado
Colorado State University’s veterinary experts recommend year-round heartworm prevention for dogs regardless of where in the state you live. While mosquito season along the Front Range runs roughly May through October, warm spells outside that window, indoor mosquitoes, and travel to lower-elevation or out-of-state areas all create gaps in seasonal-only protection. Monthly preventive medications are highly effective when given consistently, and many also protect against intestinal parasites that are active year-round.
If you’ve adopted a dog from out of state, especially from the Southeast, request a heartworm test before or soon after bringing the dog home, then test again six months later to catch any infection that was too early to detect initially. Dogs that have been treated for heartworm in the past need to stay on prevention permanently to avoid reinfection.