Heartworm is one of the most widespread parasitic diseases in dogs in the United States, with more than 100,000 cases diagnosed each year. Among dogs that see a vet regularly, reported positive cases have climbed steadily, from about 23,500 in 2016 to nearly 36,000 in 2019. In areas without regular prevention, infection rates can be dramatically higher.
How Common Heartworm Is by the Numbers
The overall prevalence depends heavily on where a dog lives and whether it receives preventive medication. Among pet dogs with regular veterinary care, the national positive rate stays relatively low because most of those animals are on monthly preventives. But in populations without that protection, the numbers jump sharply. A study of pet dogs in south Texas found that 40.8% of dogs over six months old tested positive for heartworm when multiple testing methods were used. Shelter and stray dog populations in the Southeast show similarly high rates.
Heartworm cases have been trending upward across the country, not just in traditional hotspots like the Gulf Coast and Mississippi River Valley but also in areas where the disease was once considered rare. The American Heartworm Society’s incidence mapping confirms this upward shift. Factors like pet travel, relocation of rescue dogs from high-prevalence areas, and expanding mosquito ranges all contribute.
Cats Get Heartworm Too
Dogs get most of the attention, but cats are also at risk. A Florida shelter study comparing stray cats and dogs found that 4% of cats had active adult heartworm infections, compared to 28% of dogs. That gap narrows when you account for cats showing evidence of any current or past heartworm exposure, including immature or larval infections. By that broader measure, 19% of the cats in the study had been infected at some point, a figure not statistically different from the dog infection rate.
Cats are less ideal hosts for heartworm, so they typically carry fewer worms. But even one or two worms can cause serious respiratory problems in a cat, and there is no approved heartworm treatment for cats the way there is for dogs. Prevention is the only reliable option.
Wildlife Keeps the Parasite Circulating
Even if every pet dog in a neighborhood were on prevention, heartworm would persist because wild canids carry the parasite and serve as a constant reservoir. Coyotes are the biggest concern. A study of over 300 coyotes in the Chicago area found a heartworm prevalence of 31.1%. In some rural areas, coyote infection rates reach 37%. Urban coyotes may actually face higher infection risk than rural ones, likely because of denser mosquito populations in cities and suburbs.
This means that any dog or cat exposed to mosquitoes, even briefly, has a realistic chance of encountering an infected bite. Indoor cats that never go outside still get bitten by mosquitoes that make it through doors and windows.
How Heartworm Spreads
Heartworm spreads exclusively through mosquito bites. A mosquito feeds on an infected animal, picks up microscopic larvae circulating in the blood, and carries them to the next animal it bites. The larvae need time to mature inside the mosquito before they become infectious, and that development is temperature-dependent. At around 86°F (30°C), larvae reach the infectious stage in 8 to 9 days. Cooler temperatures slow the process, and below a certain threshold, development stalls entirely. This is why heartworm transmission peaks in warm, humid months, though in southern states, mosquito season can stretch nearly year-round.
What Heartworm Looks Like in Dogs
One reason heartworm stays so common is that infected dogs can appear perfectly healthy for months. After a mosquito delivers the larvae, it takes about six to seven months for the worms to mature into adults and settle in the heart and lung arteries. During that time, a standard test won’t even detect them. The disease progresses through four recognized stages:
- Stage 1: No symptoms, or just an occasional cough. Most owners wouldn’t notice anything wrong.
- Stage 2: The dog tires more easily after moderate exercise. There may be a persistent mild cough.
- Stage 3: Fatigue after even light activity, a cough that won’t resolve, and visible difficulty breathing. Signs of heart failure begin to appear.
- Stage 4: The worm burden is so heavy that it physically blocks blood flow through the heart. This is a life-threatening emergency.
Because early-stage infection is essentially invisible, annual testing is important even for dogs on year-round prevention. The American Heartworm Society recommends yearly blood tests that check for both the proteins adult worms produce and for circulating larvae. Testing is especially critical if a dog’s prevention history has any gaps.
Why Prevention Gaps Happen
Monthly heartworm preventives are highly effective when given consistently. Most failures trace back to human behavior rather than drug resistance. Missed doses, late doses, and seasonal-only use in regions where mosquitoes are active longer than expected all create windows of vulnerability. Cost can also be a barrier, particularly for owners of larger dogs, since preventive medications are dosed by weight.
Some pet owners turn to supplements or home remedies like black walnut extract, brewer’s yeast, or essential oils as alternatives to conventional preventives. None of these have been shown to prevent heartworm infection. Dogs whose owners favor holistic approaches are statistically less likely to be on proven preventive medication, which puts them at greater risk. Intact (not spayed or neutered) dogs also have lower rates of preventive use, possibly reflecting less frequent veterinary visits overall.
The Bottom Line on Prevalence
Heartworm is common enough that veterinary organizations treat prevention as a baseline recommendation for every dog and cat in the United States, regardless of geography. The parasite circulates continuously in wildlife, mosquitoes are nearly impossible to avoid completely, and infection rates are climbing in places that used to see very few cases. In high-prevalence regions without widespread prevention, roughly one in three unprotected dogs will test positive. Even in lower-risk areas, the consequences of a single infected mosquito bite make heartworm a parasite worth taking seriously.