About 1 in 100 dogs tested in the United States comes back positive for heartworm, based on nearly 5 million tests tracked by the Companion Animal Parasite Council. That translates to roughly 47,000 confirmed cases per year among tested dogs alone. The true number is almost certainly higher, since many dogs are never screened.
Where Heartworm Is Most Common
Heartworm has historically been concentrated in the South, Southeast, and Midwest, where warm temperatures and high humidity create ideal conditions for mosquitoes to transmit the parasite. The Mississippi River Valley and Gulf Coast states consistently report the highest infection rates year after year.
What’s changed in recent years is the geographic spread. States like Idaho and Washington, where heartworm was essentially unheard of, are now reporting cases. The American Heartworm Society’s 2022 incidence map confirmed that the disease is showing up in regions of the Pacific Northwest that were previously considered safe. Warmer seasonal temperatures, pet relocation from high-risk areas, and the movement of rescue dogs across state lines all contribute to this expansion. No state in the U.S. is truly heartworm-free.
How Dogs Get Heartworm
Heartworm spreads exclusively through mosquito bites. A mosquito feeds on an infected animal, picks up microscopic larvae, and deposits them into the next dog it bites. Those larvae burrow under the skin and begin a slow migration through the body. Over about 50 to 70 days, they develop into immature adult worms, eventually reaching the heart and the blood vessels of the lungs roughly 70 to 120 days after the initial bite.
Temperature matters. The larvae need to develop inside the mosquito before they can be transmitted, and that process requires sustained warmth above about 80°F with high humidity. In ideal conditions, it takes 10 to 14 days. In cooler climates, transmission slows or pauses during winter months, but it doesn’t take much warm weather to restart the cycle.
Indoor Dogs Are Still at Risk
Keeping your dog indoors reduces exposure but doesn’t eliminate it. Research from California’s Central Valley found that dogs spending at least half their daytime hours outside were significantly more likely to test positive. That said, mosquitoes get indoors. They slip through open doors, torn screens, and garages. A single bite from an infected mosquito is all it takes, so even a mostly indoor dog isn’t protected without prevention.
Why Heartworm Takes Months to Detect
One of the tricky aspects of heartworm is the detection gap. After a mosquito bite, it takes a minimum of five months before a blood test can pick up the infection, and most dogs don’t test positive until seven months after exposure. That’s because the standard screening test detects proteins released by adult female worms, and the larvae need that long to mature.
This delay is why the American Heartworm Society recommends annual testing for all dogs over seven months of age, using both an antigen test and a separate test that checks for larval worms circulating in the blood. If you’ve missed doses of prevention or switched products, your vet will want to retest before starting something new, since an undetected infection changes the treatment approach entirely.
How Well Prevention Works
Monthly heartworm preventives are highly effective when given consistently. The standard products work by killing larvae in the early stages of development, before they can reach the heart. When given on schedule, they provide close to 100% protection.
The most common reason for “breakthrough” infections isn’t a failure of the medication. It’s missed or late doses. Even a short gap in coverage can give larvae enough time to advance past the stage where the preventive can kill them. The American Heartworm Society’s 2024 guidelines emphasize year-round prevention regardless of where you live, partly because it’s easier to stay consistent than to guess when mosquito season truly starts and ends, and partly because the geographic range of heartworm keeps expanding.
There is a growing concern about drug resistance in some heartworm populations, particularly in the lower Mississippi River region. Resistant strains can survive standard preventive doses, and continued use of the same drug class against these strains may accelerate the problem. For now, year-round prevention remains the best available strategy, but it makes annual testing even more important as a safety net.
What Treatment Looks Like
Treating an established heartworm infection is far more expensive, time-consuming, and physically taxing on your dog than prevention. The standard protocol involves a series of three injections spaced over about 90 days, designed to kill the adult worms living in the heart and pulmonary arteries. Before treatment begins, your vet will typically run imaging (X-rays, ultrasound, or both) to assess how much damage the worms have already caused. That diagnostic workup alone can cost $400 to $1,200, with the injections adding another $500 to $1,500 depending on your dog’s size.
The hardest part for most owners is the strict exercise restriction that comes with treatment. As the adult worms die, fragments can block blood vessels in the lungs. Physical activity increases blood flow and raises the risk of a potentially fatal blockage, so dogs need to be kept calm and quiet for weeks. For an energetic dog, that’s a genuinely difficult period for both the pet and the family.
By comparison, a year of monthly heartworm prevention typically costs a fraction of a single treatment cycle. The math strongly favors staying on prevention, even in areas where heartworm seems uncommon.
The Real Number Is Likely Higher
The 1-in-100 figure reflects dogs that were tested, which skews toward pets already receiving veterinary care. Dogs in shelters, strays, and pets that rarely see a vet are tested less frequently and are more likely to be infected. In some high-prevalence regions, shelter intake screenings reveal infection rates many times the national average. If your dog has never been tested or has gaps in prevention history, the odds of exposure may be higher than the headline number suggests.