Dogs get heartworm from a single source: the bite of an infected mosquito. There is no other way. A dog cannot catch heartworm from another dog, from contaminated water, or from any other animal directly. The entire transmission cycle depends on mosquitoes as an intermediary, and the biology behind it explains why prevention matters so much.
The Role of the Mosquito
The process starts when a mosquito bites a dog that is already infected with heartworm. Infected dogs carry microscopic baby heartworms, called microfilariae, circulating in their blood. The mosquito picks up these microfilariae during a blood meal, and the larvae then develop inside the mosquito over the next couple of weeks. During that time, the larvae mature into an infective stage that can actually establish an infection in a new host.
This development inside the mosquito requires warm temperatures. The larvae need roughly two weeks of temperatures consistently above 57°F to reach their infective form. This is why heartworm transmission is seasonal in cooler climates but can occur year-round in warmer regions. It also explains why the lower Mississippi River Valley remains the heaviest hot spot for heartworm in the United States, though cases are now diagnosed nationwide, not just in the South and Southeast.
More than one type of mosquito can carry heartworm. Species from the Aedes, Anopheles, and Mansonia groups are all capable of transmitting the parasite, which means nearly any region with a mosquito population has some level of risk.
What Happens After the Bite
When an infected mosquito bites a new dog, it deposits infective larvae onto the skin. The larvae then crawl into the bite wound and burrow into the tissue beneath the surface. From this point, heartworm development inside the dog follows a remarkably consistent timeline.
Within one to three days, the larvae undergo their first transformation in the tissue just under the skin. About two months (50 to 70 days) after the initial bite, they molt again into immature adult worms. These young adults, only two to three centimeters long at this point, enter the bloodstream and travel to the heart and the blood vessels of the lungs. They can arrive there as early as 70 days after infection.
By four months, the worms in the pulmonary arteries are 10 to 15 centimeters long. By six and a half months, they are fully mature. Males reach 15 to 18 centimeters (roughly 5 to 6 inches), while females grow to 25 to 30 centimeters (10 to 12 inches). Once mature, the worms begin reproducing, releasing new microfilariae into the dog’s bloodstream, where they wait to be picked up by the next mosquito. The cycle begins again.
How Long Heartworms Survive
Adult heartworms can live inside a dog for five to seven years. Because each mosquito season brings new opportunities for infection, a dog without preventive medication can accumulate worms over time. A dog with a mild case may harbor a small number of worms, while severe cases involve large numbers of worms occupying the right side of the heart and the blood vessels supplying the lungs. This growing worm burden is what makes the disease progressively more dangerous the longer it goes undetected.
Why It’s Hard to Catch Early
One of the challenges with heartworm is that dogs show no symptoms during the early months of infection. The standard blood test detects proteins produced by adult female heartworms, and the earliest this test can return a positive result is about five months after the infective bite. Microfilariae don’t appear in the bloodstream until roughly six months post-infection. This means a dog bitten in June may not test positive until November or later.
During that window, the worms are silently growing, migrating, and settling into the heart and lungs. Dogs in the early stages of infection often appear completely healthy. As the disease progresses, symptoms like a persistent cough, reluctance to exercise, fatigue after moderate activity, and weight loss begin to appear. In advanced cases, the worm burden can cause heart failure and significant damage to the lungs, liver, and kidneys.
Dogs Cannot Spread It to Each Other
This is worth emphasizing because it’s a common concern: heartworm is not contagious between dogs. An infected dog living in the same house, sharing the same water bowl, or playing at the same dog park poses no direct risk. The microfilariae circulating in an infected dog’s blood must pass through a mosquito and develop inside that mosquito before they can infect another animal. Without the mosquito step, transmission simply cannot happen.
That said, an infected dog in the neighborhood does increase the overall risk for other dogs nearby. That dog serves as a reservoir, giving local mosquitoes the opportunity to pick up microfilariae and carry them to other hosts. Areas with higher numbers of untreated or stray dogs tend to have higher heartworm rates for exactly this reason.
Geographic Risk Is Expanding
Heartworm was once considered primarily a problem for dogs in the southern and southeastern United States, but that picture has changed. Data collected from veterinary hospitals and shelters throughout 2025 shows heartworm being diagnosed in new hot spots across the country. The lower Mississippi River Valley still accounts for the highest concentration of cases, but infections are appearing in areas that were previously considered low risk.
Adding to the concern, there are strains of heartworm that have developed resistance to the standard class of preventive medications. These resistant strains were initially found mostly in heavily affected areas of the lower Mississippi region, but recent research suggests they may be more widespread than previously thought. This doesn’t mean prevention is ineffective, but it reinforces why annual testing remains important even for dogs on year-round preventive medication.
Why the Mosquito Step Matters for Prevention
Preventive heartworm medications work by killing the larvae in the early stages after a mosquito bite, before they can mature, migrate, and establish themselves in the heart. These medications are effective against the larvae during that initial period under the skin and in the tissue. Once worms reach the heart and lungs and grow into adults, prevention no longer works, and treatment becomes far more complicated, costly, and harder on the dog.
Because it takes roughly six months for infection to become detectable, and because adult worms survive for years, a gap in prevention of even a few months can allow larvae to slip past the window where medication is effective. By the time a test comes back positive, the damage has already begun. This is the core reason that consistent, uninterrupted prevention is the standard recommendation regardless of geography or season.