Dogs get heartworm exclusively through mosquito bites. There is no other way. A mosquito picks up microscopic larvae from an already-infected animal, incubates them for about two weeks, then deposits infective larvae onto the skin of a new dog during its next blood meal. Those larvae enter through the bite wound and, over the next several months, migrate through the dog’s body until they reach the heart and lungs, where they grow into adults that can be a foot long.
Heartworm is not contagious between dogs. Your dog cannot catch it from sharing a water bowl, playing at the dog park, or living with an infected dog. The parasite must pass through a mosquito to become capable of infecting a new host.
The Mosquito’s Role
The cycle starts when a mosquito feeds on a dog (or other animal) that already has adult heartworms. Infected dogs have tiny immature worms, called microfilariae, circulating in their bloodstream. The mosquito swallows these during a blood meal. Inside the mosquito, the microfilariae travel to the abdomen and develop through two larval stages over 10 to 14 days, eventually becoming infective larvae that migrate to the mosquito’s mouthparts.
This development inside the mosquito only happens when outdoor temperatures stay consistently above 57°F (about 14°C). Below that threshold, the larvae stall. That’s why heartworm transmission is seasonal in cooler climates but year-round in warm, humid regions. The lower Mississippi River Valley has the highest concentration of cases in the United States, though heartworm is now diagnosed in every state.
More than one type of mosquito can carry heartworm. Species in the Aedes, Anopheles, and Mansonia groups are all capable transmitters, which means there’s no single mosquito to avoid. If mosquitoes are present and biting, transmission is possible.
What Happens After the Bite
When an infected mosquito bites your dog, it deposits third-stage larvae onto the skin. The larvae are small enough to enter through the puncture wound the mosquito just made. Once inside, they don’t go straight to the heart. Instead, they spend weeks burrowing through the tissue just under the skin and through muscle, molting twice more as they grow.
Over roughly six months, the larvae gradually make their way into the bloodstream and travel to the pulmonary arteries (the blood vessels connecting the heart to the lungs). This is where they settle, continuing to grow into adults. Mature heartworms can reach 10 to 12 inches in length. If both male and female worms are present, they mate and release a new generation of microfilariae into the dog’s blood, completing the cycle and making the dog a source of infection for the next mosquito that comes along.
Why Testing Has a Delay
Because the larvae take months to migrate, grow, and mature, a dog that was bitten yesterday won’t test positive today. The earliest a standard blood test can detect heartworm is about five months after infection. Tests that look for microfilariae in the blood require at least six months. This lag is why veterinarians recommend testing annually even for dogs on preventive medication: a gap in protection could lead to an infection that won’t show up for half a year.
Wildlife Keeps the Cycle Going
Your dog doesn’t need to live near other dogs to be at risk. Coyotes, foxes, wolves, and raccoons can all carry heartworm. These wild animals serve as a background source of microfilariae for local mosquito populations. In areas with healthy coyote populations, heartworm can persist in the environment even if every pet dog in the neighborhood is on prevention. Surveys have found infected coyotes, foxes, and wolves across much of the United States, and recent data shows heartworm appearing in new geographic “hot spots” beyond the traditionally high-risk Southeast.
How Prevention Interrupts the Cycle
Monthly heartworm preventives work by killing the larvae that entered your dog’s body during the previous 30 days, before they can mature and reach the heart. These medications target the third- and fourth-stage larvae while they’re still in the skin and muscle tissue. At these early stages, the larvae are extremely vulnerable. Once they’ve matured into adult worms lodged in the pulmonary arteries, this class of drug can no longer eliminate them, and treatment becomes far more complicated, expensive, and risky for the dog.
This is why consistency matters so much. A single missed month during mosquito season can allow larvae to develop past the point where preventive medication can kill them. Because the window of vulnerability for the larvae is narrow and the consequences of a missed dose are serious, year-round prevention is recommended in most of the country, especially given that warm spells in winter can sustain mosquito activity in regions where transmission was once considered seasonal.