Dogs get heartworm exclusively from mosquito bites. There is no other way. Dogs cannot catch heartworm from other dogs through contact, saliva, shared water bowls, or even close proximity. A mosquito must serve as the middleman, picking up immature heartworm larvae from an infected animal and delivering them to a new host.
How Mosquitoes Spread Heartworm
The process starts when a female mosquito feeds on a dog that already has heartworm. That infected dog’s blood contains tiny larvae called microfilariae, which the mosquito ingests during the bite. Over the next 10 to 14 days, those larvae develop inside the mosquito into an infective stage. The next time that mosquito bites a dog, it deposits the mature larvae onto the skin near the bite wound. The larvae then wriggle through the wound and into the dog’s body.
This is why heartworm cannot spread directly between dogs. The larvae need that roughly two-week development period inside the mosquito before they become capable of infecting a new host. Without the mosquito, the cycle breaks.
What Happens Inside Your Dog
Once larvae enter through the bite wound, they begin a migration through your dog’s body that takes months. Within one to three days, they molt in the tissue just beneath the skin. About two months after infection, they molt again into young adults roughly an inch long and enter the bloodstream. The blood carries them to the heart and the arteries leading to the lungs, where they arrive as early as 70 days after the initial bite.
The worms continue growing in those blood vessels. By four months after infection, they’re around 4 to 6 inches long. By six and a half months, males reach 6 to 7 inches and females grow to 10 to 12 inches. Once mature, they mate and release new microfilariae into the dog’s bloodstream, ready to be picked up by the next mosquito that takes a blood meal. Adult heartworms can live 5 to 7 years inside a dog.
Why It Takes So Long to Detect
Standard heartworm tests look for proteins produced by adult female worms. Because the worms need months to mature, tests can’t reliably detect an infection until at least five months after the mosquito bite, and results aren’t consistently accurate until eight months post-infection. This is why the American Heartworm Society recommends testing a dog with unknown history immediately and then retesting six months later to catch any infection that was too young to detect the first time.
Annual testing matters even for dogs on preventive medication, because no drug is guaranteed 100% effective in every situation. A missed dose, a spit-out tablet, or vomiting after taking the medication can leave a gap in protection.
Wildlife That Keep the Cycle Going
Your dog doesn’t need to be around other pet dogs to be at risk. Coyotes, foxes, and wolves all carry heartworm and act as reservoirs in the environment. A mosquito can pick up larvae from a coyote in your neighborhood and transmit them to your dog in the backyard. This is one reason heartworm exists even in areas where most pet dogs are on preventive medication.
Where and When Risk Is Highest
Heartworm has been documented in all 50 U.S. states, but cases concentrate most heavily in the Southeast and along the Mississippi River valley, where warm temperatures and humidity support large mosquito populations year-round. The American Heartworm Society publishes updated incidence maps every three years based on testing data from thousands of veterinary clinics and shelters.
Temperature plays a direct role. The larvae inside a mosquito need approximately two weeks of temperatures consistently above 57°F to develop into the infective stage. In cooler climates, transmission season is limited to warmer months. In subtropical and tropical regions, transmission can happen year-round. Climate variability makes it difficult to predict exactly when the “safe” months begin or end in any given area, which is why year-round prevention is widely recommended regardless of geography.
What Heartworm Disease Looks Like
Early heartworm infection produces no symptoms at all. Dogs can carry worms for months before anything seems off. The first noticeable sign is usually a mild, persistent cough that shows up after exercise. As the worm burden grows and the blood vessels in the lungs become more damaged, dogs develop exercise intolerance, meaning they tire out faster than usual or seem reluctant to play or walk.
In more advanced cases, dogs may lose weight, breathe rapidly even at rest, or develop a swollen belly from fluid buildup. The most severe stage, sometimes called caval syndrome, occurs when a large mass of worms physically blocks blood flow through the heart. This is a life-threatening emergency that requires immediate intervention.
How Prevention Works
Heartworm preventives don’t stop mosquitoes from biting or larvae from entering your dog’s body. Instead, they kill the larvae that have entered over the previous 30 days, before those larvae can mature and reach the heart. This is why timing matters. The medication works retroactively, clearing out young larvae on a monthly schedule so they never get the chance to develop into adults.
Preventives are highly effective when given consistently. In controlled studies, standard doses have achieved efficacy rates above 99%. The rare failures that occur are typically linked to missed or late doses rather than drug resistance, though isolated cases of reduced susceptibility have been documented in certain regions. Options include monthly chewable tablets, topical treatments applied to the skin, and injectable formulations given by a veterinarian that provide protection for six or twelve months at a time.