Dogs get heartworm from a single source: mosquito bites. There is no other way. A dog cannot catch heartworm from another dog, from soil, from water, or from any other animal directly. Every case of heartworm disease begins with an infected mosquito depositing microscopic larvae into a dog’s skin during a blood meal.
Why Mosquitoes Are the Only Source
Heartworm (Dirofilaria immitis) requires a mosquito to complete its lifecycle. The parasite literally cannot develop into its infectious form without spending time inside a mosquito’s body first. When a mosquito bites a dog that already has heartworm, it picks up tiny immature worms called microfilariae circulating in the dog’s blood. Inside the mosquito, those microfilariae develop over roughly two weeks into a larval stage capable of infecting a new host.
The next time that mosquito bites a dog, it deposits these infective larvae onto the skin near the bite wound. The larvae enter through the puncture site and begin migrating through the dog’s body. Several types of mosquitoes can carry heartworm, including species in the Aedes, Anopheles, and Mansonia groups, which are common across the United States.
This is why two dogs living in the same house, one infected and one not, won’t spread the disease between them. Even if an uninfected dog somehow ingested microfilariae, those immature worms cannot develop further without the biological changes that happen inside a mosquito. The mosquito is not just a delivery vehicle. It’s an essential part of the parasite’s development.
What Happens After the Bite
Once infective larvae enter through a mosquito bite, they don’t head straight for the heart. They spend weeks migrating through muscle and connective tissue, molting through additional developmental stages as they go. Over time, these growing worms make their way into the bloodstream and eventually settle in the blood vessels of the lungs and the heart itself.
The full maturation process from bite to adult worm takes six to eight months. That long, silent window is one of the things that makes heartworm so dangerous. A dog can be infected for half a year before any worms are mature enough to cause symptoms or show up on a test. Adult heartworms can grow over a foot long and live five to seven years inside a dog. Once mature, they reproduce, releasing new microfilariae into the bloodstream, which means any mosquito that bites the dog can now pick up the parasite and spread it to the next animal.
Where and When Dogs Are Most at Risk
Heartworm transmission tracks directly with mosquito activity. Warm, humid climates with standing water create ideal breeding conditions for mosquitoes, which is why heartworm has historically been most common in the southeastern United States and along the Gulf Coast. But cases have been reported in all 50 states. Mosquitoes don’t need a swamp. A forgotten flowerpot, a clogged gutter, or a birdbath provides enough standing water for breeding.
Temperature matters too. Heartworm larvae need sustained warmth to develop inside the mosquito. In cooler climates, the transmission window shrinks to warmer months. In tropical and subtropical areas, transmission can happen year-round. Indoor dogs are not immune either, since mosquitoes easily get inside homes. The risk drops in winter in northern states but never fully disappears, especially with the unpredictability of warming trends and mild spells.
How Prevention Works
Heartworm preventives don’t actually repel mosquitoes or stop larvae from entering your dog. Instead, they kill the immature larvae that have entered the body during the previous 30 days (or longer, depending on the product). This is why timing and consistency matter so much. A missed dose creates a window where larvae can survive long enough to move beyond the reach of preventive medication.
The American Heartworm Society recommends year-round prevention for all dogs, regardless of where you live. Puppies should start preventive medication as early as possible, no later than eight weeks of age. Dogs older than seven months should be tested before starting prevention, since giving preventive drugs to a dog with an existing adult heartworm infection can cause serious complications. After that, annual testing with both an antigen test and a microfilariae test is recommended, even for dogs on consistent prevention. No preventive is 100% effective, and a late or missed dose is easy to overlook.
If you switch brands or types of heartworm preventive, or if there’s been any gap in doses, your dog should be tested again before starting the new product. Puppies that begin prevention on schedule should be tested six months after their first dose and then annually from that point forward. That six-month gap reflects the time it takes for an infection, if present, to become detectable.
Why Early Detection Is Difficult
The six-to-eight-month maturation period creates a built-in blind spot. Standard heartworm tests detect proteins produced by adult female worms, so a dog infected last month will test negative even though larvae are already developing inside. Microfilariae don’t appear in the bloodstream until adults are mature and reproducing, which also takes months.
Early-stage infections produce no symptoms at all. By the time a dog shows signs like coughing, exercise intolerance, or fatigue, the disease is often well established and worms may already be damaging the lungs and heart. This is the core reason prevention is emphasized so strongly: treatment for an active heartworm infection is lengthy, expensive, requires significant exercise restriction, and carries real risks. Preventing larvae from ever reaching adulthood is far simpler and safer than dealing with adult worms lodged in the cardiovascular system.