Heartworms come from mosquitoes. Every heartworm infection in dogs, cats, and other animals begins with a single mosquito bite. The parasite, a roundworm that can grow up to a foot long in its adult form, cannot spread directly from one animal to another. It requires a mosquito as an intermediate host to develop into a form capable of infecting a new animal.
How Mosquitoes Carry and Spread Heartworms
The cycle starts when a mosquito feeds on an animal that’s already infected. Tiny immature heartworms, called microfilariae, circulate in that animal’s bloodstream. The mosquito picks them up during its blood meal, and the larvae begin developing inside the mosquito’s body, specifically in structures called Malpighian tubules (the mosquito equivalent of kidneys).
Over the next two to three weeks, those larvae mature through two stages until they reach an infective form. At that point, the larvae migrate to the mosquito’s mouthparts. When the mosquito bites its next host, it deposits the infective larvae onto the skin, where they enter through the bite wound.
Around 70 mosquito species from several major groups, including common house mosquitoes, floodwater mosquitoes, and marsh mosquitoes, can carry heartworms. This means the risk isn’t limited to swampy or rural areas. Any region with mosquitoes has the potential for transmission.
Why Temperature Determines Heartworm Season
Heartworm larvae can only develop inside a mosquito when temperatures stay above 57°F (14°C). Below that threshold, development stalls completely. The warmer it gets, the faster the larvae mature. At a steady 64°F, it takes roughly 26 days for larvae to become infective. When temperatures fluctuate between warm days and cool nights, development can actually accelerate, with infective larvae appearing as early as 18 to 19 days after the mosquito picks them up.
This temperature dependence is why heartworm transmission is seasonal in cooler climates and year-round in warmer ones. In the southern United States, where temperatures rarely dip below the threshold for long, mosquitoes can transmit heartworms in virtually every month. In northern states and Canada, the transmission window is shorter but still significant during summer and early fall. The American Heartworm Society tracks incidence data from veterinary practices nationwide every three years, and the highest case concentrations consistently cluster along the Gulf Coast, the Mississippi River valley, and the Atlantic seaboard.
What Happens After the Bite
Once infective larvae enter an animal through a mosquito bite, they don’t head straight for the heart. They spend roughly two months living in the tissue just under the skin and in the chest and abdomen. During this time, they molt twice: first into a fourth-stage larva within 3 to 12 days, then into an immature adult worm around day 50 to 70.
These immature adults then enter the bloodstream and travel to the pulmonary arteries (the blood vessels between the heart and lungs) and the right side of the heart. They arrive there roughly 70 to 120 days after the initial bite. It takes about six months total from the mosquito bite for the worms to become sexually mature adults capable of producing new microfilariae in the bloodstream. This is also why heartworm tests can’t detect an infection right away. The standard blood test picks up proteins from adult female worms, so a dog needs to be infected for at least seven months before testing positive.
Wild Animals Keep the Cycle Going
Even if every pet dog in a neighborhood were on heartworm prevention, the parasite would persist. Coyotes are a major wild reservoir for heartworms across North America. As coyote populations have expanded into suburban and even urban areas, they’ve brought heartworm with them. Foxes and wolves can also harbor the parasite. These wild canids maintain a constant pool of microfilariae in the environment for mosquitoes to pick up and transmit to domestic pets.
This is one reason heartworm prevention remains important even in areas where the local pet population has high rates of preventive use. Your dog doesn’t need to encounter another infected dog. It only needs to be bitten by a mosquito that recently fed on an infected coyote passing through the area.
Dogs vs. Cats as Hosts
Dogs are the natural host for heartworms, meaning the parasite thrives and reproduces efficiently inside them. An untreated dog can harbor dozens of adult worms, and those worms can live five to seven years while continuously producing microfilariae.
Cats are a different story. Their immune systems are far more hostile to the parasite, and many infections are eliminated before worms ever reach maturity. When worms do survive to adulthood in a cat, the burden is typically low: just one to four adult worms. The larvae also tend to end up in unusual locations more often in cats than in dogs, migrating to the brain or other tissues rather than settling in the heart and lungs. Despite the lower worm burden, even a single adult heartworm can cause serious illness in a cat because of their smaller body size.
How Monthly Preventives Interrupt the Cycle
Heartworm preventives don’t repel mosquitoes or stop larvae from entering your pet. Instead, they kill the larvae that have been deposited during the previous month, targeting the third-stage and fourth-stage larvae before they can mature into adults. The drugs are effective against larvae that are less than 30 days old inside the animal. Older larvae and young adult worms are much harder to kill at preventive doses, which is why staying on a strict monthly schedule matters. A gap of even a few weeks can allow larvae to age past the window where preventives work.
Puppies under seven months old can start prevention without a heartworm test, since any infection acquired before that point wouldn’t yet be old enough to produce a positive result. For older dogs, testing before starting prevention is standard practice because giving preventives to a dog with an existing adult worm infection requires a different treatment approach.
Can Humans Get Heartworms?
Humans can technically be bitten by an infected mosquito and receive heartworm larvae, but the parasite almost never survives to maturity in people. When larvae do develop partially, they typically die and form a small nodule in the lung. A review of 24 cases in Japan found that 67% of affected people had no symptoms at all, and the infection was usually discovered incidentally when a lung nodule showed up on imaging done for another reason. The nodules are almost always smaller than 30 millimeters. Human pulmonary dirofilariasis, as it’s formally known, is rare and not considered a significant health threat.