Heartworm spreads in dogs exclusively through mosquito bites. A dog cannot catch heartworm from another dog through direct contact, shared water bowls, saliva, or proximity. The parasite requires a mosquito as a biological middleman to develop into a form capable of infecting a new host. Without that step inside a mosquito, heartworm larvae simply cannot cause an infection.
Why Mosquitoes Are the Only Route
The heartworm parasite, Dirofilaria immitis, has a two-host life cycle that depends on both a mosquito and a mammal like a dog. Baby heartworms, called microfilariae, circulate in an infected dog’s bloodstream but are not yet capable of infecting another animal. They first need to mature inside a mosquito before they become dangerous. This is why heartworm is not contagious in the traditional sense. An infected dog living in your house poses no direct risk to your other dogs. The risk comes from the mosquitoes in and around your home.
At least 23 mosquito species in the United States can carry heartworm. The most common belong to the Culex, Aedes, and Anopheles groups, which are found in virtually every region of the country. Even species traditionally considered human-biters, like Aedes aegypti, have been shown to feed on dogs frequently enough to contribute to transmission in certain areas like south Texas.
The Step-by-Step Transmission Cycle
The cycle starts when a mosquito feeds on a dog that already has heartworm. During that blood meal, the mosquito picks up microscopic microfilariae circulating in the dog’s blood. Inside the mosquito, these microfilariae go through two developmental stages over the course of roughly 10 to 14 days (depending on temperature), eventually reaching an infective larval stage. Only at this point can they infect a new host.
When that mosquito bites another dog, the infective larvae slip through the tiny wound the mosquito’s feeding creates. From there, the larvae burrow into the dog’s tissue and begin a long migration through the body. Over the next several months, they move through muscle and connective tissue, molting through additional developmental stages as they work their way toward the blood vessels of the lungs and eventually the heart. It takes approximately six to seven months from the initial mosquito bite for the larvae to mature into adult worms and begin reproducing.
Once mature, adult heartworms can live five to seven years inside a dog. Females release new microfilariae into the bloodstream, and the cycle is ready to repeat the next time a mosquito feeds on that dog.
Temperature Controls the Spread
Heartworm transmission is not possible in all climates year-round because the larvae inside the mosquito need warmth to develop. The threshold temperature is 14°C (about 57°F). Below that, larval development stalls. Scientists measure this using “heartworm development units,” a calculation based on how many degrees above that threshold the daily temperature reaches and for how long. The larvae need roughly 130 accumulated degree-days above the threshold to reach their infective stage.
This is why heartworm has historically been more common in warmer, humid regions like the southeastern United States and the Gulf Coast. But the larvae are resilient. They can pause their development during cool spells inside the mosquito and resume growing when temperatures rise again. This means even areas with variable weather can sustain transmission during warmer months. Warmer winters in recent years have expanded the geographic range where heartworm is a year-round concern.
Why Indoor Dogs Are Still at Risk
Many dog owners assume that keeping a dog indoors eliminates heartworm risk. It reduces exposure, but mosquitoes routinely enter homes through open doors, windows, and gaps in screens. Several of the mosquito species that transmit heartworm are well-adapted to indoor environments. In areas with high mosquito populations, even a single bite from an infected mosquito is enough to start an infection. There is no safe minimum number of bites, because it only takes one.
Geography also plays a less predictable role than many owners expect. While the highest prevalence is in the South, heartworm-positive dogs have been identified in all 50 states. Dogs that travel, dogs adopted from shelters that transport animals across state lines, and the movement of mosquito populations during warm seasons all contribute to cases appearing in areas where owners might not expect them.
What Happens Inside an Infected Dog
After the infective larvae enter through a mosquito bite, they spend their first few weeks in the tissue beneath the skin and in muscle. Over the following months, they migrate into the bloodstream and settle in the pulmonary arteries, the blood vessels connecting the heart to the lungs. As the worms grow (females can reach 10 to 12 inches long), they cause inflammation in the blood vessel walls and restrict blood flow.
Early infections often produce no visible symptoms at all, which is why routine testing matters. As the worm burden increases over months or years, dogs typically develop a persistent cough, exercise intolerance, and fatigue. In severe cases, the worms can physically obstruct blood flow through the heart, leading to a life-threatening condition called caval syndrome. By the time symptoms are obvious, significant damage to the heart and lungs has usually already occurred.
How Prevention Interrupts the Cycle
Monthly heartworm preventives work by killing the larval stages that entered your dog’s body during the previous 30 days, before they can migrate to the heart and mature. They do not repel mosquitoes or prevent the initial bite. Instead, they act as a backstop, eliminating larvae while they are still small, vulnerable, and confined to tissue near the bite site. This is why consistent, year-round dosing is recommended in most regions. A gap of even two or three months can give larvae enough time to advance beyond the stage where preventives are effective.
Annual heartworm testing detects proteins produced by adult female worms, which means a test won’t turn positive until roughly six to seven months after infection. This delay is another reason prevention is so much more practical than treatment. By the time a test picks up an infection, adult worms are already established in the heart and lungs, and eliminating them is a lengthy, costly process that requires weeks of strict exercise restriction to prevent complications from dying worms.