Dogs get heartworm from mosquito bites. There is no other way. A dog cannot catch heartworm from another dog, from feces, or from any other animal directly. The entire transmission chain depends on a single mosquito picking up microscopic larvae from an infected animal and depositing them into a new host.
How Mosquitoes Spread Heartworm
The process starts when a mosquito feeds on an animal already infected with heartworm. That animal’s blood contains tiny larvae called microfilariae, which are essentially baby heartworms too small to see. The mosquito ingests these larvae along with the blood meal.
Inside the mosquito, the larvae go through two developmental stages over roughly two weeks. During this time, they transform into an infective form capable of establishing an infection in a new host. This maturation only happens when outdoor temperatures stay consistently above 57°F. In cooler weather, the larvae stall or die inside the mosquito, which is why heartworm transmission peaks in warm months and in warmer climates.
Once the larvae reach their infective stage, they migrate to the mosquito’s mouthparts. The next time that mosquito bites a dog, the larvae slip through the tiny puncture wound in the skin. From that moment, the dog is infected.
What Happens Inside the Dog’s Body
After entering through the skin, the larvae begin a months-long journey through the dog’s body. Within one to three days, they molt in the tissue just beneath the skin. Over the next several weeks, they migrate through muscle and connective tissue, eventually reaching the bloodstream. Around two months after infection, they undergo a final molt into sexually immature adult worms, roughly an inch long at this point, and the blood carries them into the heart and the arteries leading to the lungs.
The worms continue growing for several more months. By about six and a half months after the initial mosquito bite, they reach full maturity. Males grow to 5 or 6 inches. Females can reach 10 to 12 inches. At this stage, the adults begin reproducing, releasing a new generation of microfilariae into the dog’s bloodstream, where they wait to be picked up by the next mosquito. The cycle starts again.
Why the Heart and Lungs Take the Hit
Despite the name “heartworm,” the primary damage happens in the pulmonary arteries, the blood vessels connecting the heart to the lungs. Adult worms physically occupy these arteries, triggering inflammation that damages the vessel walls and disrupts normal blood flow. The body’s immune response to the worms causes ongoing irritation to the surrounding lung tissue, producing a persistent cough that can mimic asthma or bronchitis.
As the disease progresses, the inflammation and physical obstruction cause blood pressure to rise in the pulmonary arteries. The right side of the heart has to pump harder to force blood through increasingly damaged vessels. Over time, this pressure overload weakens the right ventricle. In advanced cases, the heart can no longer keep up, leading to right-sided heart failure. Dogs at this stage often develop fluid buildup in the abdomen, exercise intolerance, and labored breathing. In severe infections with large worm burdens, worms can back up into the heart itself, creating a life-threatening emergency.
Why Early Infections Are Invisible
One of the most frustrating aspects of heartworm is the long silent period. A dog can be infected for months without showing any symptoms. The larvae are migrating and growing during this time, but they haven’t yet reached the size or location where they cause noticeable problems. Most dogs appear completely healthy through the entire development period.
Standard heartworm tests detect proteins released by adult female worms. Because the worms need about seven months to mature enough to produce these proteins, a dog infected today won’t test positive until roughly seven months later. This is why puppies under seven months old can start preventive medication without being tested first: even if they were bitten on the day they were born, there hasn’t been enough time for a test to pick it up. For older dogs, annual testing with both an antigen test and a microfilaria test is the standard recommendation, especially when a dog’s prevention history is incomplete.
Which Dogs Are at Risk
Any dog exposed to mosquitoes is at risk. Heartworm has been diagnosed in all 50 U.S. states, and the range of mosquito species capable of carrying the larvae is broad. Indoor dogs are not immune. Mosquitoes get inside homes, garages, and screened porches. Dogs that spend even brief periods outdoors during warm months have exposure.
Geography and climate influence the level of risk. Regions with long, warm mosquito seasons, particularly the Gulf Coast, the Mississippi River valley, and the southeastern United States, see the highest infection rates. But heartworm cases have been rising in areas previously considered low-risk, partly because infected dogs are transported across state lines through rescue organizations, seeding new transmission cycles in local mosquito populations.
How Prevention Works
Heartworm preventives don’t actually stop a dog from being bitten or from receiving larvae. Instead, they kill the larvae that have entered the body during the previous 30 days, before those larvae can mature and reach the heart. This is why timing matters: a single missed dose creates a window where larvae can develop beyond the point where the preventive can eliminate them.
The American Heartworm Society recommends year-round prevention regardless of where you live. Even in northern climates with cold winters, unseasonably warm spells can allow mosquito activity earlier or later than expected. Year-round dosing eliminates the guesswork of trying to predict exactly when transmission season starts and ends. It also accounts for the fact that it takes consistent temperatures below 57°F to fully halt larval development inside mosquitoes, a threshold that many regions don’t reliably hit for long stretches.
Annual testing remains important even for dogs on consistent prevention. No preventive is 100% effective, and a late or vomited dose can leave a gap. Catching an infection early, before significant damage to the heart and lungs, dramatically improves the outcome of treatment.