Heartworm is not contagious from dog to dog. Your dog cannot catch heartworm by sharing a water bowl, sleeping next to an infected dog, or through any form of direct contact. Heartworm requires a mosquito to complete its life cycle, making it a vector-borne disease rather than a contagious one. A mosquito bites an infected dog, picks up microscopic larvae from its blood, and later deposits those larvae into another dog during a future bite. Without that mosquito middleman, transmission simply cannot happen.
How Dogs Actually Get Heartworm
The process starts when a mosquito feeds on a dog that already has heartworm. That dog’s blood contains tiny immature worms called microfilariae, which the mosquito ingests. Inside the mosquito, these microfilariae develop over roughly two weeks into a more advanced larval stage capable of infecting a new host. This development only happens when temperatures stay consistently above 57°F, which is why heartworm is more common in warm, humid climates and peaks during mosquito season.
When that mosquito bites your dog, the infective larvae are deposited onto the skin and enter through the bite wound. From there, the larvae migrate through your dog’s body over several months, eventually reaching the heart and pulmonary arteries, where they mature into adult worms that can grow up to a foot long. The entire process from mosquito bite to detectable adult infection takes about five to six months. Several mosquito species can carry heartworm, including common varieties found throughout the United States.
Why an Infected Dog Nearby Still Matters
While your dog can’t “catch” heartworm from another dog directly, living near an infected dog does increase risk. An infected dog serves as a reservoir: any mosquito that feeds on it can pick up larvae and carry them to your dog. This is why heartworm tends to cluster in certain neighborhoods and regions. The more infected dogs (or wild canids like coyotes and foxes) in an area, the more mosquitoes become carriers, and the higher the chance your dog gets bitten by one.
Indoor dogs are not exempt from this risk. Mosquitoes enter homes through cracked doors, open windows, and gaps in screens. As experts at Texas A&M’s veterinary diagnostic lab put it, it only takes one bite. If a single infected mosquito makes it inside, an unprotected dog is vulnerable.
Where Heartworm Risk Is Highest
Heartworm has been diagnosed in all 50 states, but infection density varies dramatically. The latest incidence survey from the American Heartworm Society, based on 2025 testing data, identified several concerning trends. Texas topped the list of leading states for heartworm incidence for the first time. East Texas, the Florida Panhandle, southwest Florida, the central Carolinas, and southern Illinois all showed more than 100 cases diagnosed per clinic, even though these areas hadn’t historically been considered high-risk hotspots.
These shifting patterns mean that even dog owners in traditionally lower-risk areas should take prevention seriously. Climate changes, pet relocation, and the movement of rescue dogs from high-prevalence regions all contribute to heartworm showing up in new places.
Prevention Options
Because heartworm requires a mosquito bite and can’t spread through contact, prevention focuses on killing larvae before they mature into adult worms. Puppies should start preventive medication as early as possible and no later than 8 weeks of age.
Monthly preventives come in oral chewables or topical formulations and need to be given year-round in most regions. For owners who find monthly dosing hard to keep up with, injectable options offer longer protection. A six-month injectable is available, and the FDA has also approved a 12-month injectable formulation for dogs 12 months and older. The 12-month version uses the same active compound as the six-month product at three times the concentration, providing a full year of coverage from a single veterinary visit. Your vet can help you choose the option that fits your dog’s age, health, and lifestyle.
Testing and Detection Windows
Heartworm tests detect proteins produced by adult female worms, and these proteins don’t appear in the bloodstream until about five months after infection. Microfilariae, the tiny larvae that circulate in the blood, aren’t detectable until about six months post-infection. Because of this delay, there’s no reason to test a puppy younger than 7 months old.
Puppies that start preventives after 8 weeks of age should be tested six months after their first dose, then annually. Annual testing is recommended for all dogs, even those on year-round prevention, because no preventive is 100% effective. A missed or late dose, a vomited tablet, or a product that wasn’t fully absorbed can leave a gap in coverage. Early detection matters because treatment is far simpler and safer when the worm burden is low.
What Treatment Looks Like
Treating heartworm is a months-long process that’s hard on both the dog and the owner’s wallet. The standard protocol recommended by the American Heartworm Society spans roughly four months. It begins with an initial consultation that includes bloodwork, X-rays, and the start of antibiotics to weaken the worms. About eight weeks later, the dog receives the first of three injections spaced over two days (with the second and third injections given 24 hours apart, five weeks after the first).
The hardest part for most owners is the strict exercise restriction. As the worms die, fragments can break off and block blood vessels in the lungs. If the dog’s heart rate and blood pressure spike from physical activity, the risk of a life-threatening blood clot increases significantly. Dogs typically need to be confined to a kennel or small room for weeks, with only brief leash walks for bathroom breaks. This cage rest continues until follow-up testing around four months after treatment begins, at which point exercise restrictions may be gradually loosened depending on results. A final blood test one year after the initial consultation confirms the worms are gone.
Cost varies by the dog’s size, typically ranging from around $1,240 for a small dog to $1,990 for a large dog. Monthly prevention, by comparison, costs a fraction of that, making it one of the clearest cases in veterinary medicine where an ounce of prevention really is worth a pound of cure.
Can Humans Get Heartworm?
Technically, yes, but it’s extremely rare and plays out very differently than in dogs. If an infected mosquito bites a person, the larvae occasionally migrate to the lungs, where they lodge in small blood vessels and die. This can create small round spots on a chest X-ray that are sometimes mistaken for lung tumors. The worms don’t survive to maturity in humans, can’t reproduce, and don’t cause the progressive heart and lung damage seen in dogs. There are no blood tests available in the U.S. to diagnose it, and most human cases are discovered incidentally during imaging for something else. It’s not a reason to panic, but it is one more reason to keep your dog on prevention and reduce the number of infected animals that mosquitoes can feed on.