Cats get heartworm the same way dogs do: through the bite of an infected mosquito. There is no other route of transmission. A cat cannot catch heartworm from another cat, from a dog, or from any other animal directly. The mosquito is the essential middleman, picking up microscopic larvae from an infected animal and depositing them into a new host during a blood meal.
What Happens After a Mosquito Bite
When a mosquito feeds on an animal already carrying heartworm, it ingests tiny larvae circulating in the blood. Those larvae develop inside the mosquito for a couple of weeks until they reach an infective stage. The next time that mosquito bites a cat, it deposits these larvae into the skin.
From there, the larvae burrow into tissue and begin a slow migration through the cat’s body, eventually heading toward the blood vessels of the lungs. The entire journey from mosquito bite to the arrival of worms in the pulmonary arteries takes roughly 7 to 9 months. But here’s what makes cats different from dogs: most of the larvae die before they ever mature. A cat’s immune system is far more hostile to heartworm than a dog’s, so only a small percentage of cats that are bitten by an infected mosquito end up with adult worms.
Why Cats Are Different From Dogs
Dogs are the natural host for heartworm. Their bodies, unfortunately, provide a welcoming environment where larvae mature easily and adult worms can thrive for 5 to 7 years. A heavily infected dog can carry 30 to over 200 adult worms. Cats are a very different story. Most infected cats have just one or two adult worms, rarely more than six. Adult worms in cats survive only about 2 to 3 years.
This might sound like good news, but it’s misleading. Cats are so much smaller than most dogs that even one or two worms can cause serious, life-threatening disease. And paradoxically, the cat’s strong immune response to the larvae is itself a major source of harm.
How Immature Worms Damage the Lungs
One of the most important things to understand about feline heartworm is that cats don’t need adult worms to get sick. When immature larvae arrive in the small blood vessels of the lungs, the cat’s immune system launches an intense inflammatory attack to kill them. This inflammation affects the airways and surrounding lung tissue, causing a condition veterinarians call Heartworm Associated Respiratory Disease, or HARD.
HARD produces coughing, wheezing, and difficulty breathing that looks almost identical to feline asthma. In fact, some cats diagnosed with asthma may actually have HARD. The damage comes not from the worms thriving, but from the body’s own violent reaction to killing them. This means a cat can suffer significant lung disease even if no adult worms ever develop, and even if every standard heartworm test comes back negative.
Indoor Cats Are Not Safe
One of the most common misconceptions is that indoor cats don’t need heartworm prevention. Mosquitoes get inside homes regularly, and the data reflects this. A study of cats in the lower peninsula of Michigan found that of 25 heartworm-positive cats, 16 were housed indoors. Research in Tulsa, Oklahoma, found that 23% of exclusively indoor cats in one study group showed evidence of heartworm infection. A separate multi-state study across Florida, South Carolina, Tennessee, and Texas reported that roughly 12% of strictly indoor cats tested positive on antigen testing, and about 20% tested positive for heartworm antibodies.
These numbers make it clear that living indoors reduces exposure but does not eliminate it. A single mosquito that slips through a door or window is all it takes.
Why Diagnosis Is Difficult in Cats
Testing cats for heartworm is significantly harder than testing dogs. Two types of blood tests are used: antigen tests (which detect proteins from adult female worms) and antibody tests (which detect the cat’s immune response to any heartworm exposure). Each has limitations.
Antigen tests are highly specific, meaning a positive result is reliable. But because cats typically carry so few worms, and because a male-only infection produces no detectable antigen at all, false negatives are common. Antibody tests cast a wider net and can detect exposure even when no adult worms are present, but their real-world sensitivity varies widely. In shelter cats, sensitivity has ranged from as low as 32% to as high as 89%, depending on the study. Veterinarians often use both tests together, sometimes alongside chest X-rays or ultrasound, to piece together a diagnosis.
No Cure Exists for Cats
Dogs with heartworm can be treated with a drug that kills adult worms, but that treatment is not safe for cats. It can cause fatal complications. This means there is no approved cure for feline heartworm disease. Veterinary care for infected cats focuses on managing symptoms, reducing inflammation, and monitoring the cat while waiting for the worms to die on their own over their 2- to 3-year lifespan. Some cats survive without major problems. Others experience sudden death when even a single worm dies and causes a massive inflammatory reaction or a blood clot in the lungs.
This is precisely why prevention matters so much more in cats than in dogs. With dogs, you have a safety net of treatment. With cats, you don’t.
How Prevention Works
Preventive medications for cats work by killing heartworm larvae in the early stages of migration, before they can reach the lungs and mature. These products are given monthly (or, for one topical option, every two months) and are available as oral tablets, chewables, or topical spot-on treatments. The American Heartworm Society recommends year-round prevention regardless of where you live or whether your cat goes outside.
Most feline heartworm preventives also protect against intestinal parasites, fleas, or ear mites, depending on the product. Your veterinarian can help you choose based on your cat’s lifestyle and what other parasites are common in your area. The key point is consistency: a single missed dose during mosquito season can leave a window for larvae to establish themselves, and once they do, there’s no going back.