Two beliefs about heartworm are so common, and so wrong, that they quietly get cats killed. The first: indoor cats are safe. The second: heartworm is a dog’s disease, so cats don’t need to worry about it. Both feel like common sense. Both are false. And the reason they’re so dangerous is the fact sitting underneath them: for cats, there is no cure. Once the infection takes hold, there’s no drug to clear it, which makes prevention not just the best option but the only real one.
Start with the indoor myth. Have you ever been bitten by a mosquito inside your own home? Almost everyone has. Mosquitoes slip through windows, doors, and vents, and they are the sole way heartworm spreads. Walls don’t stop them, so walls don’t protect your cat. Now the second myth. It’s true that cats are what’s called an atypical host, meaning the worms rarely complete their life cycle the way they do in dogs. But “rarely mature” is not “harmless.” Even immature worms can inflame and damage a cat’s lungs and heart, and the handful that do reach adulthood can live inside a cat for two to three years.
What heartworm disease actually is in cats
Heartworm disease is severe inflammation of the lungs and heart caused by a parasitic worm, Dirofilaria immitis, carried by mosquitoes. When an infected mosquito bites a cat, it deposits microscopic larvae (microfilariae) into the bloodstream.
In dogs, the natural host, those larvae grow into adults and reproduce. In cats, most larvae die before ever becoming adults. That sounds like good news until you understand what those dying worms do on the way out.
Heartworm-associated respiratory disease (HARD)
The main reason heartworm is so dangerous in cats even when worms never mature is a condition called heartworm-associated respiratory disease, or HARD. When immature worms die in the lungs instead of developing further, they trigger intense inflammation that looks and behaves a lot like asthma or bronchitis. In other words, the cat’s own immune reaction to the parasite is a big part of the damage.
The symptoms are a moving target
Heartworm in cats is a master of disguise. Some cats show no signs at all. For others, the very first symptom is sudden death. In between sits a range of mostly respiratory problems:
- Respiratory distress: coughing, wheezing, labored breathing
- Vomiting, often with no connection to eating
- Lethargy and reduced activity
- Weight loss, even with a normal appetite
- Loss of appetite
- Seizures or trouble walking, rare, but possible if worms affect the nervous system
- Sudden collapse or death from acute complications
The overlap with other feline illnesses is the trap. These signs mimic asthma, bronchitis, and digestive disorders, so heartworm often gets diagnosed as something else. Feline hypertrophic cardiomyopathy, a heart condition that also causes sudden death, muddies the picture further. Since most cats who die suddenly never get a necropsy, heartworm is almost certainly underdiagnosed, which means our count of affected cats is likely well short of the real number.
How a cat gets it
To be precise: heartworm is caused by Dirofilaria immitis, and it spreads exclusively through mosquito bites, never through contact between animals. The cycle looks like this:
- A dog carrying adult heartworms has larvae circulating in its blood.
- A mosquito bites the dog and picks up those larvae.
- The same mosquito later bites a cat and passes the larvae along.
- The larvae ride along in the cat’s blood and lodge in the heart and lung tissue.
- Most never reach adulthood, but even as larvae they inflame and damage the lungs and vessels.
Which cats are most at risk
Any cat can get heartworm, indoor cats included, but risk climbs for:
- Outdoor cats, with more mosquito exposure
- Cats in warm, humid climates, where mosquito season is long
- Cats living near dogs, since dogs are the true hosts and act as reservoirs
Diagnosis is genuinely hard
Testing cats for heartworm is difficult because the available tests were designed for dogs, whose infections look different. No single test is reliable on its own, so vets usually combine several:
- Antigen test: flags proteins shed by adult female worms. Since cats seldom carry adults, and often just a single worm or two, it misses a large share of real infections, producing false negatives.
- Antibody test: detects the immune system’s response to heartworm exposure, but a positive doesn’t prove an active infection is present.
- Chest X-rays: can reveal lung inflammation consistent with the disease.
- Echocardiogram (heart ultrasound): can spot worms in the heart or vessels.
Why there’s no cure, and what treatment can do
Here’s the hard center of it. Melarsomine is the drug that kills adult heartworms in dogs. In cats, melarsomine is toxic, and there is no safe substitute. That’s the whole reason prevention matters so much: if a cat becomes infected, no drug can kill the worms.
What veterinarians can do is manage the symptoms and try to keep the cat stable while the infection runs its course:
- Corticosteroids (prednisone): to reduce lung inflammation
- Bronchodilators: to ease breathing
- Oxygen therapy: for severe respiratory distress
- Hospitalization: intensive care during a heartworm crisis
Surgically removing worms from the heart is possible but extremely risky and rarely done. Cats that survive an infection often carry long-term lung damage, and management becomes a matter of ongoing monitoring, regular check-ups, periodic chest imaging, and anti-inflammatory medication as needed. Recovery time varies: a severe case can leave lifelong respiratory issues, while a milder one may resolve over months.
Prevention is the entire game
Since there’s no treatment and no way to keep mosquitoes out entirely, a monthly preventative is how you protect your cat. Veterinarians recommend products such as:
- Selamectin (Revolution)
- Moxidectin (Advantage Multi)
- Ivermectin (Heartgard for Cats)
- Milbemycin oxime (Interceptor)
Layer in a few environmental steps: keep cats indoors to cut mosquito exposure, use window screens, and eliminate standing water where mosquitoes breed. None of these replace the monthly preventative, but they lower the odds.
“The line I repeat to every cat owner is that with feline heartworm, the medicine cabinet is empty on the treatment side,” says Dr. Priya Nair. “There’s no cure to fall back on, so the once-a-month dose isn’t optional or seasonal, it’s the whole defense. I’d rather prevent a disease I can’t treat than gamble that a mosquito won’t find my patient indoors.”
What’s new since this was written
The idea that heartworm is a shrinking or strictly regional worry is getting harder to defend. The American Heartworm Society’s 2025 incidence survey found heartworm spreading well beyond its traditional hotspots, with rising cases in states like Illinois, Michigan, Rhode Island, and Maine, and one analysis of claims data showed heartworm cases jumping roughly 201 percent between 2020 and 2025. Diagnoses are also occurring year-round, undercutting the notion that it’s a summer-only concern.
For cats specifically, the guidance is unambiguous: current recommendations call for every cat to be on year-round heartworm prevention using a macrocyclic lactone product, the drug class that includes the preventatives above. Given a disease that can’t be cured, spreads to new regions each year, and hides behind asthma-like symptoms, “all cats, all year” is the only advice that fits the facts.
References
- American Heartworm Society. “Feline Heartworm Guidelines.” Link
- Lee, Alice C. Y., and Clarke E. Atkins. “Understanding Feline Heartworm Infection: Disease, Diagnosis, and Treatment.” Topics in Companion Animal Medicine, vol. 25, no. 4, 2010, pp. 224–230. Link
- Atkins, Clarke E., et al. “Heartworm Infection in Cats: 50 Cases (1985–1997).” Journal of the American Veterinary Medical Association, vol. 217, no. 3, 2000, pp. 355–358. Link
