Heartworm in Cats

Heartworm is caused by a parasitic roundworm, Dirofilaria immitis. It primarily infects dogs, cats, and ferrets, though it can affect a range of wildlife too. It's a potentially fatal parasite, but importantly, it's also highly preventable. Humans can rarely become infected, though this is uncommon and typically doesn't cause illness.

Mosquitoes are the only way heartworms spread from animal to animal. When a mosquito bites an infected animal, it picks up young heartworms (microfilariae) along with the blood meal. Inside the mosquito, these develop into infective larvae over 2 to 6 weeks and migrate to the mosquito's salivary glands, ready to be passed on the next time that mosquito bites another animal.

Once a cat is bitten by an infected mosquito, it typically takes 6 to 7 months for those larvae to mature into adult worms - the prepatent period. The larvae spend their first 1 to 2 weeks developing under the skin near the bite site, then migrate into the chest and abdominal muscles. Around 45 to 60 days post-infection, they molt into their next larval stage, and by 10 to 15 weeks, they enter the bloodstream and travel through the heart to settle in the pulmonary artery of the lungs.

Cats are notably more resistant hosts than dogs. Microfilariae are only detectable in the bloodstream of about 20% of infected cats (compared to 80–90% of dogs), and even when present, they only survive about a month in a cat's system. This means infected cats are much less likely to pass heartworm on to a biting mosquito than infected dogs, but it doesn't make heartworm any less dangerous to the cat itself.

Adult heartworms in cats generally survive 2 to 3 years, shorter-lived than in dogs, but even a small number of worms (sometimes just one or two) can cause serious, even fatal, disease in a cat given their smaller body size and blood vessels.

Heartworm was once mostly confined to the southern United States, but it's now found in nearly every region where mosquitoes are present, as well as in many other countries. Both indoor and outdoor cats are at risk; mosquitoes can and do get indoors, so being an "indoor cat" doesn't rule out exposure. If you're planning to travel with your cat, it's worth asking your vet about heartworm risk in your destination.

Symptoms of heartworm disease in cats often overlap with other conditions, including respiratory distress, gagging, coughing, and vomiting. Signs occurring early on as the worms first arrive in the pulmonary arteries are frequently mistaken for feline asthma or allergic bronchitis. This specific early-stage presentation actually has its own name: Heartworm-Associated Respiratory Disease (HARD). In rare cases, cats can die suddenly from heartworm infection, even without prior obvious symptoms.

Diagnosing heartworm in cats is genuinely more difficult than in dogs, and results can sometimes be inconclusive even with thorough testing. Veterinarians typically run both antigen and antibody tests together to improve the odds of an accurate diagnosis, and may recommend additional imaging like chest X-rays to look for HARD-related lung changes, or a heart ultrasound to look for worms directly.

In severe cases, surgery to physically remove heartworms may be necessary. This is a high-risk procedure and is generally reserved for the most serious infections, where it offers the best chance of survival.

Given the lack of an effective treatment for cats, prevention matters even more in this species. Heartworm infection is almost completely preventable with the right medication, available in a range of formulations. Your veterinarian can help determine what fits your cat's risk factors and lifestyle, keeping in mind that indoor cats still need protection.

Preventives don't kill adult heartworms and won't clear an existing infection. Because diagnosis is more difficult in cats, additional testing is often recommended before and during a prevention program to help rule out an existing infection. Ongoing annual testing, alongside consistent use of preventive medication, remains the most reliable way to protect your cat.