Heartworm in Cats: Why Indoor Cats Are at Risk and How to Prevent It

Ask most people about heartworm and they picture a dog. Cats get it too. The parasite behaves differently in a cat, the disease is harder to detect, and, unlike in dogs, there is no approved drug to clear an established infection. That combination makes feline heartworm a condition where prevention matters far more than treatment.

How Cats Catch Heartworm

Heartworm disease is caused by a parasite called Dirofilaria immitis. It spreads only through mosquito bites. A mosquito feeds on an infected animal, picks up microscopic larvae, and when it later bites a cat the larvae enter through the skin. Over the following months they migrate through the tissues and, in a few cases, reach the blood vessels of the lungs and the heart.

The cat is not a natural host, so most larvae die along the way. A typical infected cat carries only one to three adult worms, compared with dozens in a dog. Those few worms can still do real harm because a cat's heart and lung vessels are small.

Why Indoor Cats Are Not Immune

Mosquitoes come indoors through open doors, window gaps, and worn screens. Surveys of cats diagnosed with heartworm have repeatedly found that a sizeable share of them were described by their owners as indoor-only. If you live somewhere mosquitoes are active, your cat is exposed, even if the exposure is small.

The disease is also found across a wide area. Dog heartworm is reported in nearly every US state and in many warm and temperate regions worldwide, and cats living in the same areas are at risk. Check with your vet about how common it is where you live.

What Heartworm Does to a Cat

Feline heartworm disease happens in two stages, and the first is the one many owners and vets miss.

• Arrival of immature worms. When young worms reach the lungs, they can trigger a sharp inflammatory reaction, sometimes called heartworm-associated respiratory disease (HARD). It is often mistaken for feline asthma.
• Adult worms. Adult worms live for two to three years in a cat. Their presence, and especially their death, can cause dramatic problems.

Many cats clear the infection or never develop adult worms at all, and some never show symptoms. That is part of what makes the disease slippery.

Signs to Watch For

Signs are often vague and can look like other conditions:

• Coughing, wheezing or open-mouth breathing
• Intermittent vomiting not linked to eating or hairballs
• Reduced appetite and weight loss
• Lethargy or hiding
• Fainting, collapse or seizures in some cases

In the worst cases, the first sign is sudden collapse or death. This happens when a worm dies and triggers a severe reaction, or when worms cause an embolism in the lungs. A cat that is breathing with its mouth open, or that has collapsed, needs emergency care at once.

The American Heartworm Society explains that cats do get heartworm and need prevention.

How Vets Diagnose It

Diagnosis is trickier in cats than in dogs. Because a cat carries so few worms, the standard antigen test, which detects proteins from adult female worms, can miss an infection. A single negative result does not always rule it out.

Vets therefore combine several tools:

• Antibody test, which shows exposure to larvae even when no adult worms are present
• Antigen test, which detects adult female worms
• Chest X-rays, which can show changes in the lung arteries
• Echocardiogram, an ultrasound that can sometimes visualise worms in the heart or lung vessels

Your vet will choose based on your cat's signs and where you live.

Treatment: Why There Is No Cure

The drug used to kill adult heartworms in dogs is not considered safe for cats, and no medication is approved to clear adult worms in a cat. Management is therefore about supporting the cat while the worms live out their natural lifespan, which is a couple of years.

That usually means monitoring with periodic X-rays, steroids if the cat has inflammation in its airways, and supportive care for breathing difficulties. Some cats recover entirely, others have lingering lung damage, and a few die suddenly. Your vet may also start monthly preventive medication to stop new infections piling on top of existing ones.

A veterinary clinic walks through cat heartworm, from transmission to prevention.

Prevention Is the Whole Game

Because treatment options are so limited, preventive medication is the best protection. Prevention products for cats are available as monthly spot-on liquids applied to the skin, or as monthly oral tablets and chews, depending on the region. Many also protect against other parasites such as fleas, ear mites, roundworms or hookworms.

• Ask your vet which product suits your cat. Availability and licensing vary by country.
• Give it on schedule. The products work by killing larvae picked up in the previous weeks, so a missed dose leaves a gap.
• Year-round or seasonal. Many vets recommend year-round dosing because mosquito seasons are lengthening and indoor mosquitoes do not follow the calendar. Your vet will advise for your region.
• Start early for kittens. Your vet will tell you when the first dose is suitable.
• Cut down mosquito contact. Repair window screens, empty standing water around the house, and keep doors closed at dusk.

Common Questions

Can my cat give heartworm to me or to my dog? No. It travels only through mosquitoes, not directly between animals or to people.

Should I test before starting prevention? Practice varies, so ask your vet. For cats, testing is often recommended when signs are present, though many vets will start prevention without it.

Is prevention safe? Licensed products are widely used and generally well tolerated. Use only those labelled for cats and never give a dog product to a cat, since some dog formulations contain ingredients that are dangerous to cats.

The Short Version

1. Heartworm spreads by mosquito bite and can infect indoor cats.
2. Signs are vague: coughing, vomiting, weight loss, or sudden collapse.
3. Diagnosis needs several tests, and a single negative does not always rule it out.
4. There is no approved cure for cats, so monthly prevention is the best defence.

This article is general information, not veterinary advice. If you are worried about your cat, contact your vet.