Heartworm in Cats: Rare, but Not Impossible, and Worth Taking Seriously

In this guide

A mosquito bite is all it takes, and there is no cure once a cat is infected.

Heartworm has a reputation as a dog disease, and for good reason: dogs are the parasite's natural host, and most awareness campaigns are built around them. Cats can be infected too, and while it happens far less often, the disease behaves so differently in cats, and is so much harder to treat, that it is worth understanding even if the odds are in your cat's favour.

How Cats Get Heartworm

Heartworm spreads through mosquito bites, not direct contact between animals. A mosquito picks up immature heartworm larvae after biting an infected dog, fox or other host, and passes those larvae on to the next animal it bites, cats included. Because transmission depends entirely on mosquitoes, any cat exposed to mosquitoes is at some risk, and that includes indoor cats, since it only takes one mosquito finding its way inside.

Why Cats React Differently

Cats are not a natural host for heartworm, so the parasite struggles to complete its full life cycle inside them. Many infected cats carry only a handful of worms, sometimes just one, and some clear the infection on their own. This might sound reassuring, but the immune reaction a cat's body mounts against the worms, especially as young worms arrive and later as any adult worms die, can cause serious lung and airway damage even when the total worm count is low. This is sometimes called heartworm-associated respiratory disease, and it can look a lot like feline asthma.

Signs to Watch For

• Coughing, wheezing or laboured breathing
• Vomiting that is not related to eating
• Reduced appetite and weight loss
• Lethargy
• In the most serious cases, sudden collapse or death with no prior warning signs at all

Some infected cats show no symptoms whatsoever, which is part of what makes this disease so difficult to catch early.

Diagnosis Is Genuinely Tricky

Testing for heartworm in cats is less straightforward than in dogs. Standard antigen tests mainly detect female worms, so a cat with only male worms, or very few worms, can test negative despite being infected. Vets often combine blood tests with chest X-rays, which can show telltale changes in the lungs and blood vessels, to build a fuller picture.

Treatment: Why Prevention Matters So Much

There is no approved drug to kill adult heartworms in cats. The medication used to clear adult worms in dogs is not safe for cats, so vets are left managing symptoms, such as corticosteroids to reduce lung inflammation, and waiting for the worms to die naturally, which can take up to around two years. This waiting period carries its own risk, since a dying worm can trigger a sudden, severe reaction. In rare, severe cases, surgical removal of worms may be considered, but this is a high-risk procedure reserved for specific situations. With close veterinary management, an estimated one quarter to one half of infected cats survive the infection, though outcomes vary a great deal depending on how many worms are present and how a cat's body reacts to them.

Given how limited treatment options are, prevention is genuinely the whole game with feline heartworm. Monthly preventive medications, many of which also cover fleas and intestinal worms, are highly effective at stopping infection before it starts. Ask your vet whether a heartworm preventive makes sense for your cat given your local mosquito exposure and whether your cat spends any time outdoors, including on a balcony. Vets often test a cat before starting prevention, though a negative test does not rule out infection as reliably in cats as it does in dogs, given how easily low worm burdens are missed.

A Note on Risk in India

Heartworm has had less research attention in Indian cats than in Indian dogs, but the picture in dogs is informative, since both share the same mosquito-borne parasite and the same infected-dog reservoir that keeps transmission going. Studies have found Dirofilaria immitis established in dogs in India's northeastern states, and the related Dirofilaria repens, which also affects dogs, reported widely across central and western India from Kerala up to Delhi. This does not mean every cat in these regions is at risk, but it does confirm the parasite is circulating in parts of the country, which is worth factoring into decisions about prevention in areas with a large stray dog population and heavy mosquito activity, particularly during and after the monsoon.

Article content © Ravi Teja. All rights reserved.