How does a mosquito bite turn into heartworm disease in a cat?

Heartworm disease starts entirely outside the cat, in a mosquito. When a mosquito feeds on an infected dog or other host, it picks up microscopic larvae called microfilariae, which develop inside the mosquito into an infective larval stage over roughly 10 to 14 days depending on temperature (Companion Animal Parasite Council, 2023). The mosquito then deposits those larvae onto a cat's skin during a later bite, and the larvae migrate through tissue for several months before arriving in the pulmonary arteries and lungs as young adult worms. The American Heartworm Society estimates the whole process from infective bite to mature worm takes about 8 months in cats, compared to roughly 6 months in dogs (American Heartworm Society, n.d.).

Cats are considered atypical, resistant hosts, which changes the math substantially. Only about 25% of larvae that enter a cat survive to adulthood, and most infected cats end up with just one to four worms, versus the dozens to hundreds seen in heavily infected dogs (American Heartworm Society, n.d.). Fewer than 20% of those adult worms ever produce circulating microfilariae, which is one reason standard antigen blood tests can miss feline infections — there may not be enough mature female worms present to trigger a positive result.

Why don't cats get heartworm as often as dogs in the same neighborhood?

Prevalence studies consistently show cats living in the same geographic area as infected dogs still acquire heartworm far less often — commonly cited at roughly 5% to 20% of the local canine infection rate (Companion Animal Parasite Council, 2023). Part of that gap is biological: the cat's immune system fights off most incoming larvae before they mature, and surviving worms often trigger an intense inflammatory reaction that kills them off early rather than allowing a stable, long-lived infection like in dogs.

Indoor status does not equal safety. Multiple prevalence surveys, including data compiled by the American Heartworm Society, have found that roughly one-quarter to one-third of cats diagnosed with heartworm disease were reported by their owners as strictly indoor pets, because mosquitoes routinely enter homes through doors, windows, and garages (American Heartworm Society, n.d.). That statistic is the main reason veterinary groups recommend preventives for indoor cats too, not just cats with outdoor access.

What are the first signs an owner notices when a cat has heartworms?

The clinical picture in cats is dominated by respiratory signs collectively referred to as heartworm-associated respiratory disease, or HARD, which can look strikingly like feline asthma or chronic bronchitis. Owners typically first notice intermittent coughing, wheezing, rapid or labored breathing, or episodes that resemble asthma attacks (Merck Veterinary Manual, 2022). Vomiting unrelated to eating, lethargy, and reduced appetite are also frequently reported, and because these signs overlap with several other common feline illnesses, heartworm is easy to miss without specific testing.

What makes feline heartworm especially dangerous is that dramatic signs can appear even when only one or two worms are present, because a young cat's pulmonary vasculature reacts strongly when worms first arrive and again later when they die, triggering acute lung inflammation. Cornell Feline Health Center notes that some cats show no signs at all until sudden collapse or death occurs, often when a single worm dies and lodges in the pulmonary artery, triggering a fatal embolism (Cornell Feline Health Center, n.d.).

How do veterinarians actually diagnose heartworm disease in a cat?

Because no single test is reliable on its own in cats, veterinarians typically combine several diagnostics. An antigen test detects proteins from adult female worms but can produce false negatives when a cat has only male worms, immature worms, or a low overall burden — which describes most feline infections (American Heartworm Society, n.d.). An antibody test detects the cat's immune response to any exposure, including larvae that never matured, so a positive antibody result with a negative antigen result still supports a diagnosis of exposure or early infection.

Imaging rounds out the workup. Thoracic radiographs often show enlarged, blunted, or tortuous pulmonary arteries and patchy lung infiltrates, and echocardiography can sometimes directly visualize adult worms within the right heart or main pulmonary artery in cats with a higher worm burden (Merck Veterinary Manual, 2022). Bloodwork frequently reveals eosinophilia, an increase in a specific white blood cell type tied to parasitic and allergic inflammation, which supports but doesn't confirm the diagnosis on its own.

Is there a treatment that kills heartworms in cats, and is it safe?

Unlike in dogs, there is no drug approved to kill adult heartworms in cats. Melarsomine, the arsenic-based adulticide used in canine treatment protocols, is not considered safe for cats because dying worms in a cat's smaller pulmonary vessels can trigger severe, sometimes fatal, pulmonary thromboembolism (American Heartworm Society, n.d.). This absence of an adulticide is the central reason prevention carries so much weight in feline heartworm management — once adult worms are established, veterinarians are largely managing the cat's reaction to the worms rather than eliminating the parasite.

Management instead centers on supportive care: corticosteroids to reduce lung inflammation, bronchodilators for cats with respiratory distress, and strict activity restriction, since exertion can worsen pulmonary vascular damage. In select cases where a worm is visible on echocardiography and a cat is stable, surgical worm extraction via jugular vein catheterization has been performed at specialty referral centers, though it carries real procedural risk and is reserved for specific situations (Merck Veterinary Manual, 2022).

What does long-term management look like for a cat that survives heartworm disease?

Some cats clear an infection spontaneously as their immune system destroys the worms over roughly 2 to 4 years, the natural lifespan of the adult parasite, and many of these cats need ongoing management of residual airway inflammation rather than the heartworm itself (Cornell Feline Health Center, n.d.). Because of that residual damage, cats previously diagnosed with heartworm often continue to need periodic rechecks, chest radiographs, and sometimes long-term low-dose corticosteroids or bronchodilator therapy to control HARD-related coughing and wheezing.

Prognosis varies widely depending on how the cat first presented. Cats that had a single acute embolic event and survived it can do reasonably well afterward with monitoring, while cats with chronic respiratory disease from the initial infection may need lifelong management similar to feline asthma. Veterinary internal medicine specialists generally recommend rechecking antibody status and repeating thoracic imaging on a schedule set by the treating veterinarian rather than assuming the cat is fully in the clear after signs resolve.

What's the best way to actually prevent heartworm disease in a cat?

The American Heartworm Society and the Companion Animal Parasite Council both recommend year-round heartworm prevention for cats in most of the United States, including indoor-only cats, given how easily mosquitoes get indoors and how severe even a single-worm infection can be (Companion Animal Parasite Council, 2023). FDA-approved feline preventives include monthly topical selamectin, monthly topical or oral moxidectin combination products, and monthly oral or topical ivermectin-based products, several of which also cover fleas, ear mites, or intestinal parasites in the same dose.

Consistency matters more than the specific product chosen. Missing even one monthly dose can create a window during peak mosquito season for infective larvae to establish, and veterinarians typically recommend a baseline antibody or antigen test before starting prevention in a cat with unknown status, since preventives are not labeled to treat existing adult infections. Reducing standing water around the home and using screens on windows can reduce mosquito exposure but should be treated as an addition to, not a substitute for, monthly preventive medication (American Heartworm Society, n.d.).