Can Cats Get Heartworm Disease? — Why It’s More Dangerous Than in Dogs and How to Prevent It

Infection/ParasitesQ&AMeongsiljang Veterinary Advisory Board

Heartworm disease in cats is far more dangerous than in dogs because there is no effective treatment. However, it can be safely prevented with just one monthly dose of preventive medication.

What Is Feline Heartworm Disease?

An illustration of a cat gazing at mosquitoes from the window.

Feline heartworm disease is an infectious condition caused by the parasite Dirofilaria immitis, which is transmitted through mosquito bites and takes residence in the pulmonary arteries and heart, leading to respiratory and cardiac disorders. What makes it particularly frightening is that, unlike in dogs, there is no approved treatment for adult worms in cats. Even a single heartworm can cause severe inflammation in the pulmonary arteries, potentially resulting in sudden death. Therefore, for cats, prevention is the only answer—there is no cure.

Why Is It More Dangerous Than in Dogs?

In dogs, even when dozens of heartworms grow, the disease progresses relatively slowly, allowing for gradual removal using adulticide medications like melarsomine. In contrast, cats suffer from acute lung damage even with just one to three worms due to their narrow pulmonary blood vessels and strong immune response. Furthermore, because administering adulticide drugs can cause dead worm fragments to block the pulmonary arteries, posing a significant risk of immediate death, such treatments are essentially unusable. Consequently, the only option is to manage symptoms while waiting for the worms to die naturally over two to four years.

Dogs vs. Cats — Differences in Heartworm Disease

Item	Dog	Cat
Average worm burden when infected	Multiple worms (many)	1–3 worms (few)
Adult worm treatment	Melarsomine available	No approved drug
Main symptoms	Cough / exercise intolerance	Asthma-like respiratory distress / vomiting
Risk of sudden death	Low to moderate	High
Diagnostic accuracy	Antigen test highly reliable	Both antigen and antibody tests needed
Treatment strategy	Adult worm removal	Symptom management / waiting for natural die-off

Summary of American Heartworm Society guidelines

Indoor Cats Are Not Safe Either

According to one study, approximately one-quarter (25%) of cats infected with adult heartworms were classified as strictly indoor cats. Mosquitoes can easily enter homes through gaps in window screens, when front doors are opened or closed, or even by riding in elevators. In Korea, where it is common to frequently open balcony and window doors, even indoor cats have a significant risk of being bitten by mosquitoes. In areas where heartworm disease is endemic, indoor cats remain at risk of infection, so assuming your cat is safe simply because it never goes outside is a dangerous misconception.

An indoor cat sleeping in the living room with the window open

Seek Veterinary Care Immediately If You Notice These Symptoms

Feline heartworm disease often leads to sudden death without warning, so if you notice any of the following symptoms, seek veterinary care immediately: - Sudden difficulty breathing or open-mouth panting - Repeated, spasmodic coughing resembling asthma - Recurrent vomiting (unrelated to gastrointestinal issues) - Sudden lethargy and decreased appetite - Pale or purple gums In particular, if difficulty breathing is accompanied by difficulty standing, this constitutes a medical emergency.

How Is It Diagnosed?

Because cats often harbor fewer adult worms, a single antigen test designed for dogs frequently misses infections. Therefore, the standard diagnostic approach combines an antigen test, an antibody test, a chest X-ray, and an echocardiogram. The antibody test indicates past exposure, while the antigen test detects the presence of current adult female worms. If either test returns positive, we suspect infection and proceed with further diagnostics. Since delayed diagnosis significantly limits treatment options, we recommend including these tests in regular wellness checkups.

A veterinarian examining a cat with a stethoscope at an animal hospital

Prevention Is the Only Answer

Feline heartworm disease can be prevented with nearly 100% effectiveness through monthly administration of preventive medication. The most commonly used active ingredients are selamectin, moxidectin, and milbemycin. Most products are formulated as spot-on treatments applied to the back of the neck, making them easy to administer even to cats that are difficult to give oral medications to. While the core prevention period is from April to November, when mosquitoes are active, an increasing number of veterinarians in Korea recommend year-round administration due to the warm indoor environment.

Comparison of Key Preventive Ingredients

Safe

셀라멕틴 (Selamectin)

심장사상충·벼룩·귀진드기 동시 예방

Dosage: 월 1회 스팟온

AHS 권장 1차 선택지

Safe

목시덱틴 (Moxidectin)

심장사상충·내부기생충 광범위 커버

Dosage: 월 1회 스팟온

광범위 예방 조합제 주성분

Caution

밀베마이신 (Milbemycin Oxime)

심장사상충·회충·구충 예방

Dosage: 월 1회 경구

약 먹이기 가능한 고양이에게 적합

Important Checks Before Starting Preventive Medication

It’s a good idea to test for current infection before starting preventive medication. Unlike dogs, cats rarely have circulating microfilariae (microfilariae in the blood), and when present, their levels are low. As a result, approved heartworm preventives are generally safe enough to use even in cats that test positive for antibodies or antigens. Still, we recommend testing to accurately determine the current infection status and guide future management. Never give your cat a dog’s heartworm medication; always use a product specifically formulated for cats and prescribed by your veterinarian, as the appropriate product and dosage vary by species.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

I only keep my pet indoors. Is preventive medication really necessary?

Yes, it is necessary. One study found that approximately one-quarter (25%) of cats infected with adult heartworms were classified as indoor cats. Mosquitoes can enter through gaps in window screens and when doors are opened and closed, so even indoor cats are not safe in endemic areas.

Should preventive medications be used year-round?

In Korea, mosquitoes are active from April to November, but they can survive indoors in warm apartments even during winter. Year-round medication provides consistent protection regardless of the season, which is why the American Heartworm Society also recommends it.

What should I do if my pet is already infected?

Treating adult worms in cats is challenging. Instead, we use steroids like prednisolone to calm the hypersensitivity in the lungs, provide oxygen support and manage symptoms, and then wait for the worms to die off naturally. Regular imaging is necessary to monitor progress.

Are there any side effects from preventive medications?

Approved spot-on preventatives generally have very few side effects. In rare cases, there may be temporary irritation at the application site, but this usually resolves within a few days. Please monitor your pet closely after the first application.

Can I split my dog’s medication in half?

Do not use these products interchangeably. Dog and cat formulations differ in both active ingredients and recommended dosages. Always use a veterinarian-prescribed product specifically formulated for cats.

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References

[1] American Heartworm Society, Current Feline Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Cats, 2020

[2] Textbook of Respiratory Disease in Dogs and Cats, Chapter 70 — Heartworm Disease

[3] Handbook of Veterinary Pharmacology, Drugs for Heartworm Prevention and Therapy