Most cat owners are familiar with heartworm disease as a dog problem — and it is. But one of the most common misconceptions we encounter in our clinic is the belief that cats are safe from heartworms. They are not. Heartworm disease affects cats throughout the United States, including right here in Northeastern Wisconsin, and it can be serious even in cats who never set a paw outside. The good news is that with the right information and a simple preventive medication, your cat can be fully protected.
In this post, we are breaking down everything you need to know about feline heartworm disease — from how cats get infected, to why it looks so different than in dogs, to what you can do to keep your cat safe. Let’s get into it.
What Are Heartworms?
Heartworms (Dirofilaria immitis) are parasitic worms transmitted exclusively through mosquito bites. When a mosquito feeds on an infected animal — typically a dog, fox, or coyote — it picks up microscopic heartworm larvae. Those larvae develop inside the mosquito over the next couple of weeks. When that same mosquito bites another animal, it deposits the now-infective larvae into the new host’s skin, where they begin migrating through the body toward the heart and lungs.
In dogs, heartworms complete their lifecycle, growing into worms that can reach 12 inches in length and live in the pulmonary arteries and heart for years. In cats, things play out very differently — and in some ways, more unpredictably.
How Is Feline Heartworm Disease Different from Canine Heartworm Disease?
This is the part that surprises most cat owners, and it’s important. Feline heartworm disease is fundamentally different from the canine version — so different that some veterinary cardiologists describe them as two distinct diseases caused by the same parasite.
In Dogs: A Vascular Disease
In dogs, the primary problem is physical. Large numbers of adult worms accumulate in the pulmonary arteries and heart, physically obstructing blood flow and triggering inflammation around the blood vessels. Treatment with an injectable drug (melarsomine) is available to kill the adult worms, though the process is lengthy and requires strict rest.
In Cats: Primarily a Lung Disease
Cats are not the natural host for D. immitis, and the feline immune system is extraordinarily aggressive in attacking heartworm larvae. Most larvae are killed before they reach adulthood. The problem is that this immune assault — while often effective — causes substantial inflammation in the lungs and pulmonary arteries along the way. This inflammatory response is what makes cats sick, not the physical blockage from worms as it is in dogs.
The key distinction: in cats, it is the body’s immune reaction to the worms (especially immature, larval worms) that causes disease. In dogs, it is the physical obstruction from adult worms. This means cats can become seriously ill even without a single adult worm present.
Did you know? Approximately 25–30% of heartworm-infected cats are described as exclusively indoor cats. Mosquitoes routinely enter homes through doors, windows, and small gaps — indoor-only status does not guarantee safety.
Understanding HARD: Heartworm-Associated Respiratory Disease
One of the most important concepts in feline heartworm medicine is HARD — Heartworm-Associated Respiratory Disease. HARD describes the lung disease caused by the immune reaction to migrating larval heartworms, and it can begin as early as 75–90 days after an infected mosquito bite — long before any adult worms are present.
When immature heartworm larvae die in the pulmonary arteries, the immune system fragments and attempts to remove them. The resulting inflammation thickens lung tissue, reduces oxygen absorption, and creates permanent structural changes in the delicate airways. This is what shows up as respiratory disease in your cat.
Here’s what makes HARD especially challenging to diagnose: it looks almost identical to feline asthma on chest X-rays. The coughing, wheezing, and labored breathing are the same. Without heartworm testing, the two conditions are indistinguishable by imaging alone. Many cats with HARD have been treated for asthma for months before the true cause is discovered.
Important: If your cat has been diagnosed with asthma or is receiving treatment for a respiratory condition, it’s worth asking your veterinarian whether heartworm testing has been performed. HARD and feline asthma require very different management approaches.
The Two Phases When Cats Show Signs of Illness
Feline heartworm disease tends to manifest in two distinct phases:
Phase 1 — Larval Arrival (75–90 Days Post-Infection)
When immature worms first reach the lungs and pulmonary arteries, they trigger an intense inflammatory response. Even these small larvae are disruptive to circulation and tissue health. Cats in this phase may show respiratory symptoms that are easy to dismiss or misattribute to something else.
Phase 2 — When Adult Worms Die
In the small number of cases where larvae survive to adulthood, a second and often more dangerous crisis can occur when those adult worms eventually die. The death of even one or two adult worms inside a cat’s body triggers massive inflammation, blood clots (thromboembolism), and in some cases, anaphylactic-type reactions. This phase carries a significant risk of sudden death.
It’s also worth noting that roughly 80% of cats clear heartworm infection on their own — but this does not mean the infection was harmless. The inflammation generated during the immune response, even in cats who survive, can leave lasting damage to lung tissue.
Signs and Symptoms to Watch For
Heartworm disease in cats is frequently called “the great imitator” because its signs overlap with so many other conditions. Many cats show no symptoms at all until the disease is advanced. When symptoms are present, they may include:
- Coughing: Persistent or recurring cough
- Breathing changes: Rapid, labored, or shallow breathing, especially at rest
- Wheezing: Wheezing or an audible musical sound when breathing
- Vomiting: Bringing up food or fluid, sometimes mistaken for aggressive coughing
- Lethargy: Reduced energy, reluctance to play or move around
- Weight loss: Loss of weight without a change in appetite
- Neurological signs: Wobbling, head tilting, or seizures in rare cases where larvae migrate to the brain
- Sudden death: This is a recognized consequence of heartworm infection in cats, occurring in 10–20% of cases
If you notice any of these signs in your cat, we encourage you to call us at 920.435.5000 or schedule an appointment. Early evaluation is always better than waiting.
Diagnosing Heartworm Disease in Cats
Diagnosis in cats is significantly more complex than in dogs, and this is one reason feline heartworm disease is often missed. Here’s what’s involved:
Antigen Testing
This blood test detects proteins from adult female heartworms. It is highly specific — a positive result is meaningful — but it misses infections involving only male worms, very few worms, or immature worms only. Because cats so often have small, single-sex, or larval-only infections, antigen testing alone is not sufficient.
Antibody Testing
Antibody tests detect the immune response to heartworm larvae and adult worms. They are more sensitive (better at catching early or atypical infections) but less specific — a positive result could indicate a current infection, a past infection that has since resolved, or larvae that were killed before reaching adulthood. A negative antibody test is reassuring, but a positive result requires follow-up.
Why Both Tests Are Recommended
Because no single test reliably catches all forms of feline heartworm infection, most specialists recommend running both antigen and antibody tests together when heartworm disease is suspected. Used in combination, they provide a much clearer picture. Additional tools may include:
- Chest radiographs (X-rays) to evaluate lung and arterial changes
- Cardiac echocardiography (heart ultrasound), which can sometimes visualize worms and is more useful in cats than in dogs due to worm size relative to the small feline heart
We do not typically screen healthy cats for heartworm disease the way we do dogs annually. Testing is generally indicated when a cat is showing signs of respiratory disease and other causes have been ruled out.
Treatment: Why Prevention Is So Critical
There is currently no approved treatment to kill adult heartworms in cats. The drug used in dogs (melarsomine) is not safe for cats because the death of worms triggers a severe inflammatory response that can be fatal. This is what makes feline heartworm disease so different from the canine version and why we emphasize prevention so strongly.
Managing an Infected Cat
For cats diagnosed with heartworm disease, treatment is supportive and focused on managing inflammation and symptoms. This may include:
- Corticosteroids (prednisolone): To reduce pulmonary inflammation — often the cornerstone of management in HARD
- Doxycycline: A short course of this antibiotic is often recommended because heartworms carry internal bacteria called Wolbachia, which amplify the inflammatory response. Treating these bacteria can reduce the severity of lung disease
- Bronchodilators: To help open restricted airways
- Supportive care: Oxygen therapy, IV fluids, and fast-acting steroids in emergency situations
In some cases, if a cat is stable and the worm burden is small, a conservative approach of monitoring and symptom management may allow the infection to resolve on its own over time. However, this is a guarded prognosis, and some cats do not survive the disease regardless of treatment.
A note on Wolbachia bacteria: Heartworms harbor symbiotic bacteria called Wolbachia pipientis, which live inside the worm and significantly amplify the inflammatory response in infected cats. A course of doxycycline early in treatment can weaken the worm and reduce pulmonary inflammation, which is why we frequently include it in our management protocol.
Prevention: The Best Medicine
Because there is no safe way to kill heartworms once they are in a cat, prevention is truly the only reliable strategy. The good news is that prevention is simple, affordable, and effective.
How Preventives Work
Monthly heartworm preventive medications work by eliminating infective larvae before they can develop and reach the pulmonary arteries. They do not prevent mosquito bites, but they reliably stop the infection from taking hold. Most preventives also provide protection against other common parasites, including fleas, intestinal worms, and ear mites.
Common Preventive Options for Cats
- Revolution Plus (selamectin + sarolaner): A monthly topical that protects against heartworms, fleas, ticks, roundworms, hookworms, and ear mites
- Revolution (selamectin): A monthly topical heartworm preventive that also covers fleas and intestinal parasites
- Advantage Multi (imidacloprid + moxidectin): A monthly topical option with broad parasite coverage
We will help you choose the right preventive based on your cat’s age, health status, lifestyle, and your household’s specific parasite risk. Please call us at 920.435.5000 or ask at your next visit.
Does My Indoor Cat Really Need Prevention?
Yes. We hear this question often, and our answer is always the same: indoor cats can and do get heartworm disease. Studies have found that roughly one in four heartworm-infected cats are described as exclusively indoor. Mosquitoes enter homes regularly, and it only takes a single bite to transmit infective larvae. Year-round prevention is the only way to ensure complete protection.
When Should Kittens Start Prevention?
Heartworm prevention can be started as early as 8 weeks of age and should continue for the life of the cat. There is no age at which prevention becomes unnecessary.
Risk Factors for Heartworm Infection in Cats
While all cats are susceptible, certain factors increase exposure risk:
- Outdoor or indoor/outdoor lifestyle: Cats who spend any time outdoors, even occasionally, face a higher likelihood of mosquito exposure
- Geographic location: Heartworms are found in all 50 states, but prevalence is higher in warmer, more humid regions where mosquitoes are active for longer portions of the year
- Season: Months with higher mosquito activity mean higher transmission risk, though mosquitoes can be present year-round in some climates
- Gaps in the home’s mosquito barrier: Gaps under doors, open windows without screens, and even momentary outdoor access increase exposure
- No preventive medication: Cats not on year-round preventive medication are at risk regardless of lifestyle
Frequently Asked Questions
Can I see heartworms in my cat’s stool?
No. Heartworms live in the heart and lungs, not the intestines. If you see worms in your cat’s feces, those are intestinal parasites (such as roundworms or tapeworms), which are a separate issue your veterinarian can diagnose and treat. Please bring a stool sample to your next appointment.
My cat has been coughing — could it be heartworms?
It’s possible. Coughing is one of the most common signs of HARD and feline heartworm disease in general. However, coughing in cats can also indicate asthma, bronchitis, respiratory infections, or other conditions. A physical exam, chest X-rays, and heartworm testing can help us determine the cause. Don’t wait on a coughing cat — give us a call.
Are there any natural remedies for heartworms in cats?
No. There are no safe or effective herbal, dietary, or natural alternatives to veterinary-approved heartworm preventives or treatment protocols. We understand the appeal of natural options, but in this case, they simply do not work and may delay the care your cat needs. Only approved medications have been tested for safety and efficacy in cats.
If my cat gets heartworms, how long will treatment take?
There is no curative treatment for feline heartworm disease. Management focuses on controlling symptoms and reducing inflammation, potentially for months. Some cats stabilize well; others experience serious or life-threatening complications. This is precisely why we place so much emphasis on prevention.
Can my cat give heartworms to my dog?
Heartworms are not transmitted directly between pets. Transmission requires a mosquito as an intermediary. An infected cat can theoretically serve as a source of microfilariae for a mosquito (though this is uncommon given that microfilariae are quickly eliminated by the feline immune system), but the primary reservoir for canine infection in any area is infected dogs and wildlife. Keeping both your cat and dog on heartworm prevention is the best protection for the whole household.
Protecting Your Cat Starts With Prevention
Feline heartworm disease is serious, underdiagnosed, and completely preventable. As veterinarians, one of the most straightforward recommendations we make is year-round heartworm prevention for every cat — regardless of whether they go outdoors. A single monthly medication is all it takes to protect your cat from a disease that has no approved cure.
If your cat is not currently on heartworm prevention, or if you have questions about your cat’s respiratory health, we’d love to help. Our team at Gentle Vet is here to give your cat the best possible care.
Gentle Vet Animal Hospital
2560 University Ave, Green Bay, WI
(920) 435-5000
Everything in one place: linktr.ee/gentlevetah