Heartworm in Cats: Prevention & Why There's No Treatment

Quick Answer
  • Heartworm in cats is caused by the parasite *Dirofilaria immitis*, spread by mosquito bites. Even indoor cats can be exposed.
  • Cats often carry only a few worms, but even one to three worms can trigger serious lung inflammation, breathing trouble, or sudden collapse.
  • There is no FDA-approved medication to kill adult heartworms in cats. Care usually focuses on monitoring, symptom control, and emergency support when needed.
  • Diagnosis is more complicated in cats than in dogs and often combines blood tests, chest X-rays, and sometimes echocardiography.
  • Year-round monthly prevention is the safest strategy and usually costs far less than diagnosing and managing illness later.
Estimated cost: $25–$60

What Is Heartworm?

Heartworm disease in cats is an infection caused by the parasite Dirofilaria immitis. Mosquitoes spread immature larvae when they bite. Cats are not the parasite's preferred host, so the infection behaves differently than it does in dogs. A cat may have only a few worms, or even no mature worms at the time of testing, but the inflammation caused by immature and dying worms can still be severe.

In cats, heartworm disease often affects the lungs and blood vessels more than the heart itself. This lung-centered form is often called heartworm-associated respiratory disease (HARD). It can look a lot like feline asthma, bronchitis, or other breathing problems. Some cats cough or vomit. Others show no warning signs until they suddenly become very sick.

One reason this condition is so frustrating is that there is no approved adulticide treatment for cats. The medication used to kill adult heartworms in dogs is not considered safe for cats. Because of that, your vet usually focuses on confirming the diagnosis, reducing inflammation, managing complications, and preventing new infections.

The good news is that prevention is effective and much easier on your cat than trying to manage active disease. That is why many vets recommend year-round heartworm prevention for both indoor and outdoor cats.

Symptoms of Heartworm

  • Coughing
  • Fast or labored breathing
  • Vomiting
  • Decreased appetite
  • Lethargy or exercise intolerance
  • Open-mouth breathing or collapse
  • Sudden death

Heartworm signs in cats can be vague, come and go, or appear suddenly. Some cats have no obvious symptoms until a crisis happens. Breathing changes matter most. If your cat is breathing faster than usual, seems to be working to breathe, or starts open-mouth breathing, see your vet immediately. Even milder signs like coughing, vomiting, or reduced appetite are worth discussing, especially in mosquito-heavy areas or if your cat is not on prevention.

What Causes Heartworm?

Heartworm is caused by infection with the parasite Dirofilaria immitis. Cats get infected after a mosquito bites an infected animal, picks up microscopic larvae, and later passes those larvae to another pet during a future bite. Cats do not catch heartworm directly from dogs or other cats. The mosquito is the carrier.

After entering the body, the larvae migrate and develop over several months. In cats, many larvae die before becoming adults. That may sound reassuring, but it is part of why the disease can still be dangerous. The cat's immune response to immature and dying worms can cause marked inflammation in the lungs and pulmonary blood vessels.

Outdoor cats are at higher exposure risk, but indoor cats are not fully protected. Mosquitoes get inside homes, apartments, and screened porches. Heartworm has been reported in all 50 states, so geography changes risk level but does not eliminate it.

Because cats usually have low worm numbers and may not have circulating microfilariae, feline heartworm disease can be harder to detect than canine heartworm disease. That makes prevention especially important.

How Is Heartworm Diagnosed?

Diagnosing heartworm in cats usually takes more than one test. Unlike dogs, cats often have small worm burdens, may have only male worms, or may have immature infections. That means a single negative test does not always rule heartworm out.

Your vet may recommend a combination of heartworm antibody testing, heartworm antigen testing, and chest X-rays. Antibody tests can show exposure to immature stages, while antigen tests are more likely to detect adult female worms. Since each test has limits, they are often interpreted together rather than alone.

Chest radiographs can help your vet look for changes in the lungs and pulmonary arteries that fit heartworm disease. In some cats, an echocardiogram can directly show worms in the heart or nearby vessels. Bloodwork may also be used to check overall health and look for other causes of coughing, vomiting, or breathing trouble.

Because feline heartworm can mimic asthma, bronchitis, pneumonia, and other conditions, diagnosis is often a process of building the full picture. If your cat has respiratory signs, your vet may also discuss stabilization first and more testing once breathing is safer.

Treatment Options for Heartworm

Spectrum of Care means you have options. Here are treatment tiers at different price points.

Budget-Conscious Care

$300–$900
Best for: Stable cats with mild signs, pet parents who need a focused diagnostic plan, or cats where the main goal is confirming likely disease and preventing future exposure.
  • Office exam and risk review
  • Basic heartworm testing, often starting with antibody and/or antigen testing
  • Chest X-rays if respiratory signs are present
  • Monthly prescription heartworm prevention to stop new infections
  • Home monitoring for cough, vomiting, appetite, and breathing rate
  • Symptom-guided medications your vet may consider, such as corticosteroids in selected cases
Expected outcome: Variable. Some cats remain stable for months to years as worms die naturally, while others can worsen suddenly if inflammation or embolic events occur.
Consider: Lower upfront cost, but less intensive monitoring may miss progression. This approach does not remove existing adult worms and still requires close follow-up with your vet.
Most Common

Recommended Standard Treatment

$800–$2,000
Best for: Most cats with suspected or confirmed heartworm disease that are stable enough for outpatient workup and ongoing monitoring.
  • Full physical exam and respiratory assessment
  • Combined antibody and antigen testing
  • Chest X-rays
  • Baseline bloodwork
  • Echocardiogram when available or when diagnosis remains uncertain
  • Monthly prescription prevention year-round
  • Medical management tailored by your vet, which may include anti-inflammatory therapy, bronchodilators, or supportive care depending on signs
  • Scheduled rechecks and repeat imaging/testing if symptoms continue
Expected outcome: Fair to guarded, depending on respiratory involvement and whether the cat develops acute complications. Many cats can be managed, but sudden deterioration is still possible.
Consider: More complete information helps guide care, but costs are higher and diagnosis may still remain presumptive rather than absolute in some cats.

Advanced / Critical Care

$1,500–$4,000
Best for: Cats with open-mouth breathing, collapse, severe respiratory distress, neurologic signs, or cases needing specialty-level confirmation and critical care.
  • Emergency stabilization for respiratory distress or collapse
  • Oxygen therapy and intensive monitoring
  • Injectable medications and supportive care directed by your vet or emergency team
  • Referral echocardiography and advanced imaging when needed
  • Hospitalization for severe inflammation, thromboembolic events, or acute crises
  • Specialty consultation with internal medicine or cardiology
  • Rarely, surgical heartworm extraction in select life-threatening cases with visible worms
Expected outcome: Guarded to poor in crisis cases, though some cats improve with rapid supportive care. Outcome depends on how severe the lung injury is and whether worms can be visualized and complications controlled.
Consider: Highest cost and intensity. Surgical extraction is uncommon, technically demanding, and not appropriate for every cat, but it may be discussed in select referral cases.

Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.

Questions to Ask Your Vet About Heartworm

Bring these questions to your vet appointment to get the most out of your visit.

  1. Based on where we live and my cat's lifestyle, how high is my cat's heartworm risk?
  2. Which tests do you recommend first for my cat, and what can each test miss?
  3. Do my cat's signs fit heartworm-associated respiratory disease, asthma, or another condition?
  4. Would chest X-rays or an echocardiogram change the care plan for my cat?
  5. If my cat tests positive or suspicious, what monitoring should I do at home?
  6. What symptoms mean I should seek emergency care right away?
  7. Which monthly prevention product fits my cat best, and does it also cover fleas, mites, or intestinal parasites?
  8. What is the expected cost range for diagnosis, follow-up, and prevention in my area?

How to Prevent Heartworm

Heartworm prevention in cats is centered on year-round monthly prescription medication. This is the most important step because there is no approved drug to kill adult heartworms once a cat is infected. Preventives work by eliminating susceptible larval stages before they mature and cause more damage.

Your vet may recommend a topical or oral monthly product depending on your cat's age, weight, health history, and whether you also need flea, mite, or intestinal parasite coverage. FDA-approved feline preventives include products containing ingredients such as selamectin, selamectin plus sarolaner, moxidectin plus imidacloprid, or milbemycin oxime. Some clinics recommend testing before starting prevention, especially if your cat has symptoms or an uncertain history.

Indoor cats still benefit from prevention. Mosquitoes can enter homes through doors, windows, garages, and screened spaces. Skipping winter doses can also leave gaps, especially in regions with mild weather or indoor mosquito exposure.

Beyond medication, reducing mosquito exposure can help, but it should not replace prevention. Keep window screens in good repair, remove standing water when possible, and talk with your vet if you miss a dose. In most cases, restarting promptly under veterinary guidance is safer than waiting.

Find a Vet Near You