Introduction
You’ve created the perfect indoor sanctuary for your feline friend—comfy beds, climbing trees, toys galore—and you rest easy knowing they’re safe from the dangers of the outside world. But there’s an uninvited guest that might slip through your defenses: heartworm disease.
“My cat never goes outside. Do I really need to worry about heartworms?”
If you’ve asked yourself this question, you’re not alone. Many indoor cat owners are surprised to learn that their homebody kitties could be at risk for a disease typically associated with outdoor pets. Today, we’re diving into the real scoop on heartworms and indoor cats—separating fact from fiction and giving you practical advice to keep your indoor companion healthy.
Understanding Heartworm Disease in Cats
Heartworm disease is exactly what it sounds like—worms that live in the heart (and lungs and associated blood vessels). Unlike their canine counterparts, cats are considered “resistant hosts” for heartworms, which means the parasites don’t thrive as well in feline bodies. But that’s not necessarily good news.
When heartworms do infect cats, they cause a condition veterinarians call Heartworm Associated Respiratory Disease (HARD). Even a small number of worms can cause significant damage to a cat’s pulmonary system, sometimes with fatal consequences.
7 Eye-Opening Facts About Indoor Cats and Heartworm Risk
1. Indoor Cats ARE at Risk (Yes, Really)
The American Heartworm Society reports that approximately 25% of cats diagnosed with heartworms are indoor-only cats. Let that sink in. One in four cats with heartworms never set paw outside. How is this possible? The answer is buzzing around your home right now.
How Are Indoor Cats Exposed to Heartworms?
Mosquitoes—the sole transmitters of heartworm disease—are masterful home invaders. They slip in through open doors, torn screens, or that window you cracked open on a nice spring day. Once inside, they can bite your cat and transmit heartworm larvae (microfilariae).
Did you know? It only takes one mosquito bite to infect your cat with heartworms.
2. Heartworm Disease Affects Cats Differently Than Dogs
In dogs, heartworm disease typically results in many adult worms that can live for years. In cats, the situation is quite different:
| Heartworm Disease | Dogs | Cats |
|---|---|---|
| Number of worms | Can harbor dozens | Usually 1-3 worms |
| Lifespan of worms | 5-7 years | 2-3 years |
| Main symptoms | Cough, exercise intolerance, heart failure | Respiratory issues, vomiting, neurological signs |
| Diagnosis | Relatively straightforward | Complex, often missed |
| Treatment options | Available, though risky | No approved treatment |
The key takeaway? Cats get fewer worms, but those few worms can cause severe damage. And unlike dogs, there’s no approved treatment to kill adult heartworms in cats.
3. Indoor Cats May Show Subtle or Dramatic Symptoms
What Are the Symptoms of Heartworm Disease in Indoor Cats?
Symptoms can range from nonexistent to life-threatening:
- Asthma-like symptoms (coughing, wheezing)
- Vomiting (sometimes unrelated to eating)
- Decreased appetite
- Weight loss
- Lethargy
- Difficulty breathing
- Rapid breathing
- Collapse
- Sudden death (yes, this can be the first and only “symptom”)
I once cared for an indoor Maine Coon who showed only occasional coughing—his owner thought it was hairballs. After several months, the coughing became more frequent, and diagnostic testing revealed heartworm disease. This illustrates how easily these symptoms can be attributed to other conditions.
4. Diagnosis is Challenging
How is Heartworm Disease Diagnosed in Indoor Cats?
Diagnosing heartworms in cats is like being a detective. Unlike in dogs, where a simple blood test usually provides a clear answer, cats require multiple tests:
- Antibody tests: Show if a cat has been exposed to heartworms
- Antigen tests: Detect adult female heartworms
- X-rays: Can reveal heart and lung changes
- Ultrasound: May actually visualize worms in the heart or pulmonary arteries
Even with all these tests, heartworm disease can be missed. Some indoor cats are only diagnosed during necropsy (animal autopsy) after sudden death.
5. Prevention is the Only Real “Treatment”
What are the Treatment Options for Indoor Cats with Heartworms?
Here’s the harsh reality: there is no approved medication to kill adult heartworms in cats. The medication used for dogs is deadly to cats. Instead, treatment focuses on:
- Supportive care: Managing symptoms
- Prednisone: Reducing inflammation
- Bronchodilators: Helping breathing
- Time: Waiting for the worms to die naturally (which takes 2-3 years)
In severe cases, surgical removal of worms has been attempted, but it’s extremely risky. This makes prevention absolutely crucial.
6. Prevention is Safe, Effective, and Essential
Is Heartworm Prevention Safe for Indoor Cats?
Heartworm preventatives are among the safest medications we give to cats. They work by eliminating heartworm larvae before they can develop into adults. Modern preventatives have been extensively tested and have excellent safety profiles.
The most effective heartworm prevention methods for indoor cats include:
- Topical medications: Applied to the skin, absorbed systemically
- Oral medications: Given by mouth (though fewer options exist for cats than dogs)
Many of these preventatives pull double or triple duty by also protecting against fleas, ear mites, and intestinal parasites—bonus protection for your indoor friend.
7. Year-Round Prevention Makes the Most Sense
How Often Should Indoor Cats Receive Heartworm Prevention?
For most of the US, year-round prevention is recommended, even for indoor cats. Why?
- Mosquitoes can be active indoors year-round (especially in heated homes)
- Climate change is extending mosquito seasons
- Monthly administration becomes routine, reducing the chance you’ll forget
- Most preventatives protect against other parasites that indoor cats can get any time of year
Common Questions About Indoor Cats and Heartworms
Can Heartworms Be Transmitted from One Indoor Cat to Another?
No. Heartworms require a mosquito intermediate host. A cat cannot directly infect another cat through contact or sharing litter boxes or food bowls.
What is the Cost of Heartworm Prevention for Indoor Cats?
Prevention typically costs $15-25 per month, depending on the product and your location. Compare this to the cost of treating complications from heartworm disease, which can easily run into thousands of dollars, not to mention the emotional cost of watching your cat suffer.
Do Older Indoor Cats Have a Higher Risk of Heartworms?
Age itself doesn’t increase susceptibility, but cumulative exposure does. The longer a cat lives without prevention, the more chances a mosquito has to transmit infection. Additionally, older cats may have more difficulty coping with heartworm disease if they become infected.
Is Heartworm Prevention Necessary if My Indoor Cat Never Goes Outside?
Given that 25% of cats diagnosed with heartworms are indoor-only, the evidence strongly suggests that prevention is necessary. Unless you can guarantee that a mosquito will never enter your home (hint: you can’t), your indoor cat is at risk.
Top Heartworm Prevention Options for Indoor Cats
While I can’t recommend specific products for your cat (that’s between you and your veterinarian), I can share information about the types of preventatives commonly used for indoor cats:
1. Topical (Spot-on) Preventatives
Applied monthly to the skin between the shoulder blades, these products are absorbed into the bloodstream:
- Revolution Plus contains selamectin and sarolaner, preventing heartworms while also killing fleas, ticks, ear mites, and treating some intestinal parasites.
- Advantage Multi for Cats contains imidacloprid and moxidectin, protecting against heartworms, fleas, ear mites, and intestinal worms.
- Bravecto Plus contains fluralaner and moxidectin, offering protection against heartworms, fleas, ticks, and intestinal parasites for two months per dose.
2. Oral Preventatives
Given by mouth monthly:
- Interceptor Plus contains milbemycin oxime and praziquantel, preventing heartworms and treating intestinal worms.
- Sentinel Spectrum contains milbemycin oxime and lufenuron, preventing heartworms, preventing flea eggs from hatching, and treating intestinal worms.
Important notes about these products:
- All require a prescription from your veterinarian
- Your vet may recommend testing before starting prevention
- Some products are not suitable for kittens under a certain age or weight
- Side effects are possible but generally mild
- Many provide protection against multiple parasites
Protecting Your Indoor Cat: A Holistic Approach
Beyond medication, consider these additional steps to reduce heartworm risk:
- Repair torn window screens and keep doors closed when possible
- Use mosquito control around your home to reduce populations
- Schedule regular veterinary check-ups to catch any health issues early
- Learn the signs of respiratory distress in cats so you can seek help promptly
Conclusion
While indoor living significantly reduces many health risks for cats, heartworm disease remains a genuine concern. The good news is that with simple monthly prevention, you can protect your feline friend from this potentially fatal disease.
Don’t wait until it’s too late—talk to your veterinarian about heartworm prevention at your next appointment. Your indoor cat’s health is worth the minimal cost and effort of prevention.
Have you started your indoor cat on heartworm prevention? What questions do you still have about protecting your indoor kitty? Share in the comments below!