“My cat never goes outside” is the most common reason Brooklyn cat owners give for skipping heartworm prevention. It’s also one of the more dangerous assumptions in cat ownership, because indoor cats don’t need to go anywhere for the thing that transmits heartworm to reach them.
Quick Answer: Yes, Indoor Cats Need Heartworm Prevention
Indoor cats need heartworm prevention because mosquitoes, the only way heartworm spreads, get indoors regardless of how careful you are. A single bite from an infected mosquito is enough, and cats have no approved medication to treat an adult heartworm infection once it takes hold, unlike dogs. For the full rundown of prevention products and how they compare, see our guide to heartworm prevention for cats.
How Mosquitoes Get Into a Brooklyn Apartment
This is the part most guides skip past. You don’t need a yard for mosquito exposure. Open windows during warmer months, a mosquito that rides in on a person or a dog coming back from a walk, a gap in a window screen: all of it is enough. Brooklyn’s mosquito season runs roughly April through October, and an apartment on the fourth floor isn’t automatically safe just because there’s no grass nearby.
Older buildings make this worse, not better. A brownstone with window units instead of central air means more open windows on warm nights, and older screens are more likely to have a small tear or gap you haven’t noticed. None of this means your apartment is uniquely risky. It just means the “indoor equals safe” assumption doesn’t hold up the way it seems like it should.
Why Indoor Cats Still Carry Real Risk
Indoor cats do have lower exposure than cats who spend real time outside, and that’s true. But lower isn’t zero, and heartworm disease in cats doesn’t behave like it does in dogs. Cats are considered a less natural host for the parasite, which sounds like good news until you learn what that actually means: their immune response to even a small number of worms can be severe, sometimes fatal, and it isn’t reliably caught by the antigen tests used to diagnose heartworm in dogs.
Standing water is the other piece owners tend to miss. Mosquitoes don’t need a yard to breed, a clogged gutter, an AC drip tray, or a fire escape planter that holds water after a storm is enough. In a city of brownstones and window units, that kind of standing water is more common than most owners realize, and it puts a mosquito within reach of an apartment that never had a “risk” in the traditional sense.
What Heartworm Disease Looks Like in Cats
Heartworm in cats often presents as vague respiratory symptoms, coughing, wheezing, or rapid breathing, that get mistaken for asthma or a hairball problem. Vets call this pattern heartworm-associated respiratory disease, and it can wax and wane for months before anything looks urgent. Other cats show no warning signs at all until sudden collapse, because even one or two worms dying off can trigger a fatal reaction in a cat’s smaller, more reactive cardiovascular system. If your cat develops labored breathing, especially with no clear cause, that’s not a wait-and-see situation. Know what actually counts as a pet emergency so you’re not guessing in the moment.
Starting Prevention Early
The easiest time to build a prevention habit is before it becomes urgent. If you’ve recently brought a kitten home, heartworm prevention is one of several things worth setting up at that very first appointment, alongside vaccines and deworming. Our kitten’s first vet visit checklist covers what else to expect at that stage, and starting the conversation early makes it a habit rather than a scramble later.
Adult cats coming into our practice for the first time aren’t excluded from this either. If they’ve never been on a preventive, we’ll talk through where to start rather than assuming the window has passed.
The Cost of Prevention vs. the Cost of Guessing Wrong
Monthly heartworm prevention for cats is a small, predictable cost. There’s no approved treatment for adult heartworm infections in cats, which means prevention isn’t one option among several, it’s the only real strategy available. That asymmetry is why we recommend it for every cat we see, regardless of whether they’ve ever set a paw outside.
Practical Steps for an Apartment-Bound Cat
Beyond the monthly preventive itself, a few habits lower the odds of a mosquito reaching your cat in the first place:
- Keep window screens intact and check them each spring, not just when you notice a problem
- Empty standing water from AC drip trays, planters, or anything on a fire escape after it rains
- Ask us about testing before starting a preventive if your cat has never been on one, since starting cold without a baseline test skips a step vets generally recommend
- Keep prevention going year-round rather than pausing over the winter, since a missed month is exactly the kind of gap that lets exposure slip through
None of these replace the monthly preventive itself. They just close the small gaps around it.
Final Thoughts on Heartworm Prevention for Indoor Cats
“Indoor” reduces risk, it doesn’t remove it, and cats have less margin for error than dogs if an infection does take hold. If your cat isn’t currently on a preventive, or you’re not sure what they’re on, contact The Vet Set or book an appointment online and we’ll get them protected before mosquito season catches you off guard.