Mosquito Season Never Ends Here: The Case for 12-Month Heartworm Protection

If you moved to Jupiter from somewhere with a real winter, you probably remember the ritual: heartworm pills from spring through fall, then a break when the frost hit. Maybe an old vet up north even told you it was fine. Here's the thing — that advice doesn't translate. Not even a little.

There Is No 'Heartworm Season' in South Florida

Heartworm disease spreads through mosquito bites, and mosquito activity is what dictates transmission risk. Up north, mosquitoes die off or go dormant in winter, creating a natural pause in the calendar. Along the Treasure Coast, our mosquitoes simply slow down. They don't stop.

Average winter lows in Jupiter hover in the mid-50s — well within the temperature range where mosquitoes stay active and where the heartworm parasite Dirofilaria immitis can mature inside its mosquito host. The American Heartworm Society recommends year-round prevention for every state, but in Florida it's less a recommendation and more a mathematical certainty. Skip December and January doses, and you're gambling against an insect that never left your backyard.

How a Worm Ends Up in Your Dog's Heart

The lifecycle is genuinely wild. An infected mosquito bites a dog or coyote and picks up microscopic baby worms, called microfilariae. Over the next 10 to 14 days, those larvae develop inside the mosquito to an infectious stage. When that mosquito bites your pet, the larvae enter through the bite wound and spend roughly six months migrating through the body before settling in the heart and lung arteries, where they mature into foot-long adult worms.

That six-month window matters for two reasons:

• Prevention works retroactively. Most monthly preventives kill larvae your pet picked up in the previous month. Miss doses, and the larvae grow past the point where prevention can touch them.
• Testing has a lag. A heartworm blood test detects adult worms, so a dog infected in November might not test positive until spring. That's why we test annually and keep prevention going even when a test comes back clean.

'But My Pet Never Goes Outside'

We hear this one weekly, and we understand the logic. It's just wrong.

Mosquitoes come indoors. They slip through torn screens, open sliding doors, and garage gaps. Studies have shown that a meaningful percentage of heartworm-positive cats are indoor-only. Cats are also a special case: there's no approved drug to treat heartworm infection in cats at all, which makes prevention the only option. One infected mosquito in the living room is all it takes.

Local wildlife keeps the cycle spinning, too. Palm Beach County has a healthy population of coyotes, raccoons, and foxes — all of which can carry heartworm and serve as a reservoir right in your neighborhood.

The Math Nobody Argues With

Let's talk dollars, because prevention-versus-treatment is one of the starkest cost gaps in veterinary medicine.

• Prevention: Roughly $8 to $15 per month for a dog, depending on the product. That's $100 to $180 a year.
• Treatment: Typically $1,200 to $2,500 or more in our area. The protocol involves a series of deep-muscle injections of an arsenic-based compound, strict exercise restriction for months (a serious commitment for a young, energetic dog), X-rays, bloodwork, and multiple recheck visits.

And treatment isn't just expensive — it's hard on the dog. As adult worms die, they can cause dangerous reactions, which is why crate rest and leash-only breaks are non-negotiable for months. Some dogs with advanced disease can't be treated at all and are managed with medication for the rest of their lives.

What Happens When People Skip 'Winter' Doses

Every spring, we see it: dogs who missed a dose or two in January and February, then show up in April or May with a positive test. The owners are baffled — it was 'cold' out. But a single warm week in February is all a mosquito needs, and South Florida has plenty of those.

The other common failure mode is inconsistency. Prevention works best when it's given on schedule, every month, forever. A lapsed-and-resumed pattern creates exactly the gaps the parasite exploits.

Making It Easy on Yourself

You don't need to be perfect — you need a system. Set a recurring phone reminder for the same day each month. Sign up for auto-ship through your pharmacy or clinic. Pair the dose with something you already do monthly, like paying a bill. If you use a topical or injectable option, ask us about the six-month injection for dogs, which removes the monthly step entirely.

Living in paradise comes with a few trade-offs, and a mosquito that never clocks out is one of them. The good news: this is one of the most preventable diseases we treat. Twelve small doses a year, and your best friend never has to meet a foot-long worm. If your pet is behind on prevention or due for a heartworm test, give us a call — we'll get you back on track in one visit.