One Mosquito Is All It Takes.
Heartworm disease is spread by a single mosquito bite. It's expensive to treat, dangerous to ignore, and entirely preventable. Find your regional mosquito season — and the protection plan that matches.
- Vet Recommended
- Dogs, Cats & Horses
- AutoShip 15% Off
How Long Is Your Mosquito Season?
Enter your ZIP code. We'll calculate your mosquito pressure window and the heartworm protocol that fits.
The Heartworm Life Cycle
Click any stage. Then use the Treatment Window Calculator below to schedule your next dose.
Stage 01 — The Mosquito Bite
A mosquito feeds on an infected animal, picks up heartworm larvae (microfilariae), and then bites your dog, cat, or horse. In that single bite, infective larvae transfer into the bloodstream. There is no way to know if the mosquito was carrying the parasite.
Stage 02 — Larval Migration
After the bite, larvae migrate through the body and mature over 2–3 months. They eventually reach the heart, lungs, and pulmonary arteries. This is the window when monthly preventives kill the parasite — before it matures. Skip one dose and the window closes.
Stage 03 — Adult Heartworm
Mature heartworms — up to 12 inches long — take up residence in the heart, lungs, and arteries. They can live 5–7 years in dogs. Each one damages the cardiovascular system. Once they reach this stage, treatment becomes dangerous, expensive, and sometimes unsuccessful.
Stage 04 — Cardiac Damage
Heartworms cause inflammation, block blood flow, and scar the lungs and heart. Treatment with arsenic-based injections carries serious risk — many dogs don't survive. In cats, there is no approved treatment at all. This is why prevention is the only responsible option.
Heartworm Risk by Month
Tap any month to see mosquito pressure. The pattern is why year-round prevention is the vet standard.
Year-round prevention is the American Heartworm Society standard — even in regions with short mosquito seasons, because mosquitoes survive indoors and mild winters.
Compare Heartworm Preventives
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Heartworm prevention products
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Heartworm Questions, Answered
Real questions from farm and ranch families. Structured for instant clarity.
Yes. The American Heartworm Society recommends year-round prevention for all dogs, regardless of region. Mosquitoes survive indoors in winter, and rural properties with standing water, livestock tanks, and irrigation create year-round breeding habitat.
Yes. Cats can get heartworm, and there is no approved treatment for cats with heartworm disease. Prevention is the only option. Revolution Plus for Cats is a topical monthly preventive that also protects against fleas and ticks.
Horses can get heartworm, though it is less common than in dogs. Ivermectin-based preventives are typically used along with regular deworming schedules. Talk to your veterinarian about the right protocol for your herd.
Yes. Dogs should be tested for heartworm before starting a preventive. If a dog already has heartworm, giving a preventive can cause a dangerous reaction. Your vet can run the test quickly and we fill the prescription.
If you miss a dose, give it as soon as you remember and resume the regular schedule. If you have missed more than one dose, or if it has been more than six weeks since the last dose, talk to your vet. AutoShip reminders help prevent missed doses.
Reduce standing water around the barn, kennel, and pasture. Treat water troughs, irrigation ditches, and low spots with mosquito larvicide. Property mosquito control reduces the pressure that causes heartworm transmission.