How to Prevent Heartworm in Dogs? | Year-Round Protection Plan

Heartworm disease is almost 100% preventable, yet it remains a serious threat in all 50 states because mosquitoes carry the larvae year-round, even in colder months. The American Heartworm Society (AHS) calls it the “Think 12” protocol: 12 months of prevention and a 12-month testing interval. Skipping either step leaves your dog vulnerable, and relying on seasonal prevention alone is one of the most common — and costly — mistakes owners make.

The Core Strategy: Medication Plus Testing

All preventives require a veterinarian’s prescription because the wrong dose or product can be ineffective or dangerous.

Oral options include ivermectin and milbemycin. Topical preventives use selamectin or moxidectin. Injectable moxidectin, given by a vet every 6 or 12 months, is an alternative for owners who struggle with monthly dosing. Updated AHS guidelines now also recommend combination products that pair a heartworm preventive with an isoxazoline flea/tick treatment — consult your vet about whether a combo product suits your dog. For a detailed comparison of popular brands and what other owners recommend, check out our guide to the best dog heartworm prevention options.

If a dose is missed, the AHS advises restarting monthly prevention immediately and testing the dog 7 months after the lapse to confirm no infection developed during the gap.

How Do You Start Heartworm Prevention Correctly?

The starting rules depend on the dog’s age. Puppies should begin prevention no later than 8 weeks of age; some products allow initiation as early as 4 weeks, and no pretesting is required for puppies under 7 months. For dogs 7 months or older, a negative antigen blood test is mandatory before starting any preventive — dosing an already-infected dog can cause serious complications. A second negative test must be performed 6–7 months after starting to confirm the dog was not infected prior to the first dose.

Year-round administration applies to every state because mosquitoes can survive indoors or appear during unexpected warm spells. The AVMA and Cornell University College of Veterinary Medicine both emphasize that seasonal prevention leaves dangerous gaps — and once a dog tests positive, treatment requires adulticide therapy, not just preventives.

Additional Layers: Mosquito Control and Safety

Mosquito control adds a practical second line of defense. Use EPA-approved mosquito repellents formulated for dogs (never human DEET products), remove standing water from your yard, limit outdoor activity at dusk and dawn when mosquitoes are most active, and keep screens repaired.

Safety considerations: Some macrocyclic lactones can cause neurotoxicity in dogs with the MDR1 gene mutation, which is common in herding breeds like Collies and Australian Shepherds — always discuss breed-specific risks with your vet. Oral chewables come in specific weight ranges; giving a dose meant for a larger dog can cause toxicity, while an undersized dose may not protect. Never use a preventive on a puppy younger than the product’s label minimum age.

FAQs

Can I stop giving prevention during winter?

No — the AHS recommends year-round prevention in all US states because mosquitoes can be active indoors or emerge during unseasonably warm periods. Stopping for even one month can leave your dog unprotected if a mosquito carrying larvae bites them.

What happens if my dog tests positive after I started prevention?

An infected dog needs adulticide treatment (usually with melarsomine injections) to eliminate adult heartworms. Your vet will manage the protocol, which includes strict exercise restriction.

Are over-the-counter heartworm preventives effective?

Over-the-counter products sold online or in pet stores are not regulated for efficacy against heartworm and may contain incorrect ingredients or doses. Only prescribed medications from a licensed vet are proven to prevent the disease.