Preventing heartworm in dogs comes down to one thing: giving a medication that kills heartworm larvae before they can mature into adult worms. These preventatives are highly effective, affordable relative to treatment, and available in several forms to fit your dog’s needs. The American Heartworm Society recommends giving them year-round, regardless of where you live.
How Heartworm Prevention Actually Works
Heartworm preventatives don’t create a force field against infection. They work backward. When a mosquito bites your dog, it can deposit microscopic heartworm larvae into the skin. Those larvae spend weeks migrating through tissue before eventually reaching the heart and lungs, where they grow into foot-long adult worms. A monthly preventative kills any larvae your dog picked up during the previous 30 days, before they can travel deep enough to cause damage. Think of it less like a shield and more like a monthly reset button.
This is why timing matters. If you skip a dose or give it late, larvae that were deposited weeks ago get a head start. Once they mature past a certain stage, preventatives can no longer kill them. A single missed month during mosquito season can leave your dog vulnerable.
Types of Preventatives
All FDA-approved heartworm preventatives fall into a few categories based on how they’re given. The active ingredients differ, but at standard doses they’re all effective at eliminating early-stage larvae.
Monthly Oral Tablets and Chews
These are the most common option. Products contain either ivermectin or milbemycin oxime as the active ingredient, and many are flavored to make dosing easier. You give them once a month, ideally on the same day each month. Many oral preventatives also protect against common intestinal parasites like roundworms and hookworms, which makes them a convenient two-in-one.
Monthly Topical Spot-Ons
If your dog won’t take a chewable, topical options containing selamectin or moxidectin are applied to the skin between the shoulder blades once a month. These are absorbed through the skin and circulate in the bloodstream. They also tend to cover additional parasites like fleas, ear mites, or certain ticks depending on the product.
Long-Acting Injectables
For owners who struggle to remember monthly doses, injectable moxidectin is a standout option. One version provides six months of protection, the other lasts a full twelve months. Your vet administers the injection in the clinic, so you don’t have to manage dosing at home at all. The six-month version is approved for dogs six months of age or older, while the twelve-month version is for dogs at least one year old. These injections are reserved for healthy dogs, and your vet will review your dog’s medical history before recommending them.
Why Year-Round Prevention Matters
It’s tempting to only give preventatives during warm months when mosquitoes are active. But heartworm larvae can develop inside mosquitoes whenever temperatures stay consistently above 57°F for about two weeks. In many parts of the United States, that window is unpredictable. An unusually warm stretch in late fall or early spring can sustain transmission when you wouldn’t expect it.
Year-round dosing also builds in a safety margin. If you accidentally give a dose a few days late in February, the risk is minimal because mosquito activity is low. But if you’ve already stopped prevention for the winter and restart late in spring, you’ve created a genuine gap. Staying on a twelve-month schedule removes the guesswork entirely and keeps your dog consistently protected.
Get Your Dog Tested First
Before starting any heartworm preventative, your dog needs a blood test to confirm they’re not already infected. This isn’t optional. Giving preventatives to a dog that already has adult heartworms, or microfilariae (the tiny offspring that circulate in the blood), can trigger rare but potentially severe reactions. In some cases, these reactions can be fatal.
A standard heartworm test is a simple blood draw that checks for proteins produced by adult female worms. Results usually come back within minutes at the clinic. Even dogs already on prevention should be tested annually. No preventative is 100% effective if a dose was spit out, vomited up, or given late, so yearly testing catches any breakthrough infections early, when treatment is more manageable.
The MDR1 Gene and Breed Sensitivity
Certain breeds carry a genetic mutation called MDR1 that affects how their bodies process some drugs. Collies are the most well-known, with roughly three out of four collies in the U.S. carrying the mutation. Other affected breeds include Shetland Sheepdogs, Australian Shepherds, Old English Sheepdogs, English Shepherds, German Shepherds, Long-Haired Whippets, Silken Windhounds, and various mixed breeds with herding dog ancestry.
The good news: all major heartworm preventative ingredients, including ivermectin, milbemycin oxime, and moxidectin, have been determined safe for MDR1-affected dogs when used at standard label doses. The risk with ivermectin specifically comes at much higher doses used for other purposes, not the low concentrations found in heartworm chews. If your dog is a breed at risk, a simple cheek swab test through your vet can confirm whether they carry the mutation. At prevention-level doses, you can use any FDA-approved heartworm product with confidence.
Reducing Mosquito Exposure
Preventative medication is the backbone of protection, but limiting your dog’s contact with mosquitoes adds another layer of safety. Eliminate standing water around your yard, including in flower pot saucers, clogged gutters, and kiddie pools left out overnight. Mosquitoes breed in even small amounts of stagnant water. Keeping your dog indoors during peak mosquito hours (dawn and dusk) reduces bites, and screens on windows and doors help keep mosquitoes out of the house.
These measures alone won’t prevent heartworm. Mosquitoes are too widespread and too persistent for environmental control to replace medication. But combined with consistent preventative use, they lower the overall number of larvae your dog encounters.
What Happens If You Miss a Dose
If you realize you’re a week or two late on a monthly dose, give it as soon as you remember and resume your regular schedule. A short delay doesn’t guarantee infection, but it does create a window of vulnerability. If you’ve missed two or more months, give the next dose immediately but contact your vet about retesting. Heartworm tests detect adult worms, and it takes about six months from the time of a mosquito bite for larvae to mature enough to show up on a test. Your vet will likely recommend testing six months after the gap to confirm your dog stayed clear.
With injectable preventatives, the risk of missed doses drops significantly since your vet controls the schedule. If you tend to forget monthly treatments, switching to a six- or twelve-month injection can eliminate the problem altogether.