Heartworm preventatives work by killing heartworm larvae in your dog’s body before they can mature into adults. Every active ingredient used in these products belongs to the same drug class, and they all target a specific channel in the parasite’s nervous system that doesn’t exist in mammals. The drugs don’t repel mosquitoes or stop infection from happening. Instead, they act as a monthly cleanup, wiping out any larvae deposited by mosquitoes since the last dose.
What Happens Inside the Parasite
All heartworm preventatives are built around a type of compound called a macrocyclic lactone. The four used in veterinary products are ivermectin, milbemycin oxime, moxidectin, and selamectin. Despite their different names, they share the same core structure: a 16-member ring that locks onto a very specific target in the worm’s body.
That target is a type of chloride channel found on nerve cells and muscle cells throughout the parasite. These channels are gated by a neurotransmitter called glutamate, so scientists refer to them as glutamate-gated chloride channels. When the drug binds to one of these channels, it forces it open in a nearly irreversible way. Chloride ions flood into the cell, the cell’s electrical charge drops, and the result is flaccid paralysis. The worm can’t move, and its pharynx (the muscular tube it uses to feed) stops pumping. Critically, these channels don’t exist in mammals, which is why the drugs are toxic to worms but safe for dogs and cats at standard doses.
There’s an additional layer to how these drugs work that goes beyond simple paralysis. Research has shown that the drug concentrations reached in a dog’s bloodstream after a normal dose are actually far lower than what’s needed to paralyze worms in a lab dish. This means something else is helping. The leading explanation is that the drug interferes with molecules the larvae secrete to hide from the immune system. With that chemical camouflage disrupted, the dog’s own immune cells recognize and destroy the larvae. So the preventative works as a partnership: the drug disables the parasite’s defenses, and the immune system finishes the job.
Which Larvae Get Killed
When a mosquito bites your dog, it deposits tiny L3 larvae into the skin. Over the next two months, those larvae molt into L4s and then into juvenile worms that eventually migrate toward the heart and lungs. Heartworm preventatives are effective against larvae during roughly the first 60 days after infection. After that window closes, the worms become increasingly resistant to the drugs, and by about 120 days post-infection they’re essentially untouchable by preventatives alone.
This is why the products are dosed monthly. Each dose is essentially working backward in time, clearing out any larvae your dog picked up during the previous 30 days (or, in the case of a missed dose, up to about 60 days). The American Heartworm Society has described this as a “reach-back” effect: all macrocyclic lactones can eliminate larvae up to two months old. That two-month reach-back provides a safety buffer if you’re a few weeks late on a dose, but it’s not a reason to skip months. Larvae older than 60 days enter a gap where neither preventatives nor the adult-killing treatment works well.
Why They Don’t Kill Adult Worms
Adult heartworms live in the pulmonary arteries and heart chambers. They’re large (up to 12 inches), well-established, and physiologically different from larvae. Preventatives at standard doses simply don’t affect them. Killing adult worms requires a completely different drug, an arsenic-based compound that’s injected deep into the back muscles by a veterinarian. The standard treatment protocol uses preventatives alongside the adulticide: the preventative stops new larvae from developing while the older worms are given time to reach a stage where the adulticide can destroy them. The two drugs complement each other but are not interchangeable.
Oral, Topical, and Injectable Options
Monthly oral tablets and chewables are the most common format. Products based on ivermectin or milbemycin oxime are swallowed, absorbed into the bloodstream, and cleared from the body relatively quickly. They don’t provide continuous protection between doses. Instead, each pill retroactively kills the larvae that accumulated over the previous month.
Topical (spot-on) products use selamectin or moxidectin and are applied to the skin between the shoulder blades. They absorb into the skin’s oil layer and distribute across the body over several days, working on the same principle as oral products but without requiring your dog to eat anything.
Injectable formulations take a different approach. A moxidectin-based injectable uses microsphere technology: tiny biodegradable spheres that release the drug slowly over six or twelve months, depending on the dose. Unlike monthly products, the injectable maintains a continuous level of drug in the bloodstream, killing larvae as they arrive rather than in a single monthly sweep. The six-month version delivers 0.17 mg/kg, and the twelve-month version delivers 0.5 mg/kg. Because the drug is always present, there’s no window of vulnerability between doses, which eliminates the compliance problem that comes with remembering monthly treatments.
Breed-Specific Sensitivities
Some dogs carry a mutation in a gene called MDR1 that affects how their bodies process certain drugs, including macrocyclic lactones at higher doses. The MDR1 gene normally produces a protein that acts as a gatekeeper, pumping drugs out of the brain. Dogs with the mutation lack this protection, so certain drugs can cross into the brain and cause neurological toxicity: tremors, disorientation, blindness, seizures, or worse.
At the low doses used in standard heartworm preventatives, even MDR1-affected dogs are generally safe. The risk climbs with higher doses, such as those used to treat other parasitic infections. Still, knowing your dog’s MDR1 status matters. Collies are the most commonly affected breed, with roughly 70% carrying the mutation. Australian shepherds and miniature Australian shepherds sit at about 50%. Other breeds with notable rates include silken windhounds and long-haired whippets (30%), McNabs (30%), Chinooks (25%), Shetland sheepdogs (15%), English shepherds (15%), and German shepherds (10%). About 5% of mixed-breed dogs also carry the mutation. A simple cheek-swab genetic test can identify it.
Growing Resistance
Heartworm strains with reduced susceptibility to macrocyclic lactones have been documented in the United States, originally concentrated in the Lower Mississippi River Valley. More recent surveillance has found genetically resistant isolates well beyond that region, as far north as Michigan. These resistant worms carry specific genetic markers that correlate with their ability to survive standard preventative doses.
This doesn’t mean preventatives have stopped working for most dogs. The vast majority of heartworm infections in dogs on consistent prevention still involve missed doses or lapses in coverage, not true drug resistance. But the spread of resistant strains reinforces why year-round prevention and annual testing matter, even in dogs that have never missed a dose. If a dog on perfect prevention still tests positive, resistance should be considered, and your veterinarian may adjust the approach accordingly.
Risks of Starting Prevention in Infected Dogs
If a dog already has adult heartworms producing microfilariae (tiny larval offspring circulating in the blood), giving a preventative can trigger a serious reaction. The drug kills large numbers of microfilariae at once, and the sudden release of their internal contents can provoke a shock-like response. The mechanism involves massive activation of mast cells, which dump histamine and other inflammatory chemicals into the bloodstream. This can happen through an antibody-mediated allergic pathway or through direct mast cell activation without antibodies involved. Symptoms include collapse, pale gums, rapid breathing, and vomiting.
This is the reason dogs need a heartworm test before starting or restarting preventative medication. A simple blood test detects either the protein produced by adult female worms or the microfilariae themselves. If the test is positive, treatment follows a specific sequence designed to reduce worm burden safely before full prevention begins.