The math on heartworm prevention is so absurdly lopsided that I'm still surprised when I meet a dog who isn't on it. Prevention costs somewhere in the range of a few dollars per month depending on the product and the dog's size. Treatment for an established heartworm infection — if the dog is even a candidate for treatment — runs into hundreds or thousands of dollars, carries real medical risk, and requires months of strict crate rest. In cats, there's no treatment at all.
And yet, heartworm-positive dogs walk into clinics every single day. The failure is almost always a prevention gap — months missed, products not purchased, or assumptions that the dog "isn't at risk."
How Heartworm Actually Works
The heartworm life cycle depends on mosquitoes as an intermediary host. An infected dog (or other canid) serves as the reservoir: adult heartworms living in the heart and pulmonary arteries produce microscopic larvae called microfilariae that circulate in the bloodstream. A mosquito bites the infected dog, picks up microfilariae, and over the next 10-14 days (in warm enough conditions), those larvae develop inside the mosquito to an infective stage.
When that mosquito bites another dog, it deposits infective larvae into the skin. Over approximately six months, the larvae migrate through body tissues, eventually reaching the pulmonary arteries where they mature into adults. Adult worms can live 5-7 years in dogs. A heavily infected dog may harbor 50, 100, or more adult worms.
The disease process involves both the physical presence of worms obstructing blood flow and the inflammatory response to the worms and the bacteria (Wolbachia) they carry. The result is progressive damage to the pulmonary arteries, lungs, and heart — which is why cardiac damage from heartworm is the ultimate concern.
Why Prevention Works So Well
Monthly heartworm preventives don't prevent mosquito bites or infection. What they do is kill larval heartworms in the tissue-migration stage — before the larvae reach the heart. The drug administered in January is retroactively killing any larvae deposited in December. This backward-looking action is why consistent monthly dosing is critical: a missed month means larvae from that window have progressed further in their migration, and some may escape the drug's reach.
The active ingredients used in heartworm prevention (macrocyclic lactones — ivermectin, milbemycin, moxidectin, selamectin) are effective at very low doses against early-stage larvae. They're remarkably safe in most dogs at prevention doses, though certain breed-specific genetic sensitivities exist (dogs homozygous for the MDR1 gene mutation require adjusted dosing with some flea and tick prevention products, and ivermectin at high doses — not the prevention dose — can be toxic in these dogs).
A consistent monthly dose provides a safety net. Even if you're a few days late, the accumulated larvae haven't progressed far enough to be resistant. But stretching that gap to six or eight weeks starts to push the boundary. I've seen heartworm-positive dogs whose owners swore they "usually" gave the preventive — but "usually" isn't every month.
Heartworm in Cats
Cats get heartworm. That sentence surprises a lot of people.
Cats are atypical hosts — they don't develop the large worm burdens that dogs do. A typical feline heartworm infection involves one to three worms, sometimes just one. But even a single worm in a cat can cause severe respiratory disease (called HARD — heartworm-associated respiratory disease) or sudden death.
There's no approved treatment for heartworm in cats. The drugs used to kill adult worms in dogs are too dangerous for feline patients — the dying worms cause a massive inflammatory and thrombotic response that a cat's smaller pulmonary vasculature can't handle.
Prevention is the only strategy. Monthly preventives are available for cats, and I recommend them for any cat with potential mosquito exposure — which includes indoor cats, since mosquitoes get inside homes. The cost is low, the risk of the disease is real, and there's no plan B if infection occurs.
Prevention Products
Heartworm preventives come in several forms:
Monthly oral chews or tablets — the most common format for dogs. Many combination products include flea prevention, intestinal deworming, or both alongside heartworm coverage.
Monthly topical (spot-on) products — available for both dogs and cats. Some cats tolerate these better than oral medication.
Injectable long-acting prevention — available for dogs in some markets. A single injection provides six to twelve months of heartworm protection, which eliminates the compliance challenge entirely. I've started using this for clients who travel frequently or have difficulty maintaining a monthly schedule.
Your vet selects based on species, weight, what other parasite coverage you need, and your pet's specific health profile. All of these products are prescription-only, which means they require a current veterinary relationship — and that's by design, because heartworm testing should occur before starting or restarting prevention.
Why Testing Matters Before Starting Prevention
If a dog is already heartworm-positive and you start a monthly preventive, the preventive will kill circulating microfilariae but won't eliminate adult worms. The microfilarial die-off can, in rare cases, cause a shock-like reaction. More practically, the owner may assume the dog is protected when in fact adult worms continue causing progressive damage.
The standard protocol: test annually, even for dogs on year-round prevention. No product is 100% effective, and a single missed or vomited dose can open a window. Early detection means earlier treatment and less permanent damage.
Testing is a simple in-clinic blood test that takes about ten minutes for results. I run it at every annual exam as part of a broader parasite prevention for pets approach.
For cats, testing is less straightforward. The standard canine antigen test detects a protein produced by adult female worms, and with the low worm burdens typical in cats (sometimes only male worms, or immature worms), false negatives are common. Antibody testing can identify exposure but doesn't confirm active infection. Diagnosis in cats often involves imaging and clinical suspicion rather than a definitive blood test.
The Year-Round Argument
I hear the seasonal objection regularly: "There are no mosquitoes in winter where I live."
That may be true in your yard. But heartworm transmission requires a period of sustained warm temperatures for larval development inside the mosquito, and those temperature thresholds can be met earlier in spring and later in fall than you'd expect. Microclimates — warmer pockets near buildings, bodies of water, storm drains — can support mosquito activity outside the expected season. And if you miss the cutoff by even a month, you've left a gap.
The veterinary parasitology consensus has moved firmly toward year-round prevention in all regions where heartworm occurs. The cost difference between 8 months and 12 months of prevention is trivial compared to the cost of treatment.
FAQ
Can heartworm prevention cause side effects?
Side effects from heartworm preventives at standard doses are uncommon. Mild GI upset (vomiting, soft stool) can occur with oral products. Dogs with the MDR1 gene mutation may have increased sensitivity to ivermectin-based products at higher doses, though standard prevention doses are safe for most. If you're concerned about breed-specific sensitivities, genetic testing is available.
My dog missed several months of prevention. What now?
Restart the preventive immediately and contact your vet. A heartworm test should be done approximately six months after the gap (it takes that long for larvae acquired during the gap to mature enough to be detectable). Don't wait to test before restarting prevention — get back on schedule now.
Do small dogs need heartworm prevention?
Absolutely. Small dogs are actually at higher risk of severe disease because even a few adult worms can cause significant obstruction in their smaller hearts and blood vessels. I had a 4-kg Yorkshire Terrier with just three adult heartworms who presented with severe right-sided heart failure.
Can I buy heartworm prevention without a prescription?
In most markets, heartworm preventives are prescription-only. This requirement exists because a heartworm test should be performed before starting (or restarting) prevention. Some online sources sell products without requiring proof of a current test, which I'd advise against — testing first protects your dog.
Is heartworm contagious between dogs?
Not directly. Heartworm requires a mosquito as an intermediary. Dog-to-dog transmission without a mosquito vector doesn't occur. But a heartworm-positive dog does serve as a reservoir that can infect local mosquitoes, which then infect other dogs in the area — so an untreated positive dog contributes to community-level risk.