Heartworm Prevention in Dogs: What You Need to Know

Published June 5, 2026 · 8 min read

Heartworm disease is one of the more serious preventable conditions in dogs, and the gap between prevention and treatment is stark. Prevention costs a few dollars a month. Treatment is a months-long process involving arsenic-based injections, strict exercise restriction, and costs that often run into the thousands. For a disease that is almost entirely preventable, it causes a disproportionate amount of suffering.

Understanding how heartworm spreads and what prevention actually does makes it easier to stay consistent with it, particularly for the monthly preventatives that are easy to forget.

How dogs get heartworm

Heartworm is caused by a parasitic worm called Dirofilaria immitis. It is spread exclusively through mosquito bites. When a mosquito bites an infected animal, it picks up microfilariae (heartworm larvae). Those larvae develop inside the mosquito over roughly two weeks, and when the mosquito bites again, it deposits them into the new host.

In a dog, the larvae migrate through the body over several months, eventually reaching the heart and pulmonary arteries, where they mature into adult worms that can live for 5 to 7 years. A single dog can carry hundreds of worms. As the worm burden grows, damage to the heart, lungs, and blood vessels accumulates, leading to heart failure if untreated.

The disease is present across most of the United States, with higher prevalence in southern states and along river valleys. But the American Heartworm Society recommends year-round prevention regardless of geography, because mosquito populations and climate vary unpredictably, and because prevention windows are easy to let slip.

What prevention actually does

Heartworm preventatives do not repel mosquitoes or prevent infection in the moment. What they do is kill the larval stages that have entered the dog since the last dose. A monthly preventative given on schedule eliminates larvae before they can mature into adults.

This is why timing matters. Miss a dose by a few weeks and larvae from a bite during that window may survive to develop further. Miss multiple months and you may have adult worms developing in your dog with no visible symptoms yet. Most dogs with early to moderate heartworm disease show no signs at all, which is why annual testing is recommended even for dogs on prevention.

Types of prevention

Monthly oral preventatives are the most widely used because they are convenient, cost-effective, and many combination products also cover fleas, ticks, or intestinal parasites. The injectable options are worth considering for dogs in households where monthly schedules are hard to maintain consistently.

If your dog has never been on prevention

A dog should be tested for heartworm before starting prevention. Giving a preventative to a dog with an existing adult worm burden can cause a rapid die-off reaction that ranges from mild illness to severe complications. This is uncommon but serious enough that testing first is standard practice.

If you have adopted a dog with an unknown history, or if your dog has lapsed in prevention for more than a few months, your vet will typically require a heartworm test before restarting. The test is a simple blood draw with results in minutes at most clinics.

Treatment if infection occurs

Treatment for established heartworm disease involves a series of injections of melarsomine (an arsenic-based compound) to kill adult worms, combined with antibiotics to eliminate a bacteria called Wolbachia that lives inside heartworms and contributes to inflammation. Dogs must be kept strictly calm and confined during treatment to prevent the dying worms from causing pulmonary embolism.

The full treatment protocol typically takes four to six months from diagnosis to completion. Costs vary by region and worm burden but commonly run between $1,000 and $3,000 or more. Dogs with advanced disease may have permanent heart and lung damage even after treatment. Prevention at a few dollars a month per dose is not a close comparison.

Staying consistent

Monthly preventatives are among the most commonly missed medications in pet care, because missing one dose produces no immediate visible consequence. The failure mode is subtle: a dog bitten during a lapsed month develops larvae that mature undetected over months.

Picking a fixed date each month (the first of the month works well for most people) and treating it as non-negotiable is the most reliable approach. Setting a recurring reminder on your phone or in a pet health app, and logging when each dose is given, removes the uncertainty about whether this month's dose was given or not.