Heartworm Prevention for Dogs & Cats: Options & Costs

This information is for educational purposes only. Heartworm prevention requires a prescription and a negative heartworm test before starting. Never give heartworm prevention without your veterinarian's guidance.

Heartworm disease is spread by mosquitoes and can affect both dogs and cats, even pets that live mostly indoors. In dogs, adult worms can live in the heart and pulmonary arteries for years and may cause coughing, exercise intolerance, weight loss, lung damage, heart failure, or life-threatening complications. In cats, infections are usually smaller, but even a few worms can trigger serious lung inflammation, vomiting, breathing trouble, or sudden collapse.

Prevention matters because treatment is much harder than prevention. Dogs that become infected often need months of restricted activity, repeated testing, multiple medications, and a series of injections. Cats do not have an approved drug that safely kills adult heartworms, so care is usually supportive and focused on monitoring and managing complications. That makes prevention one of the most important parasite medications your vet may recommend.

Most vets advise year-round prevention, not seasonal use. Mosquito exposure can happen outside the traditional summer months, and missed doses are a common reason pets become infected. Dogs usually need routine heartworm testing before starting prevention and then regular follow-up testing, while cats may need risk-based testing depending on the product, region, and your vet's plan.

Heartworm preventives do not create an invisible shield that stops mosquitoes from biting. Instead, they kill immature heartworm larvae that entered the body during the previous weeks after a mosquito bite. That is why timing matters so much. If doses are late or skipped, larvae may continue developing into adult worms, and adult infections are much harder to manage.

Most heartworm preventives used in dogs and cats belong to a drug class called macrocyclic lactones. Depending on the product, the active ingredient may be ivermectin, milbemycin oxime, selamectin, or moxidectin. Some products only prevent heartworm disease, while others also treat or control roundworms, hookworms, fleas, mites, or ticks.

Dogs older than about 7 months generally need a negative heartworm test before starting or restarting prevention because it takes months after infection for standard tests to turn positive. Puppies under 7 months can often start prevention earlier, then be tested later on your vet's schedule. Cats are different: diagnosis is more complicated, and prevention is emphasized because there is no approved adult heartworm treatment for them.

See your vet immediately if your pet has coughing, breathing changes, exercise intolerance, fainting, weakness, or unexplained vomiting and heartworm exposure is possible. In dogs, confirmed heartworm disease usually leads to a staged treatment plan that may include bloodwork, chest imaging, doxycycline, a heartworm preventive, exercise restriction, and melarsomine injections to kill adult worms. The commonly recommended protocol uses three injections because it is effective across disease stages and helps lower complications when paired with strict rest.

Treatment in dogs is not quick. Many dogs need activity restriction for weeks to months because dying worms can cause inflammation and dangerous clots in the lungs. Follow-up testing is also needed to confirm the infection has cleared. In severe cases, hospitalization, oxygen support, or specialty care may be needed.

Cats are much more challenging. There is no approved medication in the United States to kill adult heartworms in cats safely. If a cat is infected, care often focuses on monitoring, managing inflammation or respiratory signs, and supporting the cat while the worms die naturally over time. Some cats remain stable, while others can develop sudden, severe respiratory distress. That is why prevention is especially important for cats, including indoor cats in mosquito-prone areas.

Most heartworm preventives are well tolerated when used exactly as prescribed for the correct species and weight range. Mild side effects can include vomiting, diarrhea, decreased appetite, drooling, or temporary lethargy. Topical products may occasionally cause skin irritation or greasy hair at the application site.

More serious reactions are uncommon but can happen. Dogs with certain MDR1 gene mutations may be more sensitive to some parasite medications, especially if products are overdosed or combined inappropriately. Injectable moxidectin products for dogs can rarely cause severe allergic-type reactions or other serious adverse events, so your vet will weigh the pros and cons before using them.

Never use a dog product on a cat or a cat product on a dog unless your vet specifically instructs you to do so. Also tell your vet if your pet has had a previous medication reaction, is underweight, is pregnant, has neurologic disease, or may already be heartworm-positive. If your pet develops facial swelling, collapse, trouble breathing, repeated vomiting, or severe weakness after a dose, contact your vet right away.