Heartworm Disease: Prevention, Testing & Why Year-Round Protection Matters

Heartworm disease is caused by Dirofilaria immitis. Mosquitoes pick up immature heartworms when they bite an infected animal, then pass those larvae to another pet during a later bite. The larvae mature over months, which is why a newly infected dog may not test positive right away.

Because mosquitoes are the carrier, heartworm risk is not limited to one region or one season. Climate patterns, travel, wildlife reservoirs, and indoor mosquito exposure all help explain why your vet may recommend prevention even for pets that rarely go outside.

Year-round prevention helps close the gaps that happen with seasonal dosing. If one monthly dose is missed or delayed, larvae picked up earlier may continue developing. Keeping prevention consistent all 12 months also improves routine and makes it easier for pet parents to stay on schedule.

For cats, prevention is especially important because diagnosis can be difficult and there is no approved medication that kills adult heartworms the way canine treatment does. Even one or two worms can cause significant disease in a cat.

Dogs should be tested regularly, even if they are receiving prevention. The American Heartworm Society recommends annual testing. Puppies younger than 7 months can usually start prevention without a test, then be tested about 6 months later, again 6 months after that, and yearly after that.

Dogs older than 7 months who are starting prevention for the first time, or restarting after a lapse, generally need a test before starting or restarting and then follow-up testing based on your vet's plan. In dogs, testing commonly includes an antigen test and may also include a microfilaria test.

Heartworm testing in cats is more complicated than in dogs. Cats are less likely to carry many adult worms, so a single test may miss infection. Your vet may recommend a combination of antigen testing, antibody testing, chest X-rays, or ultrasound depending on symptoms and risk.

Cats should be tested before starting prevention when appropriate and rechecked based on exposure risk, symptoms, and your vet's judgment. Because feline heartworm can be hard to confirm and hard to treat, prevention is the most practical strategy for most households.

Many infected pets have few early signs. In dogs, signs can include cough, reduced stamina, exercise intolerance, weight loss, and in more advanced cases breathing trouble, a swollen belly, or collapse. In cats, signs may look like asthma or vomiting, with coughing, fast breathing, open-mouth breathing, low appetite, or sudden collapse.

See your vet immediately if your pet has labored breathing, fainting, weakness, or sudden collapse. Those signs can be emergencies and should not wait for a routine appointment.

Heartworm prevention comes in several forms, including monthly oral products, monthly topicals, and longer-acting injections for some dogs. Many products also cover intestinal parasites, fleas, or ticks, which can make prevention more efficient for some families.

A practical 2025-2026 US cost range is about $6 to $18 per month for many monthly heartworm preventives, with some combination products or larger dog sizes running closer to $20 to $35 per month. Long-acting injectable prevention for dogs may cost more upfront but can help families who struggle with monthly dosing. Your vet can help match the option to your pet's age, species, health status, and household routine.

A positive test does not mean panic, but it does mean your dog needs prompt veterinary follow-up. Your vet may recommend confirmatory testing, chest X-rays, bloodwork, and a treatment plan that often includes exercise restriction, preventive medication, and staged therapy. Many dogs are treated with a multi-step protocol that includes melarsomine injections, but the exact plan depends on disease stage and overall health.

Treatment is usually much more involved than prevention. Total canine treatment costs often range from about $600 to over $3,000, and some cases cost more when hospitalization, imaging, complications, or repeat testing are needed. During treatment, strict activity restriction is a major part of reducing risk.

Cats are different. There is no approved adulticide treatment for feline heartworm infection, so care often focuses on monitoring, managing inflammation or respiratory signs, and responding quickly if symptoms worsen. Some cats may clear infection over time, but the disease can still be severe or fatal.

That is one reason many vets strongly recommend year-round prevention for all cats, including indoor cats. A mosquito only needs one opportunity.