Heartworm prevention may seem simple: give a preventive, test once a year, and move on.

But heartworm risk is not always that straightforward. Mosquitoes, temperature, missed doses, travel, rescue history, regional risk, and medication resistance can all play a role.

The good news is that heartworm disease is preventable. Here are four key things every dog and cat parent should know.

1. Heartworm spreads through mosquitoes

Heartworm disease is caused by a parasite called Dirofilaria immitis. It spreads through the bite of an infected mosquito, not directly from pet to pet.

Dogs are natural hosts for heartworms, which means the worms can mature and cause serious damage to the heart, lungs, and nearby blood vessels.

Cats can get heartworm disease too, although it often looks different than it does in dogs. Even indoor cats are not completely risk-free because mosquitoes can get inside homes.

2. Why prevention is not just seasonal

Heartworm transmission follows mosquitoes, not just the calendar.

Temperature does matter. Heartworm larvae need sustained warmth inside mosquitoes before they can become infectious. Below about 57°F / 14°C, that development slows or stops.

But that does not mean prevention should be based only on today’s temperature. Mosquitoes can survive in protected areas around homes, garages, foundations, and other warmer spaces. Weather can also change quickly, and pets may travel to areas with higher heartworm risk.

That is why many veterinarians recommend year-round heartworm prevention instead of trying to guess when mosquito season starts and ends.

3. Testing still matters, even when pets are on prevention

Heartworm preventives are highly effective when given correctly, but no prevention plan is perfect.

A dose can be missed. A pet may spit out or vomit medication. A topical product may not absorb as expected. A prescription may lapse. A newly adopted pet may also have an unknown prevention history.

Resistance is also part of the conversation. Researchers have documented some heartworm strains with reduced susceptibility to commonly used preventives, especially in certain regions. This does not mean pet parents should panic or stop prevention. It means consistency, product choice, and testing matter.

For dogs, heartworm testing every 12 months is the standard recommendation, even when they are on prevention. Your veterinarian may recommend testing more often if your dog has missed doses, recently restarted prevention, traveled, or has other risk factors.

Cats are different. Heartworm disease can be harder to detect in cats, so your veterinarian can help determine the right testing and prevention plan based on your cat’s risk.

4. Natural mosquito control is not the same as heartworm prevention

Many pet parents are interested in natural or lower-intervention care. It is understandable to want to reduce unnecessary chemicals and support your pet’s overall health.

But heartworm disease is different from some other parasite concerns. Herbs, supplements, essential oils, walnut extract, and other natural remedies should not be treated as reliable stand-alone heartworm prevention.

Mosquito control can help reduce exposure. Helpful steps may include removing standing water, repairing screens, keeping pets indoors during peak mosquito activity, and asking your veterinarian about safe mosquito-repellent options for dogs.

These steps can support a prevention plan, but fewer mosquito bites is not the same as proven heartworm prevention.

What pet parents can do

TThere is not one single heartworm prevention plan that fits every dog or cat. The best plan depends on your pet’s species, age, health, lifestyle, travel, rescue history, local mosquito risk, and your veterinarian’s guidance.

Ask your vet:

• Is my dog or cat current on heartworm testing?
• What prevention option is best for my pet?
• What should I do if I miss a dose?
• Does my pet’s lifestyle, travel, or rescue history change their risk?
• Should my pet be tested more often?

Heartworm disease can be serious, expensive, and difficult to treat. Prevention is much easier on pets and families than treatment after infection.

At A.R.F., we want every dog and cat to have the chance to live a healthy, protected life. Talk with your veterinarian, stay consistent, and keep heartworm prevention on your pet care calendar.

If you are ready to welcome a new companion, view our adoptable dogs and cats.

Sources and further reading

American Heartworm Society
Companion Animal Parasite Council
American Veterinary Medical Association