More than 200 types of mosquitoes live in the United States, and a dozen spread germs that make us sick. West Nile, dengue, malaria and Zika viruses get lots of attention when they strike people. But the most common mosquito-borne disease in America attacks our pets. It’s heartworm, found mostly in dogs but occasionally in cats, ferrets or other mammals.

This quiet parasite can cause lifelong medical problems and even be fatal if left untreated. Though it can be cured in dogs, there’s no cure for cats. The key remains year-round prevention, rather than response to a disease that enters the body unnoticed.

How Do Pets Get Heartworm?

Transmission has all the characteristics of a slowed-down horror film. A mosquito bites an infected animal, ingesting baby heartworms called microfilaria. The worms incubate in the mosquito for about two weeks, developing into an infectious form.

When that mosquito bites another animal, it deposits the infected larvae. They grow in the new host for about six months, becoming mature adult heartworms that can live five to seven years in dogs and two to three years in cats. Infected animals become carriers.

Once heartworms mature, says the American Heartworm Society, they travel to the heart, lungs and arteries, where they mate and produce offspring. The worms damage and scar vital organs; if left untreated, complications can lead to heart failure, permanent lung damage and death.

Prevention Takes Careful Effort

Mosquito season in the Carolinas has run from April through October, but it’s getting longer; as temperatures rise, milder winters kill fewer pests. You can cut down on mosquitoes outside by removing standing water, cleaning up debris (some lay eggs in decaying logs and leaf piles), keeping gutters clean and planting natural repellents such as citronella, rosemary, catnip or peppermint. Sprinkling coffee grounds near standing water helps, as do some scented candles.

Owners have many reasons never to let cats outdoors – or, at least, to keep them on a well-screened porch — and avoiding the infection is one of the best. Of course, dogs have to take regular walks, and mosquitoes occasionally get inside a home.

So, you’ll want ongoing protection: topical solutions applied to the pet’s coat, monthly or tri-monthly oral medications, maybe injections given by your veterinarian. All types of prevention don’t suit all pets, so discuss options with your primary care provider. Stick with veterinary-recommended medication; some over-the-counter repellents or sprays harm pets.

How To Detect and Treat It

All breeds of cat and dog are susceptible. Nothing predisposes a pet to get heartworm, though other underlying health problems can make treatment more difficult. You’ll seldom see clinical signs early in the disease, so yearly tests (especially for dogs) remain essential. Cats can be tested, but because they typically get fewer worms, those tests may not be as accurate.

Symptoms of infection include difficulty breathing and asthma-like signs, especially in cats. You may notice intolerance for exercise, lack of appetite or weight loss. In more serious cases, pets can have fainting spells or seizure-like events, can accumulate fluid unexpectedly or may even pass away suddenly.

If your pet tests positive, first confirm the diagnosis. Then your vet will follow American Heartworm Society guidelines for treatment over several months. Treatments consist of anti-inflammatory medications, antibiotics, heartworm prevention and three injections given at set periods of time. Dogs should avoid strenuous exercise such as running, jumping and rough play during treatment, especially in the early months.

Depending on the severity of the disease, vets may introduce other medicines. Cats can’t be treated in the same way – with them, care tends to be supportive – but in more severe cases, other therapies can be tried on a case-by-case basis.

CARE Is the animal equivalent of a medical center for humans. It offers all-purpose treatment for pets, giving both 24/7/365 emergency care and board-certified specialty care based on referrals from primary veterinarians. Take a visual tour of the practice at

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