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Something Scarier Than Halloween? Ok, Heartworm It Is.

Cats face extra challenges when it comes to this parasite. A veterinarian explains.

by Jodi Helmer
October 25, 2021
Cat walking outside on the grass
everydoghasastory / Adobe Stock

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Bite. Swat. Repeat. Mosquitos are a literal pain, buzzing around in search of a blood meal and devoting their short lives to feeding off their unsuspecting victims. The little buzzers aren’t just a nuisance; mosquitoes can transmit heartworm disease to our pets. In a year with above average rainfall, mosquitos are everywhere and your cat — especially if an outdoor cat — is at an increased risk. While heartworm is much more common in dogs, affecting more than one million pups every year, compared with fewer than 200,000 cats (according to the American Heartworm Society), cats may be less susceptible, but they are not immune.

What is heartworm disease?

As the name suggests, heartworm disease is diagnosed when worms take up residence in the heart, lungs and blood vessels. Mosquitos transmit the disease, picking up microfilaria (baby worms) during their blood feast and depositing those larvae when they bite the next animal.

Not only is it gross to think of worms slithering through your cat’s heart, the parasitic worms are a major health hazard. Heartworms can cause symptoms ranging from coughing and wheezing to vomiting, loss of appetite, and weight loss. In severe cases, cats may also experience seizures, lack of coordination or fluid buildup in their stomachs, while scarily enough, some cats exhibit no symptoms at all.

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Are cats at risk?

In short, yes. Although dogs are a natural target for heartworm disease because the heartworms can complete their entire lifecycle in a single host, cats are still at risk — but the disease acts a little differently. Cats tend to have smaller and fewer worms, compared to dogs. Plus, heartworms are less likely to reproduce in cats and tend not to live as long, according to Dr. Julie Levy, DVM, PhD, DACVIM, DABVP, the Fran Marino Endowed Distinguished Professor of Shelter Medicine Education at the University of Florida. 

Fewer than 200,000 cats are diagnosed with heartworm disease each year. Among those that are infected, the parasite can take a serious toll. Many cats are asymptomatic and, in some cases, the first symptom is sudden death.

How will I know if my cat has heartworm disease?

Blood tests are most commonly used to diagnose heartworm disease in cats. Your vet may recommend a heartworm antibody test to see if your cat has been exposed to heartworms, an antigen test to look for the presence of female heartworms, or a blood test to test for heartworm larvae, though Dr. Levy warns that the tests are imperfect.

“We get a lot of false negative results in cats,” she says. “Heartworm tests only detect adult female worms or juvenile worms…and don’t detect male worms. Many cats have just one to three heartworms so there’s a good chance those are just male worms [and won’t be detected on a heartworm test].”

How can I protect my cat?

You know what they say: Prevention. Prevention. Prevention. Although heartworm might be difficult to treat, it’s easy to prevent. Monthly oral or topical medications are very effective for fending off heartworm disease in cats — but few pet owners use them.

“We’ve studied this and found that veterinarians are less likely to recommend it, even in areas where the prevalence is high in dogs and cats are at higher risk,” Dr. Levy says. “Cat owners are less likely to buy it, especially if they have indoor cats because the perception is that indoor cats aren’t at risk.” Indoor cats need preventives, too. Dr. Levy estimates that one-third of cats infected with heartworm disease are indoor only.

The same breed of mosquitoes known to transmit heartworm disease in cats, also prefer to live indoors, putting your cat at risk even if she never leaves the house. Mosquitos may seem more prevalent in the summer months but they are a year round risk. Before you decline heartworm prevention for your cat, Dr. Levy cautions, “heartworm is a constant danger.” Bonus: Many of the heartworm preventives also protect against fleas, ticks, and internal parasites like hookworm, roundworm, and tapeworm.

Not only is heartworm disease difficult to diagnose but cats that are infected with the parasite can be complicated to treat. For starters, there are no drugs approved to treat the heartworm disease in cats and attempting to kill the worms is risky. “In dogs, there are medications to kill heartworms,” Dr. Levy explains. “In cats, the death of a single worm can cause a fatal reaction so we don’t want to hasten the death of worms.” Instead of using medication to kill the heartworms, veterinarians focus on controlling heartworm symptoms in cats with steroid medications and bronchodilators to reduce inflammation and make it easier to breathe.

Jodi Helmer

Jodi Helmer is a North Carolina-based freelance writer who shares her home with an embarrassing number of rescue dogs and relies on four feral cats to patrol the barn. When she isn’t refilling food and water dishes, Jodi writes about animals for Scientific American, Sierra, WebMD, AKC Family Dog, Living the Country Life, and Out Here.