Heartworm Is on the Rise in Cats and Dogs

Heartworm is the stuff of horror stories: After a dog or cat is bitten by a mosquito, the parasite — which can grow to be a foot long — invades their heart and causes serious symptoms like coughing and weight loss. If it progresses, it can lead to blindness, convulsions, and even death. But as scary as it sounds, heartworm is extremely preventable. 

But according to a new survey by Banfield Pet Hospital, nearly 40 percent of pet parents don’t believe their dog or cat is at risk of getting heartworm, and 30 percent have taken no preventative measures whatsoever. 

These commonly held beliefs are contrary to the reality of heartworm disease. We spoke to a Banfield Pet Hospital veterinarian to set the record straight and learn how to best keep our pets safe. 

How common is heartworm, really?

Pet parents may feel that their pets don’t have to worry about heartworm, but that’s not the case. Not only is heartworm common, infecting about one in every 200 dogs each year — it’s becoming even more widespread. “According to the American Heartworm Society, more than a million pets in the U.S. have heartworm and cases are on the rise in both heartworm ‘hot spots’ and in locations where heartworm cases were once rare,” says Dr. Meaghan Gilhooly, the Vice President of Veterinary Affairs at Banfield Pet Hospital. “New data from Banfield reinforces these findings. For example, when looking at states with the highest increases in rates of heartworm in the past five years, Oregon (32 percent increase) and Washington (55 percent increase) made the top 10 list, and these areas have historically seen low heartworm rates.”

And the disease is no joke. “One bite from an infected mosquito is all it takes for your pet to become infected with heartworm,” Dr. Gilhooly says. “In unprotected pets, foot-long worms can develop in the heart, lungs, and blood vessels.”

Where is heartworm found?

Twenty-one percent of respondent’s to Banfield’s survey said they believed mosquitos in their state don’t carry the heartworm parasite — but heartworm has been found in all 50 states. “You’ll find heartworms wherever you find mosquitos — the insects transmit heartworms from infected animals to other pets, including cats and dogs,” Dr. Gilhooly says. 

Related article

Yes, Cats Can Get Heartworm. Here’s How to Prevent It

Dogs are the natural target for these worms, but that doesn’t mean your cat isn’t at risk.

Once a pet has contracted heartworm, the parasite multiplies, and it can be spread to other animals from any mosquito that sucks the pet’s blood. “There are different stages of the illness, so many pets infected with heartworms show no signs of the disease in the early stages but are capable of acting as a source of infection for other pets,” Dr. Gilhooly says. “This is why annual testing — as recommended by the American Heartworm Society — is so important.”

How to keep your pet safe from heartworm 

The silver lining is there are plenty of ways to keep pets safe. “These options include monthly oral medications, monthly topical medications, and an injection available to dogs that’s given every six or 12 months by a veterinarian,” Dr. Gilhooly says. “Pet owners should partner with their veterinary teams to determine what’s right for their pet and their lifestyle.”

Summer is known as mosquito season, but the reality is that heartworm risk persists year-round. “A common myth is that heartworm is only a ‘warm weather’ parasite, and that the risk isn’t there in fall or winter,” Dr. Gilhooly says. “Banfield’s data found rates of heartworm diagnoses in pets seen at Banfield in 2023 stayed the same across all four seasons, with thousands of pets diagnosed in spring, summer, fall, and winter.”

She recommends that pet parents use year-round prevention against heartworm — which over half of the survey respondents admitted they don’t do. 

Dr. Gilhooly notes that heartworm is a particular risk for cats; once cats are infected, there is no treatment to remove the heartworm. “In the past five years, Banfield saw a 47 percent increase in felines diagnosed with heartworms,” she says. “Even if your cat is primarily an indoor pet, they can become infected if a mosquito finds its way into the house. Year-round heartworm prevention is especially important to help keep cats protected from this deadly parasite.”