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Climate change is fueling a rise in heartworms among dogs, cats in the Pacific Northwest

FILE: With the Interstate 5 bridge in the background, a person walks a dog on Feb. 13, 2024, in Vancouver, Wash.
Jenny Kane
FILE: With the Interstate 5 bridge in the background, a person walks a dog on Feb. 13, 2024, in Vancouver, Wash.

More dogs and cats are becoming infected with heartworms in Oregon and Washington, as the parasites thrive in warming temperatures brought on by climate change.

Heavy rainfall in the spring combined with warmer summer temperatures have fueled mosquito outbreaks locally.

“When rainfall increases and temperatures rise and weather patterns become more unpredictable, the numbers of mosquitoes also go up, and then there are more mosquitoes available to spread heart disease,” said Dr. Jennifer Rizzo, president of the American Heartworm Society.

Heartworms are parasitic roundworms spread by mosquitoes. They enter an animal’s bloodstream through mosquito bites, growing from microscopic larvae up to 12-inch long adult worms infecting the lungs and heart. They commonly infect dogs, but they can also infect cats, ferrets, coyotes and other wild animals. Human infections are rare.

From 2018 to 2023, heartworm disease among pets increased by 33% in Oregon and 55% in Washington, according to patient data compiled by the Vancouver-based Banfield Pet Hospital. The private veterinary practice operates about 1,000 clinics nationwide, including some locations inside PetSmart stores.

Because the parasite has only been common in warmer southern states until recently, fewer people are accustomed to treating their pets for heartworms in the Pacific Northwest. Rizzo said about 20% of pet owners in the region give their pets heartworm medication, compared to 32% nationally.

“Then a third factor that we think is playing a role is that pets are traveling more than they ever have before,” Rizzo said.

Many states where heartworms are more prevalent also have large numbers of stray animals, and rescue organizations often move those strays to other states, like Oregon and Washington.

“So this is really good for the animals who are saved, but it can mean that heart disease becomes more common in other states,” Rizzo said.

Animals can only get heartworms from mosquito bites, so heartworm disease isn’t directly transmissible between pets. Still, mosquitoes can lay eggs in an animal, which then hatch into more mosquitoes that spread heartworms to other nearby animals.

Dog owners have a wide array heartworm medications to choose from, including monthly medications and annual injections at a veterinary clinic.

Dr. Meaghan Gilhooly, vice president of veterinary affairs at Banfield Pet Hospital, said pet owners should choose an option that they’re most likely to stick with.

“Can you give your dog a pill every month, or would it be easier for you to take your dog into the veterinarian and have them give an injection that lasts a full year?” Gilhooly said.

Either way, Gilhooly said, preventative medication is the most cost effective and least invasive protection against heartworm disease. If a dog becomes infected, they may need to undergo an expensive, painful and potentially life-threatening series of arsenic injections.

Cats are less likely to become infected with heartworms than dogs. Even so, heartworm disease appears to be rising among cats: Banfield’s national data shows a 47% increase in feline heartworm disease. Gilhooly said many pet owners don’t understand that the disease is usually fatal for cats, because there is no cure.

“It’s a really sad thing to watch,” Gilhooly said.

Copyright 2024 Oregon Public Broadcasting

April Ehrlich began freelancing for Jefferson Public Radio in the fall of 2016, and then officially joined the team as its Morning Edition Host and a Jefferson Exchange producer in August 2017.