03/11/2025
Heartworm Poses a Rising Threat to Pets in the Inland Northwest
By Matt Mason, DVM
Not long ago, a cat or dog catching heartworm disease in the Inland Northwest was so unlikely I once thought I could go my entire career without ever diagnosing one.
It used to be that unless you were vacationing with your pets in warmer areas where the parasite can thrive, heartworm medication wasn't recommended by most veterinarians in the Pacific Northwest.
Those times are changing because, as annual temperatures in our area rise, so do the number of heartworm cases at clinics in Eastern Washington and North Idaho.
At the very least, whether or not pet owners decide to treat their animals against another added parasite, they should be aware of their options and the rising threat in our region.
Heartworm is a nasty parasite that can grow up to a foot long and is known for infesting and clogging an animal's heart, lungs and associated blood vessels. The infection occurs when mosquitoes spread from an infected dog into the bloodstream of a new host via mosquito bite, but completing the parasite's full life cycle takes a perfect storm.
Temperatures must be above 57 degrees for at least 45 consecutive days, and during at least two of those weeks temperatures must exceed 80 degrees, according to the American Heartworm Society.
More than 1 million animals, mostly in the southern United States, are diagnosed with heartworm disease annually. For the Inland Northwest, what's most concerning isn't just the rising number of cases.
In recent years at WSU's Veterinary Teaching Hospital, several owners of animals that contracted the disease said they had never left the area with their pets, suggesting that Washington's and Idaho's warmer months may be suitable for the deadly worms and the mosquitoes that spread them and help them prosper.
Heartworm cases aren't just observed by local veterinarians and shared via word of mouth; they are well documented every three years by the American Heartworm Society. If trends fall in line with the past two decades, cases will only rise.
Cats and dogs with the disease may cough or wheeze, struggle to breathe, and experience a loss of appetite, vomiting, diarrhea or death.
With different pockets in the Inland Northwest experiencing different weather patterns, owners can consult their primary veterinarian on the threat in their community. If the vets are seeing heartworm disease locally, they may recommend preventatives.
As a pet owner who has only treated my dogs when traveling to warmer destinations, I am considering when the time to regularly treat, and to recommend my clients also do that, is coming.
Don't be surprised if your veterinarian suggests the same.
Matt Mason is a staff veterinarian at Washington State University's College of Veterinary Medicine.
