Should pet owners in the Pacific Northwest worry about heartworms?

More than a million pets in the United States are estimated to be infected with a deadly parasite that is spread by mosquitoes and can grow up to a foot long as it infests and clogs an animal’s heart, lungs and associated blood vessels.

While historically only an issue for pet owners in the South, Southeast, Midwest and other warm-weather regions, heartworms are now being reported in areas like Idaho and Washington where the disease had previously not been found, according to the American Heartworm Society’s recently released 2022 Heartworm Incidence Map.

Infected animals develop heartworm disease, which can lead to severe lung disease, heart failure and damage to other organs in the body if not treated. Fortunately, there are numerous preventatives available through a prescription from a veterinarian, but are they necessary for pets living in areas like the Pacific Northwest where cases are rare? 

Dr. Jessica Bell, a community practice veterinarian at Washington State University’s Veterinary Teaching Hospital, recently helped to address some of the most common questions posed by pet owners in the Pacific Northwest.

How common are heartworm cases in the Pacific Northwest?

A handful of cases have been seen at WSU’s Veterinary Teaching Hospital, however, infected animals had traveled outside of the region and to areas where heartworms are known to be present.

Dr. Bell said that is thanks to the lifecycle of the parasite. For a heartworm to complete a critical portion of its larval life stage that takes place in mosquitos, temperatures must remain above 57 degrees for at least 45 days straight and at least two weeks of temperatures must be over 80, according to the American Heartworm Society.

“Fortunately, here in the Pacific Northwest, we get a couple of cold snaps during mosquito season and the heartworm larvae can’t complete their life cycle,” Bell said.

As the effects of climate change are felt more in the region, heartworms could become more common.

“It’s a disease we need to be aware of, but in our area, pet owners should have a discussion with their veterinarian about whether preventative is necessary,” Bell said. “We don’t want to over-medicate our dogs and cats if we don’t have to, but if you travel with your pet outside the Inland Northwest, there aren’t very many pockets of the U.S. that don’t have heartworms, so your pets should be on preventatives.”

Should all dogs be placed on heartworm preventatives?

If you live in an area in which the heartworm parasite is endemic, your pet should be given a preventative year-round to ensure it is protected. Pet owners in areas where heartworms are not endemic should talk with their veterinarian to determine the best options.

A dog should also be on a preventative year-round if it is traveling to areas where heartworms are present. At a minimum, Bell said, dogs that have traveled to areas where heartworms are endemic should remain on preventatives for six months post-exposure to ensure any larvae are killed.

“It is best not to start and stop with heartworm preventatives. Once they are off preventatives, we must test them again for heartworms before resuming preventatives,” Bell said. “It’s financially and medically best if they just stay on year-round.”

Does a dog need to be tested for heartworms before taking preventatives?

While puppies younger than 6 months can be started on preventatives without being tested, all adult dogs being prescribed preventatives must be tested annually. Preventatives target and kill the parasite in its larval stages, however, adults are not affected.

Blood tests are used to detect heartworms, but tests will not accurately detect infection until 5-6 months after a dog was bitten by an infected mosquito.

What are the symptoms of heartworms in dogs?

The severity of the symptoms of heartworm disease can vary based on many factors, including how many parasites are living in the dog, how long the animal has been infected, and its activity level. Common symptoms include coughing, trouble breathing and exercise intolerance.

Can heartworms in dogs be treated?

The best form of treatment for heartworms is prevention, which is why it is critical to ensure your pet takes a preventative according to the product’s label. Treatment is expensive and difficult on dogs. Animals can have serious complications, such as life-threatening blood clots in the lungs. Treatment involves multiple visits to the veterinarian, blood tests, X-rays, hospitalization and a series of injections.        

What should I do if I have additional questions or concerns about heartworms?

You should consult with your veterinarian about any questions regarding heartworms. Veterinarians in the Veterinary Teaching Hospital’s community practice service can also discuss whether preventatives are right for your pet or address any other questions you have during an appointment. Schedule an appointment by calling 509-335-0711.