Heartworm in the United States is a worm, Dirofilaria immitis, that is transmitted in a larval stage (microfilaria) by mosquitoes.
The mosquito population in the Pacific Northwest has been heavily researched. Per OSU, mosquito surveillance data have found no heartworm within the mosquito species that could carry it in the Willamette Valley. This means heartworm is not endemic to the Pacific Northwest.
There have been positive heartworm cases in Oregon, Washington, and Idaho, however these pets have traveled to areas where heartworm is present in mosquito populations, such as the South and especially the Southeast.
“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.” (WSU). Then, once the mosquito bites a dog or other 4-legged mammal, the microfilaria can be transmitted into that animal. Many mammals are not able to accept the larval stage for many reasons, including humans. Dogs and cats are susceptible, cats less so.
Because we typically have a few cold snaps during the larval life stage for heartworm in the Pacific Northwest, heartworms cannot survive in the local mosquito populations.
With climate change, it is crucial to continue testing our pets at least annually. We want to be vigilant that our mosquito population does not carry heartworm with changing weather patterns and warming temperatures.
It does take 5 to 7 mos for an adult heartworm to develop within the heart of a dog after being bitten by an infected mosquito. Since it takes this long for adults to develop, the preventative (ivermectin) that kills the microfilaria, or larval form, can be given once every 3 mos and completely prevent the disease.
Because our region currently does not promote the heartworm larvae to fully mature in the mosquito and be transmitted, we choose to test annually and avoid giving ivermectin if at all possible. For extremely vigilant owners, testing biannually (twice per year) is more prudent.
For those overly concerned, or traveling, we suggest giving your pet a preventative once every 3 mos.
Please let us know if you would like to schedule a HW test (tech appt, about 10 mins or less) or if you have any questions.
We are having a special on HW tests for the rest of the summer (through Sep) - so consider having your pet tested (especially if you traveled in the last year)!
https://hospital.vetmed.wsu.edu/2023/06/16/should-pet-owners-in-the-pacific-northwest-worry-about-heartworms/