"Rarely is a case of heartworm detected in Washington. While the
reason is not known exactly, it is likely that this state’s climate does not support an active population of mosquitoes long enough for the parasite to maintain its complex life cycle. There’s also no evidence there is any increase in new cases. Professor William J. Foreyt, a parasitologist at WSU’s College of Veterinary Medicine and a heartworm authority, has studied the subject for more than 30 years.
...
Dr. Foreyt has concluded a multi-year heartworm incidence survey
of harvested, free-ranging coyotes primarily from eastern Washington and the areas where an occasional case is reported in dogs. He examined more than 600 susceptible coyotes after death and found no heartworm.
Typically, Washington dogs with heartworm have relocated to the
state from known heartworm regions. Diagnosis has still been very
uncommon and occurs most often on or around military installations and communities with a migrant population."
Community Practice Newsletter, Summer '09
Washington State University College of Veterinary Medicine
http://www.vetmed.wsu.edu/ClientED/c...y_practice.asp