Is Your Dog At Risk For Heartworm?

One of the advantages of our temperate Seattle weather is that it does not foster the development of particular parasites that are endemic to the warmer climates. One of those parasites is Dirofilaria immitis otherwise known as heartworm.

Heartworm is a parasite that is transmitted through the bite of a mosquito. Up until a few years ago, we had no concerns about this parasite threatening pets in the Pacific Northwest. However, we are now seeing the incidence of heartworm creeping northward through the Willamette Valley of Oregon and it could some day be a threat in Eastern Washington.

When a mosquito carrying the heartworm parasite bites an animal and transmits the infection, the larvae develop and migrate through the host’s circulatory system over a period of several months to become sexually mature adult worms. These adults will eventually end up in the heart, lungs and associated blood vessels.

After reproduction, they release their offspring, (microfilaria), into the blood stream.  The microfilaria can then be transmitted back to another mosquito through a blood meal. Once ingested by the mosquito, the immature parasite has to develop within the insect’s abdomen to become infective. This specific part of the life cycle is climate dependent and requires a minimum ambient temperature of 57° F to continue. The warmer the climate, the more efficient the maturation process is.

Microfilaria can be detected in the blood about six to seven months after the infective larvae from the mosquito enter the dog. Male heartworms (four to six inches in length) and females (10-12 inches) fully mature about one year after infection, and their life span in dogs appears to average up to five to seven years. Detection of Heartworm disease can be accomplished through numerous means but the most common method of diagnosis is through a simple blood test that identifies the adult form of the parasite.

Most dogs with heartworm disease show no clinical signs. As the disease progresses, the most common early signs are coughing and respiratory changes. The severity of the clinical signs usually corresponds to the number of adult heartworms present, the timeline of the infection, and the activity of the dog.

Dogs with higher numbers of worms are generally found to have more severe heart and lung disease due to inflammation that occurs in and around the arteries of the lungs. Later, the heart may enlarge and weaken due to an increased workload and congestive heart failure may occur. This can manifest further in more severe respiratory changes, weight loss, collapse, abdominal swelling (ascites) and exercise intolerance. 

Heartworm disease is treatable; however treatment is not always successful. And depending upon the worm burden it can prove to be deadly.Occasionally, a dog with a large number of heartworms may not only have worms in the heart, but also in the caudal vena cava (the large primary vein running from the body back to the heart).  If these worms are not removed surgically, this syndrome causes sudden collapse and death within two to three days.

Prevention is the key and there are many options available – most of which are a monthly treatment. All treatments are prescription medications and a discussion with your veterinarian is important to determine your pet’s level of exposure and the timing of treatment.

Cats are also susceptible to heartworm infection and although their worm burden is usually lower, it can prove quite deadly and there is no real safe treatment for infected felines. Prevention is the absolute key with cats and it is recommended to treat indoor and outdoor cats if you live in an endemic area.



Dr. Mortimer earned his degree from Iowa State University. He received a B.A. in Biology from Baylor University, and completed an internship in Small Animal Emergency Medicine and Surgery in Rochester, New York. His interests include internal medicine and surgery, including minimally invasive surgery (endoscopy and laparoscopy).

“Above all, I have a strong interest in people and animals. The relationship between my patients and their owners is the reason I am in this profession.”

When away from the practice, he spends most of his time having fun with his family (his wife Kristie, three children, three cats and one “Labrador non-retriever”. Personal interests include fly-fishing, hiking, gardening and travel.

Seattle Veterinary Associates was founded in 1971 by Dr. Sanford Olson, Dr. Donald Canfield, and Dr. Stephen Jones. In 1971, the original practice opened its doors at the current location of Queen Anne Animal Clinic. Coupled with our considerate expansion and the advancement of our medical capabilities at all our hospitals, we are proud to be providing the highest quality veterinary care to pets and their families. SVA is comprised of four locations: Queen Anne Animal Clinic, Green Lake Animal Hospital, Ravenna Animal Hospital, and Northwest Veterinary Hospital.

With respect to the State of Washington, we currently do not see an incidence rate that justifies routine use of preventatives. However, if you are traveling out of state or live in the southern part of the state a conversation with your veterinarian is strongly advised.

For more information on Heartworm, please visit the Companion Animal Parasite Council website  or the American Heartworm Society’s website.


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