Heartworm in the Pacific Northwest

Our laboratory has noticed an interesting trend this year. We have had several canine cases in which the patient had evidence of heartworm disease, either microfilaria found on a blood film, or a few adult heartworms found in the pulmonary vessels on necropsy. These patients were presented for conditions unrelated to their active heartworm disease, but this was still an important comorbidity.

In our area of the country, the general veterinary consensus has been that heartworm disease is either not present or is very uncommon. As we are finding with changes in climate, our growing Seattle population with families bringing dogs from other areas of the country, and the common practice of rescues distributing animals to areas of higher adoption rates, infectious diseases are becoming more and more common in this region. 

Heartworm disease is considered endemic in all 50 states, and the American Heartworm Society recently released a map showing the distribution of cases in 2016. The map can be found here: https://www.heartwormsociety.org/newsroom/in-the-news/347-ahs-announces-findings-of-new-heartworm-incidence-survey. It is interesting to note that Alaska is missing from this map, but given that cases have been noted in the past, the lack of recorded cases for 2016 may be due to lack of reliable data and the likely relatively low number of incidences in this state.

We have had veterinarians ask if this is a reportable disease in our state and also where do we find the list of reportable diseases.  The Washington State Department of Health provides the list of reportable conditions in veterinary species and there are several categories based on severity. The list can be found here: https://agr.wa.gov/FoodAnimal/AnimalHealth/ReportableDiseases.aspx

Heartworm disease (dirofilariasis) is not a reportable disease in the state of Washington, but it is on the list of monitored conditions. This means that the state veterinarian’s office keeps record of cases that are reported to their office, but a veterinarian is not legally required to report the case. The state veterinarian’s office may then have the ability to post a monthly report of these conditions that is accessible by the public, but in reality, this varies by state and a variety of internal and external factors.

Testing and prevention are other two important topics to understand with any infectious disease. The standard in house screening test is an ELISA and results are often available in minutes. This test can vary in the sensitivity and specificity depending on the manufacturer, storage, etc, but both are generally quite high in cases of significant adult worm burden. Test instructions insert should be strictly followed to ensure minimization of errors. The ELISA test traditionally detects the female adult heartworm uterine antigen. So this means that the patient must have a living adult female heartworm in order to be positive on the screening test. False negatives can result from patient’s that only are infected with male adult heartworms, have a low worm burden (usually cats and small dogs), or with infections with immature worms. These tests are not specific to Dirofilaria immitus and also detect antigen from the other filariad, Acanthocheilonema reconditum. Other diagnostics include microfilaria testing (direct, buffy coat evaluation, or Knott’s test), antibody screening, thoracic radiographs, and echocardiography and are often used in staging cases of positive ELISA screening or in cases with negative ELISA screening but heartworm disease is still strongly suspected. Administration of a topical or oral preventative is necessary for proper prevention of heartworm infections and the efficacy can vary by product used. For a much more detailed explanation of testing and prevention, please refer to the American Heartworm Society’s website and official guidelines: https://www.heartwormsociety.org/veterinary-resources/american-heartworm-society-guidelines.

Also, we cannot forget that cats are also susceptible to this serious condition and that does include pets that are indoors only! All it takes is one carrier mosquito to slip inside the door or through the small gaps in screens for this vector borne disease to reach those pets. There have also been heartworm disease cases in Western Washington affecting indoor only cats, so it is important to provide prevention to our feline friends as well!

All in all, the moral of the story is to not completely discount heartworm disease and other infectious diseases in our ever changing world. As veterinarians, we are on the front lines of public health and this is most important with general practitioners. Though it may seem that heartworm disease is rare here, recommendations of prevention and routine testing according to the American Heartworm Society should be followed. Should a veterinarian diagnose a case of heartworm disease, it is of importance to take 5-10 minutes and report it to the state veterinarian’s office, regardless of the patient’s travel history. This public sharing of knowledge helps provide better epidemiologic monitoring and awareness to fellow veterinarians and pet owners. Even this complex vector-borne parasitic disease can be considered highly contagious among the right population and Seattle has one of the fastest growing human and pet populations in the country. It is important to remain vigilant for other communicable conditions as well, but that is another post for another day!

The pathologists at SpecialtyVETPATH are happy to answer questions from veterinarians regarding this or other infectious conditions and can help direct you to useful resources. If you are a pet owner, please contact your veterinarian and refer to www.heartwormsociety.org for specific recommendations on prevention for your furry family member.