Canine heartworm cases in Washington continued their upward trajectory in 2020 according to data from the Companion Animal Parasite Council (CAPC).
The number of heartworm cases in Washington climbed from 461 in 2019 to 733 in 2020, an increase of 59%. Since 2014, the number of cases increased from 108 to 733, an increase of 579%.
Seattle DogSpot obtained the data from CAPC’s Parasite Prevalence Maps.
The maps show that the number of cases for various parasites Washington counties reported to CAPC for every year from 2014-2019. Here are some of the highlights regarding canine heartworm:
- Canine heartworm cases in Snohomish, Pierce, and Skagit Counties more than tripled from 2019 to 2020.
- The number of heartworm cases in Snohomish County increased from 5 in 2014 to 305 last year, an increase of 2020%.
- In King County, heartworm cases reported in 2020 (305) were almost 9 times higher than the number reported in 2014 (36).
- Only 3 cases of heartworm were reported in Spokane County in 2014. Last year, 41 cases were reported, an increase of 1267%.
Obviously, the number of cases in some counties like Spokane is small, but they still indicate a significant upward trend reflected in many parts of the state.
A chart showing the changes in heartworm cases for all Washington counties from 2014-2019 is at the end of this post
Canine Heartworm Can Be Fatal
Mosquitos transmit the heartworm parasite by biting an infected dog and ingesting its blood which contains microscopic worms called microfilaria.
Once inside the mosquito, the worms change into larvae after about 2 weeks. The mosquito transmits the larvae when it bites an uninfected dog and leaves them on its skin.
The larvae enter the dog’s bloodstream through the bite and migrate through its circulatory system. About 6 months later they mature into adult worms.
These adults will eventually end up in a the dog’s heart, lungs and associated blood vessels where they can live 5-7 years.
A dog may not show any signs of heartworm in the early stages of the disease. Eventually, it will develop one or more of these symptoms:
- a mild persistent cough
- reluctance to exercise
- fatigue
- decreased appetite
- weight loss
Left untreated, heartworms can multiply to the point where they cause inflammation in a dog’s arteries and around its lungs.
The inflammation is more severe in dogs with higher numbers of heartworms.
As heartworm progresses, excess fluid in a dog’s abdomen can cause heart failure. It can also cause sudden blockages of blood flow within the heart leading to a life-threatening form of cardiovascular collapse called caval syndrome.
Heartworm can be fatal if it isn’t identified and treated in its early stages.
Why Have Canine Heartworm Cases in Washington Increased So Significantly?
When we got our first dog in 2002, our vet told us we didn’t need to give him heartworm prevention medication because so few cases were ever reported in Washington.
But in 2020 she told me that due to the increase in heartworm cases in Washington she will most likely start recommending heartworm preventative treatments for all the dogs in her practice.
In 2017 Dr. Brian Joseph, the State Veterinarian at the Washington State Department of Agriculture, told me the factors he believed caused heartworm cases to rise so sharply in Washington over the last few years:
- climate change
- lack of prevention
- rescue groups that bring heartworm positive dogs in that state with inaccurate, fake or no certificates of veterinary inspection (health certificates)
Dr. Joseph also told me that at a recent United States Animal Health Association meeting, people from the US Department of Agriculture said that “there may be as many as one million dogs brought into the United States every year from foreign countries, but as few as 10,000 may have appropriate health documentation.“
That means approximately 99% of the dogs brought into the United States from other countries don’t have proper health documents.
This doesn’t mean all dogs brought to the United States from other countries have heartworms or other parasites. Still, you can’t ignore the fact that the increasing number of heartworm cases in Washington have occurred as more rescues import dogs
Climate Change and Lack of Prevention
Mosquitos need a warm climate to reproduce. The warmer the weather, the faster they propagate.
In an interview with KUOW in 2014, Washington State University entomologist Sharon Collman said, “In hot weather, it can take a mosquito egg four to five days to mature to adulthood. In cool weather, it takes two to three weeks.”
Data from the climate.gov shows that temperatures in Washington in 2020 were above the average temperature from 1981-2020.
And according to the NOAA National Centers for Environmental Information, the average annual temperatures in Washington “have risen approximately 1.5°F and temperatures over the past three decades have been warmer than any other historical period” since the beginning of the 20th century.
Based on this information, it’s reasonable to assume that above normal average temperatures in Washington for the last few decades created a more favorable environment for mosquitos to reproduce quickly.
Hot weather = More mosquitos = Greater Potential for Heartworm to Thrive
I think the explanation is straightforward regarding the lack of prevention.
For years, Washington veterinarians told their clients they didn’t need to use a preventative because heartworm was so rare here.
Veterinarians embedded this mantra into the minds of Washington dog owners for years; as a result, we checked heartworm off our list of potential threats to our dogs’ health and didn’t worry about taking steps to prevent it.
Some Dog Rescues Bring Heartworm Positive Dogs to Washington
In the last 10-15 years, dog rescues, municipal shelters, and animal transporters created a massive network to move dogs from states with overcrowded, high kill shelters to areas like the Pacific Northwest “where euthanasia rates are lower and pets are in greater demand.”
Rescuers built this infrastructure in 2005 “when Hurricane Katrina left more than 250,000 pets stranded.” Their effort showed dog rescues “a new way to find placements for unwanted dogs.”
Western Washington is a popular destination for these dogs.
Our shelters often have space for them due to our high spay/neuter rates and a greater demand for adopting shelter dogs.
This transportation network saved the lives of tens of thousands of dogs. It also created the opportunity for people to start fake rescues that bring dogs to Washington from other areas and sell them for hundreds of dollars.
Many of these dogs come from states with high rates of heartworm infections like Texas. And overcrowded shelters there eagerly give dogs to rescues that will take them.
The American Heartworm Society notes that transporting dogs can spread heartworm. Specially, AHS says:
“Transporting and relocating dogs is an increasingly common practice. Whether the situation is an owned pet accompanying emigrating or traveling caretakers, the relocation of homeless animals for adoption, or the movement of dogs for competition, exhibition, research or sale, this process carries the risk of spreading infectious diseases. This includes the transmission of Dirofilaria immitis when infected dogs are microfilaremic.”
In addition, an article on the Galena Veterinary Hospital website notes:
“Canine heartworm disease is another disease, that while prevalent in certain areas of North America, in other areas it’s uncommon/does not occur. It’s important to be aware that a rescue dog coming from an area of high heartworm incidence could be infected with heartworm disease and may require specialized, expensive, and risky treatment. Additionally, bringing a heartworm positive dog into an area where there are few heartworm cases and few dogs are on heartworm preventives also brings the risk that the “environment” becomes contaminated and heartworm disease becomes a bigger problem in the area.“
Unscrupulous Dog Rescues Skirt Health Testing Rules
Washington requires that before any dog comes into the state it must be “tested negative for heartworm or are currently on a heartworm preventative” (they must also have a current rabies vaccination).
People who run fake dog rescues want to make as much money as possible, so they often import dogs to Washington that haven’t been tested for heartworm because:
1. Heartworm tests aren’t cheap. Initial tests cost $35-$75, and confirmation tests, which most vets require due to the length and expense of treating heartworm, cost $20-$40.
2. Heartworm treatment is time consuming and expensive. Treating a heartworm positive dog can take several months and cost thousands of dollars.
Inevitably, some of these dogs have heartworm, and every one of the growing number of mosquitos in Washington that bites one of them can potentially infect other dogs here.
Furever Homes Rescue near Olympia is one of the rescues that brought dogs from Mexico to Washington without the required health certificates.
In 2018 the Washington Department of Agriculture citied them for bringing dogs into Washington without health certificates. Another dog had a health certificate without the heartworm testing requirement.
When you adopt a dog brought to Washington from another state or county, ask the rescue to see its health certificate. If it doesn’t have one, find another rescue because the dog you want to adopt could potentially have heartworms.
How to Protect Your Dog from Heartworm
While the number of heartworm cases in Washington increased significantly since 2014, the percentage of heartworm cases vs. the number of dogs tested is still much lower than other states.
In 2019, 0.6% of the dogs in Washington checked for heartworm tested positive (733 out of 129,555).
Compare that to the percentages of positive cases in the 5 states with the highest incidences of heartworm in 2020 according to the American Heartworm Society:
MS 8.2%
LA 6.6%
AK 4.4%
AL 4.2%
TX 3.1%
However, just because Washington’s heartworm rate is much lower than these states doesn’t mean you shouldn’t have your dog tested for heartworm or put it on a preventative.
The upward trend in Washington shown on the Parasite Prevalence Maps indicates our heartworm rates will continue to increase, particularly if the increased transport of rescue dogs from states with high heartworm states to Washington continues.
That’s why Washington dog owners should ask their vets if their dogs should be on a heartworm preventative; in addition, if the dog you want to adopt from a rescue came from another state ask to see the its health certificate to see if a vet tested it for heartworm. If they can’t provide it, don’t adopt a dog from that organization.
For more information on heartworm, please visit the Companion Animal Parasite Council website or the American Heartworm Society’s website.
About the Numbers
CAPC notes that the data in the map “is statistically significant and it serves as a strong representation of the parasite activity for each area. However, it does not represent the total number of positive tests. Instead, we estimate it represents less than 30% of the activity in the geographic regions.”
The numbers are underreported because not all veterinarians send the results of their heartworm testing to CAPC. The blank spaces in the chart means CAPC had no information from that county.
These counties didn’t send any test results to CAPC from 2014-2020 : Columbia, Pacific, Skamania, Wahkiakum, Okanogan, Ferry, Pend Oreille, Lincoln, and Garfield.
Some counties like Whitman didn’t send test information for one or more years, but did send it for other years. A zero means a county did heartworm testing but had no positive results.
These numbers don’t include every heartworm test for every dog in every county. But they affirm the that the number of heartworm cases reported in Washington has spiked dramatically since 2014.
* = There is no formula for calculating the percent change if the value for 2012 is 0.
Heartworm cases in Washington state counties ranked by percentage increase from 2014-2019
County | 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | Change from 2019 | Change from 2014 | ||||||||
| Whatcom | 1 | 7 | 5 | 4 | 7 | 14 | 22 | 2100.0% | 57.1% | ||||||||
| Snohomish | 5 | 13 | 8 | 32 | 24 | 35 | 106 | 2020.0% | 202.9% | ||||||||
| Spokane | 3 | 4 | 4 | 11 | 11 | 16 | 41 | 1266.7% | 156.3% | ||||||||
| Clallam | 1 | 1 | 0 | 1 | 0 | 1 | 10 | 900.0% | 900.0% | ||||||||
| King | 36 | 61 | 71 | 121 | 119 | 242 | 305 | 747.2% | 26.0% | ||||||||
| Kitsap | 4 | 7 | 17 | 13 | 23 | 16 | 29 | 625.0% | 81.3% | ||||||||
| Pierce | 11 | 14 | 13 | 26 | 27 | 21 | 64 | 481.8% | 204.8% | ||||||||
| Thurston | 7 | 12 | 11 | 10 | 24 | 18 | 34 | 385.7% | 88.9% | ||||||||
| Clark | 19 | 17 | 8 | 14 | 38 | 57 | 66 | 247.4% | 15.8% | ||||||||
| Skagit | 6 | 2 | 6 | 8 | 11 | 5 | 19 | 216.7% | 280.0% | ||||||||
| Island | 2 | 1 | 0 | 3 | 1 | 9 | 5 | 150.0% | -44.4% | ||||||||
| Chelan | 2 | 0 | 2 | 6 | 2 | 6 | 4 | 100.0% | -33.3% | ||||||||
| Benton | 3 | 1 | 3 | 6 | 4 | 7 | 5 | 66.7% | -28.6% | ||||||||
| Gray’s Harbor | 2 | 0 | 0 | 0 | 1 | 0 | 3 | 50.0% | |||||||||
| Cowlitz | 3 | 0 | 1 | 3 | 6 | 2 | 3 | 0.0% | 50.0% | ||||||||
| Mason | 1 | 0 | 1 | 0 | 0 | 0 | 1 | 0.0% | |||||||||
| Yakima | 1 | 0 | 2 | 0 | 2 | 1 | 0 | -100.0% | -100.0% | ||||||||
| Adams | 1 | 1 | 1 | 0 | 0 | 0 | -100.0% | ||||||||||
| Jefferson | 0 | 1 | 0 | 0 | 1 | 6 | 500.0% | ||||||||||
| Lewis | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 0.0% | |||||||||
| Walla Walla | 0 | 0 | 0 | 0 | 0 | 4 | 2 | -50.0% | |||||||||
| Franklin | 0 | 0 | 0 | 0 | 2 | 1 | 0 | -100.0% | |||||||||
| San Juan | 0 | 0 | 0 | 0 | 0 | 2 | 0 | -100.0% | |||||||||
| Whitman | 0 | 0 | 0 | 0 | 2 | 0 | -100.0% | ||||||||||
| Asotin | 0 | 0 | 0 | 1 | 2 | 0 | 1 | ||||||||||
| Clickitat | 0 | 1 | 0 | 2 | 2 | 0 | 1 | ||||||||||
| Columbia | 0 | 0 | |||||||||||||||
| Douglas | 0 | 0 | 0 | 0 | 0 | 0 | 1 | ||||||||||
| Ferry | |||||||||||||||||
| Garfield | |||||||||||||||||
| Grant | 0 | 0 | 0 | 1 | 0 | 0 | 1 | ||||||||||
| Kittitas | 0 | 1 | 0 | 1 | 0 | 0 | 3 | ||||||||||
| Lincoln | |||||||||||||||||
| Okanogan | |||||||||||||||||
| Pacific | 0 | 0 | |||||||||||||||
| Pend Oreille | 0 | ||||||||||||||||
| Skamania | 0 | ||||||||||||||||
| Stevens | 0 | 0 | 1 | 2 | 0 | 0 | 0 | ||||||||||
| Wahkiekum | |||||||||||||||||
| Total | 108 | 143 | 155 | 266 | 307 | 461 | 733 | 578.7% | 59.0% |
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