Canine Heartworm Cases in Washington Almost Quadrupled Since 2012

The number of canine heartworm cases reported in Washington state last year was more than four times higher than the number of cases reported in 2012 according to the Companion Animal Parasite Council (CAPC).

And in King County, the number of heartworm cases reported in 2019 was almost ten time higher than the number reported in 2012.

King County reported 242 heartworm cases last year. That’s twice as many as 2017 and a 764% increase since 2012 when the county reported only 28 heartworm cases.

Clark County had a 307% increasein the number of heartworm cases reported from 2017 (14) to 2019 (57), the highest percentage increase of any Washington county.

Snohomish County only had 6 heartworm cases reported in 2012 compared to 35 in 2019, an increase of 483%.

A chart showing the changes in heartworm cases for all Washington counties from 2012-2019 is at the end of this post.

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:

Adult heartworms after they develop from larvae. Photo from American Heartworm Society.

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 Heartworm Cases in Washington State 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 last month 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.

rescue groups that bring heartworm positive dogs in that state with inaccurate, fake or no certificates of veterinary inspection (health certificates)

And last week, Dr. Joseph 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 that 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 from other countries.

Climate Change and Lack of Prevention

Mosquitos need a warm climate to reproduce. The warmer the weather, the faster they propagate.

Hot weather increases the rate at which mosquitos reproduce. Temperatures in Washington have been above historical norms since 2012, and 2015 was the hottest year on record. Image from the National Centers for Environmental Information.

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.”

Average annual temperatures in Washington have increase significantly over the last few decades. Image from American Heartworm Society.

Based on this information, it’s reasonable to assume that above normal average temperature in Washington for the last few decades created a more favorable environment for mosquitos to reproduce quickly.

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 preventativebecause 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.

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.

Others come from Mexico where these fake rescues have people who snatch dogs off the streets send them to Washington.

However, the heartworm rate was much higher in areas that border the United States. The Northeastern region, which borders Texas, had a rate of 8.9%, the highest in the country.

The Northwestern region had a rate of 6.1%, the third highest in the country. It borders Texas, New Mexico, Arizona, and California.

Unscrupulous Rescues Skirt Health Testing Rules

Washington requires that all dogs must be “tested negative for heartworm or are currently on a heartworm preventative” before they can come into the state (they must also have a current rabies vaccination).

The Washington Department of Agriculture cited Furever Homes Rescue earlier this year for bringing dogs in Washington without health certificates. The rescue also had a violation in 2015.

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, then 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 2012, 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 (461 out of 76,816).

Compare that to the percentages of positive cases in the 5 states with the highest incidences of heartworm in 2019 according to the American Heartworm Society:

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.

That’s why Washington dog owners should do the following to protect their dogs from heartworm:

Ask your vet to test your dog for heartworm annually.

Ask your vet if your dog should be on a heartworm preventative.

Find out if the dog you want to adopt from a rescue came from another state. If it did, 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 from that organization.

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.

Comments

Is there any data on whether these new heartworm cases were native dogs that don’t travel or dogs that traveled to other areas and came back? Or were they dogs that came from other areas? I find it hard to believe that heartworm survives long enough in the mosquito in say, Spokane, to be passed on to a dog that never went anywhere in its life.

There isn’t data but it’s reasonable to assume most came from out of state as Spokane has become a destination for out of state dogs in recent years. I can’t speak to mosquito survival rates in the Spokane area but as more heartworm positive dogs are brought to WA and temperatures continue to warm it will create a more favorable environment for mosquitos to survive.

We adopted a dog from Texas who was on heartworm preventative medicine. We had him tested 6 months after adoption and the test was negative. Should we keep giving him the preventative medicine for the rest of his life?

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