Should I worry about heartworm if I live in the PNW?
When I first started practicing in Oregon, heartworm disease was luckily not something we had to deal with…like at all. No testing, no prevention, no treatment. What a dream. In the last two years, that has changed. This post is meant to help elucidate some myths and truths around heartworm disease particularly in the Pacific Northwest and Columbia River gorge.
The question I hear all the time is: Is heartworm prevention really necessary? Now it is – emphatic yes! Why? Let’s discuss.
WHAT IS HEARTWORM DISEASE?
Heartworms are actual worms that can live in your dog’s and cat’s heart, and they cause damage to the valves and arteries in and around the heart and lungs. They are probably the most dangerous parasite that dogs and cats can get in the United States.
HOW DOES MY DOG OR CAT GET HEARTWORM DISEASE?
Mosquitos. More specifically, a mosquito will bite an infected dog, pick up the microfilaria then harbor the microfilaria until it grows into the infective larval stages of heartworm over 2 weeks. Then when they bite another dog or cat, they transmit it to the bloodstream of the dog or cat. After that, it takes about 70 days for the larvae to grow into adult worms and migrate to the heart. By 6 months after the first bite, the adult worms are reproducing and now your dog has microfilaria that another mosquito can pick up.
When I was in vet school, we learned that in northern Oregon, the reason we didn’t have heartworm disease is because our mosquitos did not live to 2 weeks, so they could therefore not be infective. Some investigation around the mosquito life cycle implies that this may not be the case anymore. More on this later!
WHAT HAPPENS IF MY DOG GETS HEARTWORM DISEASE?
Seeing the positive test dot light up on the heartworm test leaves a pit in every veterinary staff member’s stomach. Heartworm treatment is a long, expensive process. Here is a table on the exact treatment regimen but basically you are looking at 4-9 months of severe exercise restriction – these guys are on all the sedatives. The reason for this is that as the worms are dying, we don’t want them embolizing somewhere deadly like the brain, lungs, heart or anywhere else. They get three injections of melarsomine, all given in a veterinary hospital while actively monitored and deeply sedated. The total cost of treatment usually ends up being between $1000-1500 depending on the size of the dog. Often, despite treatment most dogs will end up with long term damage to the heart and pulmonary arteries because of heartworm disease. As with most diseases, if it is caught early, there is less long-term damage.
WHAT HAPPENS IF MY CAT GETS HEARTWORM DISEASE?
Unfortunately, there are no good cures for feline heartworm disease. Cats are not the intended host for heartworm (dogs are) so they manifest the disease a bit differently. They tend to get more inflammation and more sudden death from heartworms. They also don’t tolerate the melarsomine treatment, so therapy is aimed at decreasing the inflammation the worms produce rather than achieving a cure. Because of this, prevention tends to be the key for cats.
I’VE BEEN TOLD HEARTWORM IS NOT IN THE GORGE, WHAT CHANGED?
Forever, we have had 0 positive cases in the Columbia River Gorge. You heard right, ZERO. In 2019 that started to change. There is a great surveillance website here that has compiled testing data for the last several years for a variety of parasites, heartworm being one of them. In Hood River County we have had 1 positive test for every 200 negative tests. And we have not been testing very much when compared to our neighbors in Multnomah County. They also have a 1 positive per 200 negative tests on average.
So, what changed? The American Heartworm Society suspects that we are seeing two phenomena. The big driver of the nationwide and statewide spread was likely Hurricane Katrina. Thousands of dogs were shipped out of Louisiana, the state with the highest incidence of heartworm, and sent all over. Additionally, climate change has produced a change in the mosquito life cycles, causing them to complete the heartworm life cycle. The statement we have all been telling ourselves that “We just don’t have heartworm disease here” is just plain false.
WHY IS TESTING SO COMPLICATED, AND WHY DO I HAVE TO GET AN ANNUAL HEARTWORM TEST IF MY DOG OR CAT IS ON MONTHLY PREVENTION?
Good question! Testing for heartworm disease is very complicated. There are four heartworm tests:
1. Antigen test
2. Antigen test with heat treatment
3. Microfilaria test
4. Antibody test
The routine screening test for heartworm disease in dogs is an antigen test. This means that it is looking for tiny bits of proteins that adult heartworms make and secrete into the bloodstream. You probably remember that adult heartworms take 70 days to grow, and tests have shown that it likely takes just over 150 days for the antigen to be produced. So that means that a dog can have heartworms for over 150 days before they will have a positive test. Yikes! Furthermore, if your dog or cat starts mounting an immune response to the heartworm and produces antibodies that bind up all the antigen, they can also have a negative heartworm test. This can be remedied by heat treatment but is more expensive to run. The test goes from about $25 to $60-70 with heat treatment. A third test is the microfilaria test. They are great at detecting the microfilaria which are in the blood even later in the course of disease. This test will only be positive when the adult heartworms are old enough to be reproducing. The fourth test is the antibody test, commonly used in cats over dogs. This just proves that the cat has been exposed to heartworms at some point, not that they are having an active infection.
The reason a heartworm test is recommended every year is because the prevention only kills the larval stages before they turn into immature adults. So that means that if a dose gets missed or delayed or if a little larva turns into an adult, it is immune to the prevention. Testing is required prior to starting heartworm prevention, but remember that there are 150 days when an animal has heartworm disease but tests negative? You could be using prevention in your dog for a year when they actually have heartworm disease but test negative prior to starting prevention. For this reason the yearly heartworm test is used to make sure no worms slip through and set up in the heart.
Here is the testing protocol recommended by the American Heartworm Society:
1. Antigen test prior to starting prevention monthly
2. Antigen test 6 months later for at risk dogs – this means all the pups coming up from Mexico, Texas, and California.
3. Antigen test annually while giving monthly heartworm prevention
HOW DO I PREVENT HEARTWORM DISEASE IN MY DOG OR CAT?
There are a variety of heartworm preventatives on the market. The canine versions are usually oral and given monthly. Most of these will kill larval stages and few kill immature adults. The feline versions are usually topical and monthly and are also flea preventatives as well. In general, you can anticipate paying about $100-150 per year in heartworm prevention for dogs and around $350 per year for heartworm and flea prevention in cats.
