Canine Heartworm Disease: Veterinarians Share Insights About Heartworm Disease And Prevention in Dogs

Do you think your dog might be at risk of contracting canine heartworm disease? Do you think it’s something to worry about or not so much? Is your dog on preventative, or do you think they don’t need one?

I asked my veterinary friends.

Canine Heartworm Disease: Veterinarians Share Insights About Heartworm Disease And Prevention in Dogs

Question: Have you seen heartworm-positive dogs in your area? Do you feel that dog owners underestimate the seriousness of the issue?


Yes, we do see heartworm-positive dogs in our area. 

Some of them are rescues coming up from the south, but some are residents of Rhode Island.

I do feel that many owners underestimate the seriousness of this issue, and I believe that internet sites that advise people that their dogs don’t need heartworm or that the preventive medication is dangerous are a big part of the problem.

I believe it is irresponsible for a blogger or writer to be giving that type of advice.

—Dr. Lorie Huston, DVM, Rhode Island


Mosquitoes transmit heartworms.

This means that canine heartworm disease is a regional issue that correlates to climate. When I lived in central Pennsylvania, I saw very few cases of heartworm disease.  I now live on the coast of South Carolina, where the locals joke that the mosquito is our state bird.  Unfortunately, our practice has seen ten heartworm-positive dogs this year.

A few of those dogs were from shelters, presumably not on preventative. Still, the rest were beloved companions whose owners were sporadic in administering preventative or had stopped giving it through the winter months.  Their owners, like most, underestimated the seriousness of the issue.

Treating canine heartworm disease is neither simple nor benign.  Treatment involves killing adult worms (which can be up to 14 inches long) residing in the dog’s heart and pulmonary arteries. It is expensive for the client and painful for the dog, not to mention that the dog must be very strictly confined for one month following the final treatment.  Thankfully, prevention is simple, safe, and comparatively inexpensive!

—Dr. Julie Buzby, South Carolina, ToeGrips
Dr. Julie on Facebook and on Twitter


Poor compliance

In light of the last few years of a struggling economy and many people being squeezed so tightly to try to make ends meet among a long list of demands from all sides, canine heartworm disease has become more prevalent.

I have spoken to many clients who have had to choose between “ideal care” for their pet, including flea & tick prevention, heartworm prevention, all recommended vaccines, and high-end food. Unfortunately, many of my clients’ concerns about feeding their families and keeping a roof over their heads meant foregoing my veterinarian’s recommendations. As a result, I saw heartworm prevention sales decrease, and routine visits followed. Unfortunately, I also saw an increase in advanced illness because many people took a “watch-and-wait approach,” which in some cases caused more advanced, expensive, and more challenging to resolve diseases and illnesses.

Expense of prevention versus treatment

Between the decrease in available funds for pets due to the economy and a false sense of safety about the prevalence and consequences of heartworm disease, I have seen more dogs test positive for heartworm disease in the last few years.

I do think that many people do not understand how easily and unknowingly the disease is spread, how simply and economically it can be prevented, and the expense and danger treating this disease pose to their pet.

It seems that no matter how many times I tell clients about this potentially life-threatening disease, the danger and expense of treating it, and the advances of modern medicine in preventing it so effectively, I still see dogs that test positive.

One mention of advice (my hashtag is, after all, “FreePetAdvice), Please buy your heartworm prevention from your veterinarian. If you do, your dog is protected by the heartworm manufacturer’s guarantee, IF you give it monthly (as prescribed). And please test your dog yearly. If your pet tests positive, you need to know ASAP.

—Dr. Krista Magnifico, DVM, Pennsylvania,  Diary of a Real-Life Veterinarian,
Dr. Krista on Twitter 


We saw three dogs with heartworm disease last year.

I don’t think that dog owners realize that the NY metro area is seeing an increase in canine heartworm disease due to several factors.

  1. The natural spread of this parasite over the years
  2. The importation of infected dogs by rescue groups that mean well but don’t test dogs before bringing them north
  3. Potential resistant strains of the parasite are emerging in the south.
  4. Mild winters increase the number of insect vectors

All dogs in endemic areas are at risk, as mosquitoes will come in, even if they do not go out. And cats are at ink as well, but that is another question for another time

—Dr. Keith Niesenbaum, VMD, New York, Crawford Dog and Cat Hospital
Dr. Keith on Facebook and on Twitter


Variable numbers of infections

First, the OVMA website asks, ” How prevalent is heartworm in Ontario? Do I really need to worry about it in my pet?

The actual number of heartworm-positive pets in Ontario varies from year to year.

While there is currently no system to track every case of heartworm in the province, a survey conducted in 2010 found that the number of dogs with heartworm in Ontario increased by 60% between 2002 and 2010.

Of the dogs that tested positive for heartworm in Canada, nine percent were confirmed as having been imported from the Southern United States (Katrina dogs), and 12 percent had been imported from other parts of the United States or other countries. Fifty-one percent had never left their local area.

The take-home message is that Ontario pets are vulnerable from various sources, and prevention is the best approach.

The best information resource for this information is a local veterinarian.

On the internet, one can find opinions and information worldwide. The problem with parasites is; that they behave differently depending on the local climate and density of the hosts available to them.

Within my practice, the risk of heartworm is measurably greater 10 miles away than outside my front door.  Go another 10 miles, it increases again, and go 60 miles, and the risk is now as close to 100 % as mother nature allows us to get.

Dog owners that have lost a dog to heartworm stay on top of testing and prevention. Those that have not been that unlucky may feel it is not a concern and underestimate the impact. The science and epidemiological information is there for heartworm. Unfortunately, humans pay attention to things they think are important to them and ignore those things they feel are unnecessary.

—Dr. Rae Worden, DVM, Ontario,  Fergus Veterinary Hospital
Dr. Rae on Facebook and Twitter


Most of my clients understand heartworms are bad news. 

They also understand there is easy monthly prevention. The only problem we ever get into is this business of “you can skip giving heartworm and flea prevention in the winter” that some clients have been raised with.

Almost invariably, that logic causes a gap in prevention that begins innocently enough as a plan to skip December, January, and February. Yet somehow, it always seems that May rolls around, the snowmen are all melted, Christmas is long forgotten, the flowers are blooming, the dog is swarming with mosquitoes, crawling with fleas, and has no heartworm or flea prevention on board because the owner was so busy enjoying spring that he forgot to protect his furry loved ones. So then we get to spend the rest of the year fighting nasty fleas that could have been easily prevented and worrying if Fido is growing heartworms inside until that six month post-exposure recheck heartworm test.

I realize prevention is a hassle and can get expensive. The alternative is worse.

Train yourself to a habit of giving your dogs their monthly flea and heartworm stuff on the same day every month forever, and you’ll prevent a world of hurt.

—Dr. Greg Magnusson, DVM (Leo’s Daddy), Indiana, Leo’s Pet Care
Dr. Greg on Facebook and Twitter 


Low prevalence in our region

No, I have not seen (an increase in numbers of) heartworm-positive dogs in my area of veterinary practice in southern California (Los Angeles).

There was only one occasion when I diagnosed a dog as being positive in my seven years of SoCA practice.  This occurred in a dog brought to Los Angeles from Louisiana (where heartworm disease runs rampant) after being rescued in the aftermath of Hurricane Katrina.

As our desert-adjacent climate typically sees very little rain and is generally quite arid, the conditions that support the mosquito’s lifecycle are not as available as in other parts of the country.

Yet, such conditions do exist, and mosquitoes can prosper.

Plus, wild populations of changes (coyotes, etc.) have been reported by the Los Angeles County Department of Public Health to carry heartworm disease. So they could be a vector for transmission into our domesticated companion canines (and other species).

Dr. Patrick Mahaney, Los Angeles, The Daily Vet
Dr. Patrick on Facebook and Twitter 


No, we don’t see it in our area.

As for prevention in endemic areas, yes, owners underestimate the issue.

Heartworm disease doesn’t show itself until the dog is quite sick.

It is easy for owners to ignore prevention, and most owners don’t realize the seriousness of the disease and how risky it is to treat it once it is diagnosed. Unfortunately, all those things lead owners to underestimate the condition.

—Dr. Karel Carnohan, British Columbia

Did these answers change your mind regarding heartworm prevention?  

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Jana Rade

I am a graphic designer, dog health advocate, writer, and author. Jasmine, the Rottweiler of my life, was the largest female from her litter. We thought we were getting a healthy dog. Getting a puppy from a backyard breeder was our first mistake. Countless veterinary visits without a diagnosis or useful treatment later, I realized that I had to take Jasmine's health care in my own hands. I learned the hard way that merely seeing a vet is not always enough. There is more to finding a good vet than finding the closest clinic down the street. And, sadly, there is more to advocating for your dog's health than visiting a veterinarian. It should be enough, but it often is not. With Jasmine, it took five years to get a diagnosis. Unfortunately, other problems had snowballed for that in the meantime. Jasmine's health challenges became a crash course in understanding dog health issues and how to go about getting a proper diagnosis and treatment. I had to learn, and I had to learn fast. Helping others through my challenges and experience has become my mission and Jasmine's legacy. I now try to help people how to recognize and understand signs of illness in their dogs, how to work with their veterinarian, and when to seek a second opinion. My goal is to save others the steep curve of having to learn things the hard way as I did. That is the mission behind my blog and behind my writing. That is why I wrote Symptoms to Watch for in Your Dog, which has turned out being an award-winning guide to dog owners. What I'm trying to share encompasses 20 years of experience.

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