Veterinarians Answer: Heartworm Disease And Prevention

Do you think your dog might be at risk of contracting 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.

Question: Have you been seeing 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 as a blogger or writer to be giving that type of advice.

—Dr. Lorie Huston, DVM, Rhode Island, Pet Health Care Gazette
    Dr. Lorie on Facebook and Twitter


Heartworms are transmitted by mosquitoes.  This means that heartworm disease is a regional issue which 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 10 heartworm positive dogs this year.

A few of those dogs were from shelters, presumably not on preventative, but 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 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 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


I think that 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 that heartworm disease has become more prevalent.

I have spoken to many clients who have had to choose between "ideal care" for their pet which included flea & tick prevention, heartworm prevention, all recommended vaccines and high end food. For many of my clients their concerns about feeding their family and keeping a roof over their heads meant foregoing the recommendations of me, their veterinarian. I saw heartworm prevention sales decrease, and routine visits followed. 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 difficult to resolve diseases and illnesses.

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 poses 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 being able to prevent 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 manufacturers 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 heart worm disease last year.

 I don't think that dog owners realize that the NY metro area is seeing an increase in heart worm disease due to a number of factors.

  1. The natural spread of this parasite over the years
  2. The importation of infected dogs by rescue groups tat mean well but don't test dogs before bringing them north
  3. Potential resistant strains of the parasite emerging in the south.
  4. Mild winters increasing the numbers of insect vectors
All dogs in endemic areas are at risk, even if they do not go out, as mosquitoes will come in. 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


To first quote the OVMA website " 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 in place 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 per cent of them were confirmed as having been imported from the Southern United States (Katrina dogs) and 12 per cent had been imported from other parts of the United States or other countries. Fifty-one per cent had never left their local area.

The take home message is that Ontario pets are vulnerable from a variety of sources, and prevention is the best approach."

Now the best information resource for this information is one's local veterinarian.

On the internet one can find opinions and information from around the world. The problem with parasites is; 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 under estimate the impact. The science and epidemiological information is there for heartworm, humans pay attention to things they feel are of importance to them and ignore those things they feel are not important.

—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. 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 


No, I have not been seeing (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 a positive in my seven years of SoCA practice.  This occurred in a dog that was brought to Los Angeles from Louisiana (where heartworm disease runs rampant) after being rescued from the aftermath of hurricane Katrina.

As our desert-adjacent climate is typically sees very little rain and is generally quite arid, the conditions that support the lifecycle of the mosquito are not as available as 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 and 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 on our area. As for prevention in endemic areas, yes, owners to 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. All those things lead owners to underestimate the disease.

—Dr. Karel Carnohan, British Columbia, Animal Nutrition and Wellness Services
    Dr. Karel and Facebook and Twitter

Did these answers change your mind regarding heartworm prevention?  

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