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Jenna Stregowski, RVT

Is Your Dog's Heartworm Prevention Working?

By June 22, 2011

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Heartworm prevention has been on my mind this week, and it's not just because the mosquitos are biting. Merial, the manufacturer of the leading heartworm prevention Heartgard, is being accused of misleading veterinarians and pet owners about the product's efficacy. Dr. Kari Blaho-Owens, former scientist for Merial in Gwinnett County, Georgia says she was fired after she refused to destroy certain evidence. According to a story on wsbtv.com, "Blaho-Owens said a supervisor told her to hush her own study that showed Heartgard had a 20 percent failure rate." Blaho-Owens has filed a lawsuit against Merial, claiming that the company deliberately withheld information about the potential ineffectiveness of Heartgard.

I have used Heartgard Plus on numerous dogs over the last 15+ years. I have seen it prescribed for thousands of dogs in the vet clinics where I have worked over the years. Never have I seen a case where a dog developed a heartworm infection that was not related to owner error (e.g., forgetting to give the dose or under-dosing the dog). That's not to say it's right to withhold evidence about the product's efficacy (if that did happen). But, to be honest, I cannot say that this case will change my opinion of the product.

Whether you choose to give your dog Heartgard or one of the many other heartworm prevention products, just make sure you are giving something. In most of the US, mosquitoes have been out in full force, nibbling away at us and our dogs. Mosquitos also happen to transmit heartworms, a serious parasite that affects the heart and lungs of dogs (and sometimes cats as well). This life-threatening disease can easily be prevented by using heartworm prevention AND having your pet checked yearly for heartworms (a simple blood test).

Do you use Heartgard? Does this pending lawsuit change your opinion of the product? If so, will you be changing to a different product? Tell us why or why not by leaving your comments below.


June 23, 2011 at 1:00 am
(1) Peggy Juliann says:

It is a bit capricious to write in support of HeartGuard and lead others to think that it is perfectly fine to use based on anecdotal evidence and choose to ignore the scientific data
of Dr. Blaho-Owens. It is also slightly, whether purposefully or by intent, misleading to refer to Dr. Owens as being a former scientist when instead the correct phrasing should be a scientist formerly with Merial.

Whether it is product, procedure or just poor PR there is something inherently wrong about a company that wants to cover up data that might prove to be harmful to said company, particulary at the cost of potential harm to the clients and users.

I will not be using Heartgard, nor will a number of my colleagues and associates who are involved with canines in one way or another.

June 23, 2011 at 9:36 am
(2) Judy says:

Peggy, I couldn’t agree with you more! Anytime an entity attempts a cover-up, something is definitely amiss.

I will not use Heartgard on principal. Thanks for your comments.

June 23, 2011 at 12:51 pm
(3) Jenna - Dogs Guide says:

I would not say that I support Heartgard – only that I have never personally seen a problem with it. The fact is that no product is 100% effective. Heartgard was wrong to ever claim it was.

You can click through on the link to the full story to read more about it. I brought up this case to challenge dog owners to learn more about the products they give their dogs.

June 23, 2011 at 8:13 am
(4) daria says:

Always used Heartgard with no problems. My dog just died after 16 years and I rescued another one. Now my vet put her on a new medicine called Trifexis which is a 3 in 1 pill.

June 23, 2011 at 11:19 am
(5) VICKI says:

That explains why my dogs have to be tested every year for heartworm even though they have been on Heartguard ever since I have had them. I asked the vet technician one time if the yearly testing was because they weren’t sure the Heartguard was working or if it was just to get more money. I think now maybe it was a little of both.

June 23, 2011 at 1:00 pm
(6) Jenna - Dogs Guide says:

Actually, all dogs should be tested for heartworms yearly. No product is 100% effective. It doesn’t matter if you switch to a different brand of ivermectin or change to a different drug completely. Some heartworm preventatives can be dangerous if given to a heartworm positive dog. That’s the primary reason for the testing. Honestly, vet hospitals don’t make that much money on heartworm tests.

Also, FYI – if your dog does come up heartworm positive and you have been giving prevention, the company will typically pay for treatment. Obviously, it would still be terrible if it happened, but it’s good to know that these companies stand behind their products. Fortunately this is not very common.

June 23, 2011 at 6:15 pm
(7) Maureen says:

Deer ticks and heartworm are endemic in my area. All of my dogs have received heartgard and none have developed heartworms. I think I just won’t question the efficacy of the drug and keep using it.

June 23, 2011 at 11:13 pm
(8) Mary says:

My two dogs have been using Heartguard for 10 years and so far so good! However, the company shouldn’t be hiding results (if they did) nor should they claim 100% (not sure if it does) as nothing is 100%. As of last year my vet changed to doing the blood test for heartworm to every two years. I live in ontario, canada and i wonder if the bi-annual blood test is done elsewhere as well?

July 4, 2011 at 3:55 pm
(9) Paloma says:

I also have been using heartgard on all my dogs since about 1989. None of my dogs have ever had heartworm problems, and I don’t think that Merial’s product is any less effective than it ever has been, nor do I believe it is less effective than any other product out there. I switched vets a while ago, and they only supplied Interceptor, which supposedly worked better that Heartgard, but I cant say that it was any more effective. Regardless, my dogs are back on Heartgard and couldn’t be healthier!

Anyway, to answer your question, I live in El Paso TX, and yes we also do bi-annual testing here as well. The vets told me that while its up to the owner to decide if they want it done 2x a year, if your dog is on heartworm prevention year round and you do not skip their doses, then they are protected from the worm and the testing is really only necessary to perform every other year for monitoring and maintenance.

June 26, 2011 at 7:59 am
(10) dot says:

i have never tested my dogs for heart worm, nor given them any heart worm prevention. its arsenic based. heart worm has been around as long as dogs have been domesticated. there were papers published as far back as ’47 discussing heartworm. i think its big pharm want us to spend money on their drugs.

June 24, 2013 at 10:51 am
(11) Marylou says:

Hey there, You have done an excellent job. I will definitely digg it and personally suggest to my friends.
I am confident they’ll be benefited from this website.

February 24, 2014 at 10:39 pm
(12) John says:

I doubt anyone will read this but wanted to provide some direct testimonial. We have an 11 year old rat terrier in perfect health. Has always been given Heartguard Plus …. never, ever a dose missed. Went to the vet for annual checkup and surprise – we have heartworms. I used it on my Pug before the Rat Terrier too with no issues. But now I’m starting to dig to see what happened. Treatments will begin soon. She is healthy and hopefully will tolerate them well, but the vet said it’s not out of the realm of possibility that the treatments could be fatal depending on how they are tolerated.

I really and truly hope this company has been fair and right in their representation of this product. I’m worried now about going forward with this product if she makes it through treatment.

So to the poster above who claims they’ve never seen it happen….well, it happened. I sure hope it never happens to you or anyone else. We love this little girl dearly and will be devastated if we lose her.

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