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Ask a Vet: Does Trifexis Really Kill Dogs? Or Is It Just an Internet Rumor?

A TV news report says a common heartworm and flea preventative may be killing dogs -- but it has all the hallmarks of an unfounded rumor.

 |  Dec 3rd 2013  |   9 Contributions


A while ago I was speaking with my editor at Dogster. I mentioned that I would like to write a piece on canine Internet rumors and how to detect them. These rumors are common. They also have a great deal in common with each other.

Most canine Internet rumors involve websites dedicated to stating that a product kills dogs. These sites usually contain heart-wrenching stories of tragic loss. The perpetrators of the rumors, in most cases, have not intentionally set out to start a false rumor. Rather, they honestly believe their stories. They have attributed or more frequently misattributed their dog's death or illness to a product that their dog consumed or was exposed to (while ignoring many other possible causes for the illness or death).

The websites tend to be very emotional and emphatic. They exist for just about every product out there. I have seen such sites for Rimadyl, Advantage, Frontline, Purina, Science Diet, Vectra 3D, K9 Advantix, most vaccines, Nutro, and countless other products. In fact, if you want to experiment, hop on over to Google and search for "X" killed my dog. X can be anything. Something will probably come up (although I tried searching for "water killed my dog" and came up surprisingly short).

Now, emphatically, I am not making light of the potential for any product to cause illness or death in dogs. Dogs can have idiosyncratic reactions to any product. So yes, every product ever invented (as well as every natural product out there) has the potential to cause a fatal reaction in an especially susceptible individual. But for most products that are generally safe, reactions are far from the norm. Peanuts have the potential to cause fatal reactions in sensitive people, but that does not mean that peanuts kill people in general.

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The veterinary profession is a tough one. There just aren't enough pets to support all those vets. Photo of Dr. Eric Barchas by Liz Acosta

I understand why people are motivated to create sites warning of products that they perceive to be dangerous. If a person believes a product killed his dog, he will have a strong desire to spread the word. The millions of people whose dogs benefited from that same product don't have such a strong incentive.

Another way to sniff out an unfounded rumor is to look at problems the offending product is purported to cause. Toxic agents act through specific mechanisms. Anticoagulant rodenticides cause uncontrollable bleeding. Non-steroidal anti-inflammatory agents can cause liver, kidney, and GI tract damage. Tainted melamine in dog food causes kidney failure. But many of the products about which Internet rumors spread are said to cause just about every problem under the sun. To quote toxicology expert Sharon Gwaltney-Brant on the Veterinary Information Network, "Oftentimes, especially for the drugs, you will see that the animals are having all kinds of problems; they are having cancer, cancer of the liver, cancer of the brain. They are having respiratory issues. They are having GI issues ... If you have five animals all with five different types of disease, I don't think I am going to be thinking too much about having a common toxic issue in those situations."

So, to recap: Internet rumors generally start with true stories of sad loss, often incorrectly attributed to a product that was used. They tend to be highly emotional, they often aren't backed up by facts found on autopsy, and the suspect product is often believed to lead to all manner of unrelated problems. There is one more thing: The people behind these rumors generally have unwavering belief in them.

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Gross. Just ... gross. Fleas by Shutterstock.

Lo and behold, look what fell into my lap as I was contemplating this article. A television station in Florida ran a report on dog owners who believe that Trifexis (a combination flea and heartworm preventative) is killing their dogs.

The report has all of the hallmarks of an Internet rumor. It mentions a Facebook page titled "Trifexis Killed my Dog." It is highly emotional -- one of the people interviewed for the story was quoted as saying, "It's like a piece of your heart is being torn out." Trifexis is purported to cause many different problems in dogs. One dog suffered neurological symptoms including vertigo, lethargy, and seizures. Others suffered inflammation of the heart. And peoples' belief in the menace of Trifexis is unwavering, even in light of evidence to the contrary: An independent autopsy on one of the dogs mentioned in the story "stated a bacterial infection likely caused the dog's heart failure. It ruled his symptoms were not typical of drug toxicity." The owner was not convinced.

Again, emphatically, I am not making light of the true suffering of the people in the story, nor the tragic loss of the dogs mentioned in the story. Nor am I saying that it is impossible that Trifexis was involved in any of the deaths.

Trifexis contains two active ingredients. One, milbemycin, has been used for many years as a heartworm preventative sold under the brand names Interceptor and Sentinel. It can, in fact, cause neurological symptoms in especially sensitive dogs. All heartworm preventatives have this potential, but a dog would need to be extraordinarily sensitive to experience any side effects at the doses used in Trifexis.

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Puppy scratching by Shutterstock.

The second ingredient in Trifexis is the flea preventative spinosad. This product also is available as Comfortis. It is widely used, and it causes a not insignificant proportion of dogs to vomit. For the large majority of dogs who do not vomit (including my pal Buster), it appears to be by far the most effective flea preventative out there.

Could Trifexis cause heart failure? Anything is possible, I suppose. But I've never heard of a confirmed case. In fact, Trifexis is much more likely to prevent heart failure than to cause it, since it prevents heartworm disease.

Is Trifexis rampantly killing dogs? The answer to that is clear. It is not. And that is why I don't like Internet rumors. Just as the now completely discredited purported link between childhood vaccines and autism is estimated to have caused the death of over 1,000 children, I worry that the Trifexis rumor could cause dogs to die of heartworm disease and to suffer from miserable skin problems as a result of flea allergies.

Everything in life carries risks and benefits. Walking your dog comes with the risk of him slipping his harness and getting lost. Playing fetch carries the risk of choking on the ball. Every medication and treatment carries the risk of side effects. Before starting your dog on Trifexis or any medication, explore those risks. And also explore the benefits. But do it responsibly, and avoid websites with agendas.

Note to readers: I do not own stock in Elanco. I do not even receive a discount on Buster's Comfortis.

Read more by Dr. Eric Barchas:

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