As an American veterinarian living in Europe – I have been intrigued by the concern and internet rumours surrounding the flea and heartworm product called Trifexis® (spinosad – milbemycin oximne).

As an American veterinarian living in Europe – I have been intrigued by the concern and internet rumours surrounding the flea and heartworm product called Trifexis® (spinosad – milbemycin oximne).
I moved to Europe before this product was on the North American market. I have never sold it and I have never prescribed it to a patient. It is not in use in my country because there is no risk of heartworm infection here in Northern Europe. However, heartworm disease is very real for many dogs back home in the USA.  Resistance to common flea and heartworm preventatives is becoming a problem in the Deep South and newer generations of prevention medication, such as Trifexis®, are needed.

A TV news outlet and ABC affiliate in Atlanta, Georgia ran a special about Trifexis® in November 2013. Three puppies from the same litter died within 3 weeks of receiving Trifexis® and it gained media attention. Soon after, other claims that “Trifexis® killed my dog” cropped up on Facebook and other social media. These concerns prompted me to look a little bit closer.  Is Trifexis® really dangerous?  Could dogs be dying from this heartworm preventative?  What does the FDA have to say about this?  Are adverse effects being reported to the FDA and local veterinarians?

I’ve decided to comb through the published studies, Facebook pages, complaints and information from other veterinarians to help you, the consumer, determine the truth.

What is Trifexis®?

Trifexis® is a monthly heartworm, intestinal parasite and flea preventative made by the pharmaceutical company Elanco.  It is made up of two different drugs. One, spinosad, is used for flea control. It has been used safely for many years in a product called Comfortis® in dogs 14 weeks and older. Spinosad is also used in humans as a topical treatment for head lice. Dose tolerance studies showed spinosad to be safe in adult Beagles when dosed 16.5 times the label dose and it was given on a daily basis for 10 consecutive days. (1) The most common side effect, although uncommon, is vomiting – often right after the dose was given.
The second drug in Trifexis® is milbemycin oximne. This is the drug that prevents heartworm infection and controls intestinal parasites (roundworm, hookworm and whipworm). This drug is also found in products such as Interceptor® and Sentinel®.  It has been used in these products for over a decade with few side effects. Tolerance studies showed that Beagles tolerated a one-time dose 200 times higher than the monthly label dose and rough-coated Collies tolerated 10 times the monthly label dose.

Why all the fuss?

I was able to obtain copies of the 3 original autopsy (necropsy) reports from the November 2013 news story.  All 3 puppies died of complications from heart failure and had evidence of heart disease.  One puppy had been diagnosed with heart problems before he died. It is unclear when these problems started.  Three different pathologists in two states examined the bodies. Each one suspected either a genetic problem or early infection of parvovirus, which can cause significant heart damage. None of the pathologists thought that the heart problems and death were linked to Trifexis®. No other adverse effects associated with the heart or cardiovascular system have been reported after use of Trifexis®. 

My own dog had a reaction!

While there is no evidence that Trifexis® killed the puppies, adverse reactions can occur after taking any medicine.  My own dog, an Australian Shepherd, had adverse reactions to the product Sentinel® when he was a puppy.  He would have a single seizure within 6 hours of taking the once-monthly Sentinel®. This type of side effect is fairly unusual but has been reported with Sentinel®, especially in herding breeds. We stopped the Sentinel® and switched to Comfortis® and Heartgard® Plus. He has never had another seizure. Sentinel® contains one of the same ingredients as Trifexis®.  I personally would not put my Aussie on it – but only because he reacted to Sentinel®.  But this would not stop me from using Trifexis® with my other dog, if we were to move back to Mississippi.

Since Aussies are relatives to Collies, they can have reactions to ivermectin (Heartgard®) – we watched our boy very closely when we switched products.  He did fine and we were able to prevent heartworm infection and fleas successfully with two oral products instead of one.

Talk to your veterinarian

Your veterinarian wants nothing but the best of health for your dog.  If you think that your pet has had an adverse reaction to a drug, call your veterinarian immediately.  You also have the power to report reactions to the FDA and the company that made the product. This is not just limited to Trifexis®, but to any prescription or over-the-counter drug.

It is mid-2014 and the rumours about Trifexis® are still going strong.  I’m not convinced that Trifexis® is unsafe but it may not be appropriate for all dogs.  The risk of the documented side effects (vomiting, depression/lethargy, itching, decreased appetite and diarrhea) is low and the risk of contracting heartworms in the Deep South is much, much higher. 

My heart goes out to all who lose a pet – for whatever reason. My final message to you is – don’t believe everything you see on TV or on the internet. Seek out reputable, scientific sources when doing your homework.  Safety studies are done for a reason and correlation does NOT equal causation.  Remember that you and your veterinarian know your dog best and together you can decide what product is best for your fur baby.

Resources for Dog Owners
Questions about Trifexis®?  Talk to Your Veterinarian. American Veterinary Medical Association.
Pathology Review of Three Cases of Suspected Trifexis® Intoxication. Engelhard Consulting, Inc.

This news story is independently sourced and does not specifically endorse products or services offered by any company referenced in this article, or benefit from any association with any companies referenced.