Last week I shared a list of currently marketed products for heartworm prevention. The last two I mentioned were Trifexis and ProHeart 6. If you do a Google search with either of these search terms, you will find a mix of official websites for the products, sites selling the products, and sites with negative press about the products. You don’t have to read for too long to realize the situation is complicated and people have taken sides. Let’s start with some history about Trifexis.
Trifexis (containing milbemycin oxime and spinosad, for prevention of heartworm and fleas and treatment of round-, hook-, and whipworms), was released into the marketplace in early 2011. The product insert lists adverse reactions (in decreasing order) as vomiting, depression/lethargy, itchy skin, decreased appetite, diarrhea, trembling/shaking, ataxia, seizures, hypersalivation, and itchy/red skin. The insert recommends giving the monthly dose with food, and if vomiting occurs within one hour of dosing, owners should give another full dose immediately. During safety studies performed by the manufacturer, dogs were given 1, 3, and 5x the upper therapeutic dose. Adverse reactions seen in dogs included vomiting, salivation, tremors, and lethargy but there were no deaths.
Last November, Trifexis was investigated on TV in Atlanta, with headlines claiming that owners blamed the death of their dogs on this drug. This prompted the American Veterinary Medical Association (AVMA) to send out an alert to its member veterinarians about the Trifexis rumors circulating in the media. Necropsies (the animal version of an autopsy) were performed on the dogs that had died, and the conclusion was there was no correlation between use of Trifexis and the death of these dogs.
I’m a member of the Veterinary Information Network (VIN), which is an on-line community of thousands of veterinarians around the world. There has been much discussion on VIN about Trifexis. The most frequent complaints are that dogs 1) refuse to take the medication, 2) vomit after taking it, and 3) feel lethargic for up to a day. There are some reports of tremors and seizures, and also of some dogs that have had immediate (within 30-60 minutes) allergic reactions requiring treatment.
So what’s my position? First, I am somewhat conservative when it comes to using new drugs. Trifexis falls into this category, having been available now for only three years. When a drug is being tested for release into the market, there are only hundreds of dogs included in the studies (in this case, there were 352 dogs in the field study, 176 of which received Trifexis). Often it’s only after the drug is used thousands of times that we get a real sense of the safety and efficacy profile. Second, a lot of dogs really, REALLY don’t want to eat this tablet (that is supposed to be chewable). I have no idea what repulses them, but I’m taking notice of it. They don’t like it, and that makes owner compliance go down quick. Then I have dogs that aren’t being protected against parasites, which is the whole point. Third, vomiting is gross. Who wants to deal with puke once a month, and then the dilemma of do I give another full dose (because I’m not sure if he vomited within one hour or 2.5 hours later)?
ProHeart 6 contains moxidectin in sustained release delivery, meaning it is given as an injection under the skin once every 6 months. It was first released in 2001 but was removed from the market in September 2004 after the FDA received reports of thousands of adverse reactions, including deaths. An investigation concluded that solvents used in manufacturing the product were at least partly to blame.
After revamping the manufacturing process, the drug returned to the market in 2008, with restrictions. First, veterinarians using the drug had to register with the company and undergo training before using it. Use of the drug was restricted to healthy dogs between the ages of 6 months and 7 years. Vets were required to provide handouts to clients discussing the drug and obtain written consent before administering it.
Last September, the FDA removed the upper limit on age and vets no longer have to obtain written consent from owners. Also, veterinary technicians and assistants may give the drug under a vet’s supervision, if they have completed on-line training first.
Again, if you do any searching on-line you will find some very sad stories about dogs that have died, the vast majority of them from the 2001-2004 time period. My colleagues on VIN who are using the product since it was re-released are reporting very few issues with it. Some of them choose to separate the ProHeart injection from vaccinations, although this isn’t required in the product labeling. Many vets living in the South where heartworm is rampant are enthusiastic about this drug, because they know the dog is covered for a 6 month time period.
What do I think? I’ve never given ProHeart 6 to a dog. Some of that has to do with the timing of it being on the market, then off, and the fact that I was not working in small animal practice during some of that time. The biggest pro I see in using it is the compliance factor: you don’t need to remember to give your dog a pill once a month, and he can’t go behind the couch and spit it out! The cons include 1) giving a drug that stays in the body for 6 months means you can’t take it back if it causes an issue and 2) dogs will need to have fecal exams/deworming at least every 6 months since ProHeart doesn’t prevent intestinal parasites like monthly heartworm preventatives do.
I hope this helps give you some perspective on two drugs that have received a lot of bad press. Please use caution when you read things on the interwebz. If you find something that troubles you, I encourage you to discuss it with your vet, or drop me a line and I’ll help you research it. The answer is out there…