This week I’m going to tackle a controversial type of heartworm prevention. The hospital is considering the 6-month heartworm prevention injection for regular use again (ProHeart 6). This product was temporarily pulled from the market in 2004 due to concerns about safety. I’m going to spend some time today going over the history on ProHeart 6 and sharing some research data that examines the weight of any concerns about adverse reactions to the product.
ProHeart 6 is a specialized medication that allows the slow release of its active ingredient from the injection site over 6 months. The active ingredient is moxidectin. That same medication is found in Advantage Multi (made by Bayer Pharmaceuticals). The product was initially introduced by Fort Dodge in 2001. Fort Dodge had gone through all of the required FDA studies to have the product licensed for use in the US. Proheart 6 didn’t show any significant adverse effects, so it was put on the market. Veterinarians began using it extensively within a very short time. Millions of doses were administered all over the US. Many other countries also used ProHeart 6, including those in Europe, Asia, and Australia. In some places, ProHeart was licensed for 12-month protection (the dosing was different in those places).
Any time a new medication is released, a far larger number of patients receive it than during the safety and efficacy studies. Inevitably, this wide range of patients is a different population than in the safety studies. Dogs that may have had other illnesses, dogs of various ages, a wider range of breeds, use of the drug when vaccines are administered, other concurrent medications, etc. are all factors that contribute to differences ‘in the field’ as opposed to in the lab.
Concerns began to show up that ProHeart 6 was causing severe and serious reactions. These reactions included liver damage, autoimmune anemia, vomiting, anaphylactic reactions, and in some cases, deaths. The internet exploded with web pages (many of which are still up if you search) discussing the thought that “Proheart 6 is killing dogs.” At that time, no one was sure whether there was merit to this association or not. Bear in mind that there’s a difference between association and causation.
Fort Dodge voluntarily pulled ProHeart 6 off the market in the US in 2004. The equivalent oversight boards in most other countries did NOT recommend a recall of the product. Only South Korea took it off the market. Canada did NOT take it off the market. Four million doses were given in Australia. Europe had 3 million, and Japan 2 million.
A study published in 2005 used data from the Banfield vet hospital chain to examine whether ProHeart was associated with any increased risks, more reactions, or was causing deaths. The study used data from 1.93 million dogs, which is a very large sample size. It compared reactions from 2 oral heartworm preventions, ProHeart 6, and vaccinations.
This study led to a LOT of interesting comparative results. Overall, though, ProHeart was not implicated as a reason for increased rises in illness or disease or major reactions. The death rate per 10,000 encounters was NOT higher with ProHeart 6 patients, with or without concurrent vaccines. Concurrent drug use (NSAIDs, steroids) were associated with increased risk. There was an increase in some one type of cancer with dogs that received ProHeart 6 (mast cell tumor). The highest increase in problems came from pets receiving vaccinations. That may not surprise you if you slogged through the immunology posts with me!
The FDA allowed ProHeart 6 to come back on the market in 2008. As far as I’m aware, no change was made to the formula. Some very specific rules were placed that limited the types of patients that could have ProHeart 6. Pets had to be under 7 years old to get the product. Bloodwork had to be done BEFORE getting ProHeart 6. It had to be given on a separate visit from vaccines. Caution was advised for any dog with other types of allergies. Any animal with a history of weight loss could not get ProHeart 6. Vets had to have a 1-hour training session online and pass a small test to be allowed to order ProHeart 6 for use. We briefly used it at Pet Authority at that time, but clients were not very interested in jumping through all of the hoops, so we stopped offering it.
At no time did we see severe reactions to ProHeart 6 (even before it was recalled). We had no deaths that we can attribute to ProHeart 6.
Recently, Pfizer bought Ford Dodge. Pfizer now owns and manufactures ProHeart 6. The conditions I described above no longer HAVE to be followed, though Pfizer is, of course, recommending that veterinarians carefully choose which pets get ProHeart 6. We don’t routinely vaccinate animals that are sick or debilitated, so ProHeart 6 won’t be given to patients who shouldn’t be getting it, either.
We’re very likely to start stocking and offering ProHeart 6 again at Pet Authority. We feel the product is safe, and it’s a tremendous asset for some clients and patients. Only about 46% of clients remember to give their dogs heartworm prevention every month, so ProHeart 6 gives us an opportunity to make sure those dogs aren’t at risk. We’ll discuss the risks and benefits with all clients, the same way that we do for any other medications we give.
If you’re interested in that study that used the Banfield records, click here.
If you’d like to see the 2008 “Risk Minimization Action Plan” by Fort Dodge for the reintroduction of ProHeart 6, click here.
If you’d like to see the ProHeart 6 web page, click here.