Evidence Update: Seresto Flea & Tick Collars

A couple of years ago, I wrote about the wave of concern over the safety of Seresto flea and tick collars, prompted by a sloppy bit of fear-mongering journalism. My conclusion at the time was:

There are undoubtedly some risks to this product, and it is possible that they are greater than has so far appeared in the scientific literature. The research done on this product has mostly come from companies and investigators with a financial interest in it, and that always raises some concern about the potential for bias in the data. On the other hand, such concern doesn’t somehow make collections of unsubstantiated anecdotes a reliable source of data…. Unfortunately, rigorous scientific investigation takes time; a lot more time and work that sensationalist medial reporting. My hope is that reasonable people will respond to this latest example of poor-quality reporting in a reasonable way. We likely should take a careful, objective look at the safety data for this product, and perhaps conduct further research if warranted. 

The EPA responded to the concerns raised about this product by requiring more extensive monitoring of reported problems and then analyzing the relevant evidence. This included controlled scientific research already available as well as the flood of reports from owners that always comes when the media raises the alarm about a pet healthcare product. 

As I discussed in my previous post, simply having a bunch of such reports doesn’t tell us if a product is actually safe or not. The number of reported problems has as much or more to do with public awareness and anxiety about a product as about its actual biological effects. The EPA attempted to control for this by comparing reports regarding Seresto to those for other similar products and for alternatives, such as spot-on flea control products. The agency has produced a report which, in true government agency fashion, is a compromise unlikely to satisfy anyone but easily claimed as vindication by both camps.

The bottom line is that the EPA found mild adverse events consistent with those reported in controlled studies were most common- itching and hair loss. A small proportion of dogs (about 10%) were reported to get lethargic with use of the product. More serious events were rare and could not be clearly linked to the ingredients in the product, which previous research has shown to be generally quite safe. 

The rate of these kinds of adverse effects was not significantly different from those reported for other similar collars. For mild events, there was also no difference between Seresto and spot-on products, but there were more repost of “moderate” events with Seresto than with the spot-ons.

It was generally impossible to show that any of the uncommon serious adverse events were actually caused by the collar, and many of these cases had other factors that could just as easily have been the cause, such as existing illness or other medications. The agency could not show a high level of confidence that Seresto was responsible for any of the serious adverse events with one exception- 4 dogs and 9 cats died as a likely result of being strangled by the collars when they failed to release appropriately under tension.

The manufacturer claims the report supports the safety of the product. This is largely true, but being unable to determine if many of the negative events reported were or were not related to the collar due to lack of adequate information isn’t a ringing endorsement. Information suggesting the ingredients are probably safe already existed, but this report doesn’t add much confidence to that conclusion. 

Critics of the collar also claim victory, even though the agency didn’t revoke the product approval as they wanted. They base this not on the finding of actual evidence of harm, because there wasn’t any, but on the actions the agency took in response to the review. These actions were reasonable but largely aimed at getting additional information and making the agency look like it is taking the public concerns seriously even though the existing evidence isn’t particularly strong. So what did the EPA actually do?

• The collar is approved for only 5 years, instead of the usual 15, and additional reviews will need to be done
• The company must collect and submit additional, more detailed information about possible adverse events associated with the collar
• The company must develop and distribute educational materials for vets and owners discussing possible safety concerns and adverse reactions to the product
• The company must add a warning label to the product information identifying possible adverse reactions that have been reported
• The company must study the release mechanism and report on this to the agency

At this point, the report doesn’t do a lot to change the state of evidence regarding this product. There is reason to believe the ingredients are pretty safe; there have been a lot of reported adverse events; most of these are minor and it isn’t clear that there are proportionally more than for other pest-control products; more information is needed to determine if the benefits of the product in preventing parasites and parasite-transmitted diseases outweigh the risks. 

None of this is likely to satisfy critics of the product, but the state of the evidence is evolving, as it does, and we always have to make the best decisions we can based on the evidence available now, not the perfect evidence we would like to have. As I said previously, I don’t actually prescribe this product in my area, but I also don’t discourage owners from using it, though I do discuss the concerns and uncertainties with any who express and interest or who are using it already. This is, I believe, a reasonable compromise as we, hopefully, gather more evidence to strengthen our understanding of the risks and benefits of the Seresto collar.