Seresto flea collar & imidacloprid

Earlier in March, USA Today reported that bestselling Seresto flea collars have been linked to 1700 pet deaths in the US – and that’s just going by the number reported to the Environmental Protection Agency (EPA); the actual number is expected to be higher. The story originated from Johnathan Hettinger at the Midwest Center for Investigative Reporting, a non-profit newsroom focused on agribusiness and related topics.

Since Seresto flea and tick collars were introduced in 2012, the EPA has received incident reports of at least 1,698 related pet deaths. Overall, through June 2020, the agency has received more than 75,000 incident reports related to the collars, including nearly 1,000 involving human harm. – USA Today

I actually had first-hand experience with this, two years ago when taking care of a friend’s cat, Bagheera, for a week. On about day 2, Bagheera switched from being playful to looking listless and clearly unhappy. Thankfully, we figured out that the likely cause was the new flea collar that Bahgeera was wearing – she recovered pretty fast after I removed the collar. After seeing the story, last week, I checked and it turns out that the collar was Seresto brand.

Now I have to admit that I didn’t even think of reporting the incident to the EPA, which underscores the likelihood that the 75,000 incident reports that the agency received about this collar is a large underestimate. Why has the EPA done nothing about this? The Midwest Center reported that is “looking into” whether to continue to stock the product, but the #1 bestselling flea collar is still listed as the “editorial top pick” on Amazon. Three reviewers on Amazon posted some pretty nasty photos of their dogs’ reaction to the collar, and more disturbing are the reports of “very severe neurological symptoms.” The product gets an average score of 4.6 out of 5 on Amazon, but I suggest caution with putting your trust in Amazon

But more immediately relevant to this post is: Can you trust Bayer? The company “has indicated that reports of adverse events have occurred at a rate of less than 0.3%” and that most of these events were limited to hair loss or skin irritation. Assuming that the company’s number is accurate, is this an acceptable risk? You’d have to decide that for yourself (and your pet). I’ve heard from two vet sources that it’s not even the most effective flea treatment, making the risk/benefit breakdown even less attractive. Perhaps a more imperative question is: why did it take investigate reporting to reveal that the flea collar appears to be killing so many cats and dogs? The product leaflet mentions some side effects (itching, redness and hair loss) but neglects to mention neurological symptoms or death.

Pet collars are big business. In its 2019 annual report, German agribusiness and pharmaceutical company Bayer reported revenue of more than $300 million on Seresto alone. The company sold its animal health division to Elanco Animal Health, a former subsidiary of Eli Lilly and Co., for $7.6 billion in 2019. – Midwest Center for Investigative Reporting

What are the active ingredients in the Seresto flea collar?

The active ingredients in the Seresto flea collar are two insecticides, imidacloprid (10%) and flumethrin (4.5%), that are believed to work synergistically. It’s not common knowledge which of the ingredients in the Seresto collar is causing pets to die, but the reports of neurological symptoms in dogs wearing the collar fits with the mode of action of either of these insecticides, which damage the nervous system.

Seresto flea collar and imidacloprid. A package containing a Seresto flea collar is shown, bearing the Bayer logo. The product is now sold by Elanco, which bought  Bayer's animal health division in 2019.

I noticed some neurological problems. He was weak in the rear legs, lethargic, had a head tilt to the left and unable to control his BM. At first I thought it was his age, he will soon be 12 years old. I removed the Seresto collar just to see if it was a reaction. I gave him a bath with Dawn dish soap to remove any of the chemicals from the Seresto collar. After 12 hours, he was much improved, after 48 hours he was almost back to normal. I just called the Bayer Company to report what had happened and the rude woman on the phone told me that there was NO WAY the Seresto Collar caused any of the symptoms. Almost like she was shaming me for blaming the collar. – Review of the Seresto collar on Amazon.

Risk–benefit analysis of the Seresto flea collar

Treatments for fleas, like the majority of pharmaceuticals, don’t come without risks. Let’s do a quick risk-benefit analysis of the Seresto flea collar. Of course, life would be a lot easier if Bayer and Elanco had published public data on the likelihood of a pet dying as a result of using the Seresto flea collar (preferably on the product page and packaging insert) and, frankly, this lack of transparency is unacceptable. In the absence of this data, I’ll do a rough estimate based on what’s known:

25 million Seresto collars have been sold since approval in 2012, according to a spokesperson from Elanco. Each collar lasts for 8 months, so let’s assume that each pet who wore the collar went through 10 of them (about 6 years’ worth of collars) so that’s around 2.5 million pets, in total, who wore the collar. In case that’s an overestimate, I’ll be generous and estimate that the US represented 40% of the market over this period – that would be 1 million pets who wore the collar in the US. 1700 pet deaths in the US were reported to the EPA – that’s a fatality rate of 0.17% (or almost two deaths per 1000 pets). Bear in mind that this is just a rough estimate and that it’s based only on the number of people who contacted the EPA. It also doesn’t include non-fatal harm, such as the Amazon reviewer, quoted above, who figured out the problem in time, or my own experience with Bagheera.

So, I wanted to share that story with you in case you happen to buy this product for your pet. And also because it ties in with a recurring theme here on the Green Stars Project: neonics in agriculture. The main active ingredient in the Seresto flea collar, imidacloprid, belongs to a group of insecticides known as neonics (short for neonicitinoids) and is one of the most abundantly used agricultural pesticides. To be continued…

13 thoughts on “Seresto flea collar & imidacloprid

  1. And, having gone through a terrible flea infestation in the place I was staying as the person was letting their cat be indoor/outdoor (another neighbour back in my hometown was also plagued), I finally relented and tried a couple of pesticide flea treatment, which didn’t even work because, as the vet said, the fleas are adapting to the pesticides we create (of course) and are becoming resistant to them. So I tried this natural product that has a few methods of application. One is attaching it to a hose and spraying it around the outside of the house and the entryways. The second is an indoor spray. The third was wipes (the topical treatment I don’t think had been created yet). Within one month the fleas were diminished. By the third month, the fleas seemed to have been eliminated. (One of my cats had flea allergy dermatitis so I would know if fleas were still around. Might be good for a Green Star Review candidate!


    1. Thanks for sharing the tips, Willow!
      I may be investigating natural solutions this summer – I’ve heard that a diluted lemon juice spray is one of the best ways to repel fleas so I may try that. I’ll make sure that it’s diluted enough that it doesn’t irritate my cat, of course. She runs a mile whenever she smells the flea treatment now, so I imagine that a light citrus mist will be a lot more pleasant for her 🙂


      1. Ha, cats are so smart! LOL Let me know how it works! I was so relieved that the Vet’s Best or whatever worked so we didn’t have to have the house bombed with pesticides. The fleas were so bad that year for the whole neighbourhood!


      2. Will do! I just took a look at the Vet’s Best products. There’s a different formulation for cats and dogs because essential oils are not without risk. If making a homemade product with essential oils (or using them on a pet’s collar) it’s important to know that essential oils need to be diluted A LOT before use. The active ingredients in the Vet’s Best product for cats are: 0.2% Peppermint Oil and 0.46% Eugenol, which comes from Clove oil.
        So that’s just a message to anyone who is thinking a homemade remedy of peppermint or clove oil – 0.2% peppermint oil is a 1/500 dilution. You’d have to also make sure that it’s well-mixed, each time before use.
        So it would be best to start with a commercial product (like your Vet’s Best product) if not comfortable making at home.


  2. All four of our dogs use these, and I’ve always been skeptical, but we’ve never had any reactions thankfully. The fact that they’ve caused deaths is alarming! Ugh. Thanks for the article.


  3. This is such important information, Jay. I stopped using flea collars many years ago because of the strong chemical smell my animals quickly developed. As a chemistry and biology major in my first two years of college, I knew it was not healthy. I then tried the topical treatments. When my dog at the time rolled on the varnished wooden floor after the first application, I stopped. The chemicals dissolved the varnish where she rolled. Not a good sign! I knew that chemicals that can dissolve varnish could do great damage to all of an animal’s internal organs when traveling via the circulatory system. Since then, I have tried an herbal blend, “tick tonic,” an expensive alternative, and more recently, aromatic essential oils applied to my dog’s collar or harness. I especially like clove scent, and it seems to work here in a residential neighborhood of a small city. In the wooded north country, though, Lyme Disease is a serious risk. I’m not sure aromatic oils would be effective.


    1. Thanks for the comment, Carol – apologies for the late response. I was in the weeds researching the follow-up post that came out today. The commercial treatments are not 100% effective, for sure, so whatever course of action you take it’s always a balance of risk versus benefit. I think you can provide a similar level of protection with a combination of general precautions (vacuuming, grooming, etc.) and more benign deterrents like essential oils (making sure not to apply them neat to the animal) or dilute lemon juice. I’ve read that introducing a little apple cider vinegar to your dog’s food or water is also effective at repelling ticks.


  4. Since neonics are known bee-killers, I would never use them for any purpose. But I gave up on flea collars, generally, several decades ago. I find that regular combing with a flea comb keeps fleas and ticks under control. It’s a general rule of thumb in our household that we don’t poison our loved ones.


    1. Agreed. As mentioned above, I’m going to try using a dilute lemon juice spray on my cat soon and see how what works out. Based on her reaction last time I opened a vial of flea treatment, she’d rather deal with a few fleas than the chemical on her neck.


      1. I comb the cats, at least weekly during fleas season. It’s a lovely way to connect (once they get used to it) and, doing so, we have never had a flea problem.


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