Seresto collar

Photo by Toni Gunnison

Atreyu, a blue heeler mix in Wisconsin, sports one of the more than 25 million Seresto collars sold since 2012. A news story questioning the safety of the collar has provoked skepticism among veterinary toxicologists.

Veterinarians across the U.S. fielded calls from worried pet owners this week after USA Today reported that a popular flea and tick collar was implicated in the deaths of nearly 1,700 cats and dogs. But whether the collar is the true cause of the deaths is unclear, prompting veterinary toxicologists to sound a note of skepticism.

Published online Tuesday, the article said that the U.S. Environmental Protection Agency, which is charged with regulating products that contain pesticides, had received complaints linking Seresto brand collars to hundreds of pet deaths, tens of thousands of injured animals and hundreds of harmed humans.

"Yet the EPA has done nothing to inform the public of the risks," according to the story, which was published in collaboration with the Midwest Center for Investigative Reporting.

Two independent veterinary toxicologists interviewed by the VIN News Service said the EPA's seeming lack of responsiveness was likely due to the fact that consumer reports, as described in the article, do not present a consistent or coherent picture of heightened risk.

"Looking at these reports, these are very random things, ranging from ruptured eardrums — which I can't make fit really at all — to liver failure, to heart problems, to kidney failure," said Dr. Tina Wismer, medical director at the ASPCA Animal Poison Control Center and a toxicology consultant for the Veterinary Information Network, an online community for the profession and parent of VIN News. "The fact that the signs are very random makes me think that probably [the collar] is not involved," she said.

Dr. Sharon Gwaltney-Brant, another VIN toxicology consultant, noted that consumer reports to the EPA are unverified and often anecdotal. "Anyone can report anything to regulatory agencies — that doesn't mean it's true or accurate," she said. "This is why looking at the raw data from these agencies is so dangerous — they reflect only the reports, not any ancillary information required to determine if there's actually any merit to the report."

More than 25 million Seresto collars have been sold in the U.S. since it was introduced in 2012, according to Keri McGrath, a spokesperson for Elanco Animal Health. The collars were developed by Bayer Animal Health, which was sold to Elanco in 2020. 

The collars are valued by the veterinary community for their ability to rapidly kill disease-carrying fleas and ticks. Some practitioners have found the collars more apt than other types of treatments, such as oral medications, at preventing the spread of infectious diseases such as tick-borne canine ehrlichiosis.

Veterinarians from Florida to Connecticut to California reported they were contacted by clients worried about the Seresto collars after seeing the story, according to posts on a VIN message board. Several said they were telling pet owners that the incident reports alone fail to show the collars caused the reported illnesses or disorders. For the time being, the veterinarians said they would continue to recommend Seresto.

As pesticide products, the collars, like other parasiticides, carry some inherent risk; whether the risk overshadows the benefit of preventing vector-borne disease is the question.

Wismer said the ASPCA Animal Poison Control Center has fielded calls about Seresto collars since the collars first came on the market, mostly from people whose pets ate them. Her records of these reports show vomiting in 26% of cases; ataxia, or wobbliness, in 2.7%; and tremors in 1.3%. 

Nothing in these proportions suggests a cause for alarm, she said. Wismer added that she expects to see vomiting and neurologic signs from some animals that ingest Seresto's active ingredients, flumethrin and imidacloprid. But the wide variety of signs attributed to the collar in the consumer reports leads Wismer to doubt that the collars are to blame.

She explained that a poison tends to elicit the same signs, along a continuum, in every animal, offering ibuprofen as an example. "We can see vomiting; we can see stomach ulcers; and, at higher doses, we see kidney failure. But it's the same progression in every animal," she said.

Gwaltney-Brant noted that pet owners sometimes make assumptions about what caused a downturn in their pet's health, often attributing internal changes to external causes such as flea-control collars and heartworm preventives. "[These are] the first things that come to mind as a cause when suddenly the pet becomes ill and/or dies for any reason," she said.

But, as is often stated in science, correlation does not mean causation. "Without veterinary evaluation [via necropsy or examination] to rule out other potential causes for the illness/death, you cannot make a cause-effect connection," she said.

Gwaltney-Brant said fellow veterinary toxicologists with whom she communicates on a listserv shared her surprise at the disquiet over Seresto collars. "[They] have confirmed that even with ingestion (a fairly common occurrence with dogs), the toxicity of these collars is extremely low, and they have no 'red flags' on this particular product," she said by email.

When consumers report adverse effects of products to federal agencies such as the EPA and the U.S. Food and Drug Administration, Gwaltney-Brant said, specialists including toxicologists review the data to determine whether there is potential for concern, and if there is, they generally take action. "I trust that they have vetted the reports and determined there is no cause for concern," she said about the Seresto-associated cases.

Wismer echoed that confidence, saying that in instances that consumer and veterinary reports about a pesticide show worrying trends, the EPA has taken action. She offered as an example rabbits and fipronil, the active ingredient in Frontline spot-on treatment. In the early 2000s, veterinarians noticed that when Frontline was applied to rabbits, some developed seizures. The EPA investigated the reports and determined that Frontline was the cause. The agency required a change in the labeling to indicate it not be used on rabbits.

"There really is a mechanism to protect the consumer," Wismer said.

The EPA also in 2010 called for an array of changes in the way topical pet flea and tick prevention products are tested, labeled and monitored after the agency evaluated health problems, including deaths, associated with use of the products. The agency's review was prompted by a dramatic rise in adverse-event reports between 2007 and 2008. 

Regarding concerns about Seresto, Wismer and Gwaltney-Brant said the existence of counterfeit collars could complicate interpreting incident reports. Fake collars, which reportedly are widely available, may contain ingredients that are harmful to pets.

"You don't necessarily know where these collars are coming from and what actually is in them," Wismer said. "And that could explain a lot of the different kinds of clinical signs we are seeing."

VIN News was unable to determine whether the EPA distinguished between authentic and counterfeit collars in reviewing reports. The agency did not respond to questions by publication time. 

It can be hard for consumers to tell the difference between a real and a fake collar. "Counterfeiting operations are very smart, but consumers can protect themselves by only buying products ... from a trusted source," Elanco's McGrath said. "For Seresto, the best bet is to check out the 'Where to Buy' page on petbasics.com. And, if the price is too good to be true when comparing to other trusted retailers, it likely is not an authentic product."

McGrath confirmed that, as reported in the article, one in 568 users has reported an adverse experience with the Seresto collars. However, she said, "The significant majority of these incidents relate to non-serious effects such as application site issues — reddening of the skin or hair loss below the collar."

Gwaltney-Brant said those are common issues with too-tight collars, even unmedicated ones. For collars containing pesticides, local irritation also is usually not a big concern, she added.

"Just as there are some humans who cannot 'wear' certain brands/types of cosmetics, there are animals that develop hypersensitivity to one of the ingredients in the collar and will develop a rash," she said. "This is not a toxicosis, just an individual hypersensitivity."

March 19 update: The U.S. House Subcommittee on Economic and Consumer Policy called on Elanco this week to immediately temporarily recall the Seresto collar, and requested a variety of documents and information from the company, including consumer complaints related to harm to pets or people, internal communications about collar safety and sales volume. Elanco spokesperson Keri McGrath said the company will not recall the collars but will provide the requested information. The company released a statement outlining its reasons for staying the course.