Visiting Vet: Adverse reactions

Flea and tick products can impact some pets.


Saturday night I stayed up too late. My daughter was returning on the last boat, having been off for four days at a big horse show, and I had to hear all about it before going to bed. As a result I was still in my jammies Sunday morning when I got a call about Alice, a 2-year-old Boxer Labrador cross. “There’s something wrong,” her owner said. “Alice’s head is very jerky, back-and-forth and back-and-forth.” She had taken a video and wanted to know where to send it and what to do. The video showed a happy, albeit somewhat confused, dog sniffing around the room, head tremoring, ears fluttering like aspen leaves in the wind. She had normal mentation, which means she knew who she was and where she was. She walked well, steady on her feet. It was just her head, shaking constantly.

By the time I received and viewed the video, the tremors had stopped. “Could she have eaten anything unusual? Been exposed to any toxins?” I asked. There are many substances that cause tremors or even seizures when ingested. Her mom thought for a minute then reported that two days earlier, Alice had gotten hold of a Thermacell “tick control” tube. These tubes contain cotton treated with the insecticide permethrin. The idea is that mice use the cotton for nesting, then the permethrin in the cotton kills the deer ticks on the mice, thus breaking the tick life cycle.

Was there any chance Alice could have eaten enough permethrin-treated cotton to cause tremors? Cats are exquisitely sensitive to permethrin, which is one reason many flea and tick products for dogs cannot be used on kitties. Too often some thrifty owner, trying to save money, puts a dab of a dog-only parasiticide on the cat. The cat ends up shaking uncontrollably, often seizuring, sometimes dying. The reaction can last for days and requires aggressive medical intervention. But dogs usually tolerate permethrins well, unless they ingest huge amounts.

The Thermacell website dispelled any concerns. The amount of insecticide per tube was too minute to cause problems for this 50-pound pup. Out of curiosity, I continued reading. The FAQs included the question What happens if a cat eats a mouse with permethrin on its fur? “There are no reported cases where cats have been harmed . . . a toxicity study shows that a small cat of 3 or 4 pounds would have to eat more than 100 mice per day to show any adverse effects. Most cats don’t eat more than six mice a week, indicating the risk of exposure is acceptable.” I wondered how they got those mice-per-cat statistics, finding it all very interesting, but no help for Alice. “She gets Nexgard once a month,” her owner volunteered, “but I gave it weeks ago.”

Nexgard,  Bravecto, Credelio, and Simparica, are all FDA-approved oral flea and tick products containing isoxazolines, a new class of synthetic pesticide first marketed for dogs in 2013. These products go into the host’s blood and are distributed throughout the body. When fleas and ticks bite, they ingest the isoxazolines, which selectively target certain nervous system receptors of insects and ticks. This causes the parasites to become paralyzed and die, theoretically fast enough to minimize risk of transmission of diseases such as Lyme.   

When new veterinary products come out, I tend to be conservative about using them until they have been on the market for a while. In 2015 I started recommending isoxazoline class parasiticides to many clients, taking into consideration individual medical needs, as well as each family situation. Perhaps Princess, the pampered toy poodle who rarely goes outside, is fine with a topical product that stays on the skin and does not get absorbed systemically, e.g., K9 Advantix or Frontline. On the other hand, Laddy the Labrador spends hours in the yard roughhousing with the kids. His family prefers oral parasiticides so the children are not exposed to chemicals on Laddy’s fur. Other factors to be considered include a pet’s age, general health, and pre-existing medical conditions.

Since their introduction, isoxazoline class parasiticides have become very popular. Of course the more animals taking them, the more side effects reported. This month the FDA released a bulletin — “Neurologic Adverse Events Associated with Certain Flea and Tick Products.”  Veterinarians have always known to avoid certain products in pets with pre-existing history of seizures. This is nothing new. But the report, widely shared through social media, has provoked great concern from lay people. It is not uncommon for new information about side effects to emerge after marketing products to larger populations. Please don’t overreact. In most cases, the danger to our pets of tick-borne diseases far outweighs the risk of adverse reactions.

Here are the facts. Some animals receiving Bravecto, Nexgard, or Simparica have experienced adverse neurological events such as muscle tremors, ataxia, and seizures. The FDA states these products continue to be safe and effective for the majority of animals. I agree. The FDA “is providing this information so that pet owners and veterinarians can take it into consideration when choosing flea and tick products for their pets.” Manufacturers are being asked “to make the changes to the product labeling in order to provide veterinarians and pet owners with the information they need to make treatment decisions for each pet on an individual basis.”

Talk it over with your veterinarian. Don’t believe everything on the Internet. If your pet has an adverse reaction, call your veterinarian. They can report it to the appropriate agencies. But, please, let’s not go back to the days of dunking dogs weekly in vats of nasty tick dip. These oral isoxazoline class products still have a useful place in veterinary parasite control. Alice? She had another brief bout of tremors but seems fine. Was it the Nexgard? The manufacturer will reimburse her owner for diagnostic tests but there is no way to know for sure. All we can do is stop giving the product, then wait and see if the symptoms resolve.