Dangers from flea and tick medicine?


Q: We have three small dogs — two cavalier King Charles spaniels and a scnoodle. There appears to be a growing body of claims that flea and tick medicines, including Bravecto, NexGuard, and Simparica, are harmful. Apparently, their chemical compounds impact an insect’s nervous system, causing death. The question then becomes: Do they also impact a dog’s nervous system? I am looking for either reassurance or a good alternative. We live in a high-tick area, and I have already had Lyme disease once.

Catherine Sotomayor
Big Oak Valley, California

Dear Ms. Sotomayor,

A: We get this question from many people during the summer months — which is unfortunate, because flea and tick medicine should be administered all year long.

As for the answer to your query, it’s essentially a reassuring one. The preventatives that you mention belong to a class of drugs known as isoxazolines, which the Food and Drug Administration considers safe and effective for dogs (and cats). In fact, says Tufts veterinary dermatologist Ramón Almela, DVM, “isoxazolines have been and are currently used in millions of dogs and cats around the world.”

Adverse reactions do sometimes occur, Dr. Almela notes, but they seem not to be more common or more severe than reactions to other flea and tick preventatives. “There is no zero risk for any drug,” he says.

Because of the small chance for a negative reaction, the FDA alerts pet owners and veterinarians of the “potential for neurologic adverse events in dogs and cats when treated with drugs that are in the isoxazoline class.” These include muscle tremors, ataxia (a loss of balance and coordination resulting from a lack of muscle control), and seizures. In the rare case that a dog suffers such effects, it should be reported to the pet’s veterinarian, who will file a report with the FDA.

But Dr. Almela points out that “the benefit almost always far outweighs the risk of a dog getting Lyme disease or other tick-transmitted diseases, which can be life-threatening. As with any treatment, the patient should be considered in making decisions about which drug to prescribe. For instance, we avoid these preventatives in dogs that already have a history or suspicion of a neurological disorder.”

Tufts small animal veterinary intern-ist Michael Stone, DVM, agrees, saying, “Since the FDA started keeping tabs on this issue five years ago, there has been no confirmation of a problem that’s greater for isoxazolines than for other drug ingredients. I live in a tick-endemic area and use a flea and tick preventative on all my pets — and recommend it to all my clients.” 


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