DEAR DR. FOX: I have an 18-month-old mini-labradoodle. He was bred for good temperament, and in fact, many dogs from his breeder work as therapy dogs.

When he was a wee pup, the vet treated him with NexGard. Quickly the dog became aggressive and a biter. I discontinued the NexGard after reading about side effects online, and my dog became less aggressive.

Then this summer, the vet recommended Frontline Gold. I had used the original version of Frontline on a previous (large) dog with no side effects, so I tried it on my mini-doodle. Instantly, the mini became aggressive again, and bit me! Since discontinuing the Frontline Gold, he has begun to return to his gentle self.

I would like to see more publicity about this dreadful side effect, and I applaud your efforts to spread the word. — K.J., Tampa, Florida

DEAR K.J.: I regret what you and your dog have gone through. These insecticidal products’ labels say we should avoid contact with the substance if applied topically (although family members often make contact when petting their animals), which is a sign that all is not necessarily well. Pets treated with these products seem OK for a while. But what of the long-term consequences, including possible carcinogenicity, immunosuppression, epilepsy and behavioral changes like aggression and depression? I wonder about the dogs who are not so reactive to these insecticides, who may suffer chronic, unnoticed side-effects.

This insanity should stop. The veterinary profession should adopt an integrated approach to dealing with fleas, ticks and internal parasites — even more so in the livestock sector, where insecticides and parasiticides often end up in the soil and water, damaging the essential micro-organisms found there, and potentially contaminating what we eat and drink.

Please ignore those veterinarians who offer these quick-fix (for a big profit) products, except in dire emergency situations. These products are not needed as a routine preventive for dogs and cats, most of whom are at very low, if not zero, risk of exposure for infestation. The exception is heartworm, for which oral preventive medication is advisable during the mosquito season (which means year-round in only a few states).

For an integrative approach to preventing fleas and ticks, which can transmit diseases to humans, see my article “Preventing Fleas, Ticks and Mosquitoes” on my website (

I would like to hear from readers whose animals have had adverse reactions to these insecticides. While walking my dog this morning, I talked to a woman whose sister had one of her dogs euthanized because of seizures. When her other dog started to have seizures, too, she did some research about a product the vet had given both dogs — Bravecto — and found that seizures can occur.

Veterinarian Rosemary Perkins, in her article “Are pet parasite products harming the environment more than we think?” (Veterinary Record, September 2020), notes that few studies have been done on this issue. She presents some references that indicate that the neonicotinoid insecticides applied externally to cats and dogs, or in impregnated neck collars — notably neurotoxic fipronil and imidacloprid — are highly toxic to a wide range of invertebrate species and have high environmental persistence.

The European Commission has banned fipronil and imidacloprid for agricultural use to kill insects because of the impact on honeybees and other beneficial insects, yet their widespread use as a pet flea treatment has been ignored. Perkins references studies showing the dust in homes with pets receiving these topical insecticides contains sufficient amounts of these chemicals to kill bees; also, these chemicals are found in urban rivers at levels exceeding published environmental toxicity limits. This likely comes from dogs and their beds being washed in homes, and from treated dogs being allowed to play in outdoor waterways.

Send questions to or to Dr. Michael Fox in care of Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO 64106.

© 2020 United Feature Syndicate

Distributed by Andrews McMeel Syndication for UFS

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