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Dear Dr. Fox • After reading your column about what to do when a gentle dog turns aggressive, I decided to share the following saga with you:

My dog, P.D., a 6 ½-year-old 27-pound neutered mixed-breed dog, had always been a sweetie. When we got him, he was about 10 to 12 weeks old, and had 52 ticks on his little body. As a pup and young dog, topical flea and tick preventives made him sick, so for the ensuing years we have fed him brewer’s yeast and garlic tablets.

He has never had fleas or ticks during that time. However, after applying Frontline Tritak on him — as required by a boarding kennel — his behavior suddenly and drastically changed for the worse. The scientific literature search below will explain what we learned and how P.D. returned to his normal, sweet self. Approximately 60 days after the Frontline was discontinued, his aggressive behavior completely stopped. It has not recurred.

I’m including part of what I have learned about fipronil, the main active ingredient in Frontline Tritak for dogs.

An Australian study, conducted on behalf of the Australian Pesticides and Veterinary Medicines Authority, reports about dogs treated with fipronil: “Neurological clinical signs included ataxia, lethargy and two instances of biting or aggression. Gastrointestinal signs included vomiting and diarrhea.”

From the Journal of Pesticide Reform: “fipronil is a relatively new insecticide. It is used in cockroach baits and gels, flea products for pets, ant baits and gels, termite control products, turf and golf course products, and agricultural products. ... In pets, poisoning symptoms include irritation, lethargy, incoordination, and convulsions. ... In tests with laboratory animals, fipronil caused aggressive behavior, damaged kidneys, and ‘drastic alterations in thyroid function.’ The fipronil-containing product Frontline caused changes in the levels of sex hormones.

“The offspring of laboratory animals exposed to fipronil during pregnancy were smaller than those of unexposed mothers. They also took longer to mature sexually.

“The U.S. Environmental Protection Agency classifies fipronil as a carcinogen because exposure to fipronil caused benign and malignant thyroid tumors in laboratory animals.

‘’One of fipronil’s breakdown products is ten times more toxic than fipronil itself.

“People can be exposed to fipronil when they pet an animal that has received a flea treatment. Fipronil persists for at least 56 days on pets.

“Studies of fipronil contamination of water are limited, but it has been found in rivers near rice fields where it is used in Louisiana. It has also been found in an urban stream in Washington.

‘’Fipronil is toxic to birds, lizards, fish, crawfish, shrimp, bees, and other animals. Minute concentrations (as low as five parts per trillion) have caused adverse effects.”

I believe, and my vet agrees: Fipronil is what caused P.D.’s aggressive behavior. — B.B., St. Louis

Answer • I very much appreciate your observations and possible confirmation of the anti-flea product making your dog become aggressive. The aggression could have been fear-induced because the Fipronil made your dog more fearful or anxious. He could engage in so-called defensive fear-biting, with possible involvement of the adrenal glands, or be more irritable and prone to offensive aggression, which could have been due to the drug’s effects on the thyroid gland or central nervous system.

Fiprinol is one of a class of chemicals that block nerve conduction (so-called GABA-gated chloride channels) to which many species of insect are highly susceptible. Merck drug company is promoting Bravecto, a new oral drug for dogs that kills fleas and ticks, available only from veterinarians. The company claims that one dose works for 12 weeks, and that it is safe for pregnant and lactating dogs. It reports that the most common adverse reactions recorded in clinical trials were vomiting, decreased appetite, diarrhea, lethargy, polydipsia and flatulence. The active insecticide in this product is an isoxazoline called fluralaner, which is in the same isoxazoline class of insecticides as Fiprinol. The same is true of the drug afoxolaner in Merial’s NexGard.

Your research summary findings on Fiprinol, available to anyone with Internet access, support my quest for a sane, safe and effective approach to flea control, as detailed in the review article posted on my website, DrFoxVet.com. Consumers in general, and the pet-owning public in particular, need to be more mindful and questioning rather than trusting what they are told by manufacturers and our government regulatory authorities.

Visit Dr. Fox’s website at DrFoxVet.com. Send mail to animaldocfox@gmail.com or to Dr. Michael Fox in care of Universal Uclick, 1130 Walnut Street, Kansas City, Mo. 64106.