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.

P.D., our 27-pound, 61 / 2-year-old, 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. When he was a pup and young dog, topical flea and tick preventatives made him sick, so for years we have fed him brewer’s yeast and garlic tablets.

He has never had fleas or ticks in all those years. However, after we applied Frontline Tritak on him — as required by a boarding kennel — his behavior suddenly and drastically changed for the worse.

The scientific literature excerpts below will explain what we learned, and how P.D. returned to his normal, sweet self.

About 60 days after we discontinued the Frontline, his aggressive behavior stopped and 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, reported 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 think, and my vet agrees, that fipronil is what caused P.D.’s aggressive behavior.

B.B., St. Louis

DF: 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 because of the drug’s effects on the thyroid gland or central nervous system.

Fiprinol is one of a class of chemicals that block nerve conduction and to which many species of insects 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 Web site, 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.


Dear Dr. Fox:

My dog was diagnosed with gastritis. What dog food is easily digested? Our vet suggested Hill’s Prescription i/d. Do you have a recommendation?

T.D., Fort Myers, Fla.

DF: “Gastritis” is a term describing a condition without an identifying cause — part of the magic and obfuscation of medical terms derived principally from Latin nouns and adjectives.

A costly endoscopic probe might confirm that there is inflammation and possibly erosion and ulceration of the lining of the stomach.

But what is the cause, and what are the signals or symptoms that lead to this diagnostic conclusion? Is the dog vomiting immediately after eating or some time after? Is there blood or yellowish bile in the vomit? Is your dog bloated and belching?

It could be acid reflux, which treatment with an antacid should quickly resolve. It could be a gastric or intestinal bacterial or viral infection — yes, dogs can show such symptoms with salmonella “food poisoning” or have food allergies.

If there is bile being vomited, there could be a problem with the liver. Other causes of vomiting can be associated with conditions such as pancreatic disease and kidney disease, especially in older dogs.

Simply trying out a special and costly manufactured prescription diet such as the one suggested by the veterinarian is one approach. If it works, then the problem is solved. But again, what is the cause?

It could possibly be an ingredient in your dog’s regular food or a contaminant, or even jerky treats made in China. It is also possible that one or more ingredients in the prescription diet are also in your dog’s regular food and could be responsible for his gastric malady, such as soy, corn or wheat. For details, check out the book that I co-authored with two other veterinarians, “Not Fit for a Dog.”

I would give him a regular human Tums antacid before meals and gradually switch him to my home-prepared diet, posted on

If this dietary change does not improve his condition, then we can rule out diet as a possible factor and consider other possible causes. Let me know the outcome.

Michael W. Fox, author of a newsletter and books on animal care, welfare and rights, is a veterinarian with doctoral degrees in medicine and animal behavior. Write to him at United Feature Syndicate, 1130 Walnut St., Kansas City, Mo. 64106.

2014 United Feature Syndicate