The cat flea (Ctenocephalides felis felis) is the most important external parasite of cats and dogs worldwide. A 17-year study that monitored for resistance among cat fleas to the insecticide imidacloprid finds no evidence of decreased susceptibility. The study involved researchers in 10 countries, who tested more than 1,800 cat flea egg samples. (Photo credit: Michael K. Rust, Ph.D.)

By Ed Ricciuti

Although it may not match the astounding longevity of legendary experiments on fertilizers on-going in the United Kingdom since 1843 or the famed 68-year Framingham study on cardiovascular disease, a 17-year study of cat flea resistance to insecticide, reported in June in the Journal of Medical Entomology, is still one for the ages.

Ed Ricciuti

Given that most studies of insect resistance to insecticides span only a couple of years and sample a couple of dozen populations, testing about a quarter-million eggs from thousands of flea populations in 10 countries at opposite ends of the Earth in five laboratories has been an undertaking of epic proportions. “We are unaware of any other similar effort to monitor the activity of any parasiticide of veterinary importance,” write the authors.

Organized and funded by the animal health division of the multi-national German corporation Bayer beginning in 1999, the international project established a way of monitoring potential resistance to the widely used insect neurotoxin imidacloprid among different populations of the cat flea (Ctenocephalides felis felis), worldwide the most important external parasite of cats and dogs. “The methods outlined in this paper,” say its authors, “should provide an acceptable protocol for testing many of the new active ingredients that have been registered for cat flea control.”

Ending in 2017, the study recruited veterinarians at clinics in the United States, the United Kingdom, Australia, Germany, France, Austria, Canada, Denmark, Italy, and the Netherlands to collect and send flea eggs to laboratories for testing. The laboratories were located at Auburn University in Auburn, Alabama; Bayer Animal Health laboratories in Monheim, Germany; Kansas State University in Manhattan, Kansas; the University of Queensland in Gatton, Australia; the Royal Veterinary College in Hertfordshire, United Kingdom; and the University of California Riverside in Riverside, California.

The researchers found no evidence that the flea developed resistance to the insecticide during almost two decades of research and, the paper states, the methods used in the study provide a model for testing resistance to new flea-control agents. Technically speaking, resistance is the ability developed by a strain of an organism to tolerate doses of an agent that would prove lethal to majority of individuals in a normal population of the same species.

Since resistance results from genetic changes that occur selectively over a relatively long period, research on the subject takes time. “Most studies of the development of insecticide resistance on a given insect are completed within two to three years,” says lead author Michael K. Rust, Ph.D., of UC Riverside and lead author on the report. “Depending on the insect and the tests conducted, maybe 20 to 30 populations might be tested. The bottom line is that it is hard to find funding for such efforts.”

In a 17-year study of cat fleas (Ctenocephalides felis felis), researchers tested egg samples—adult fleas being too difficult to collect and delicate to maintain—for signs of decreased susceptibility to the insecticide imidacloprid. The results showed no evidence of resistance to the insecticide among the tested cat flea populations, which came from 10 countries across the globe. (Photo credit: Michael K. Rust, Ph.D.)

Eggs from more than three thousand populations of fleas from cats and dogs were collected in the 10 countries involved. Researchers opted for eggs because adult fleas are too difficult to collect and delicate to maintain. Collecting the eggs required considerable hand labor and so much skill that some clinics had to make several tries before success. Veterinarians and clinic staff brushed them off infested animals in cages with a grated floor. Below the cage was paper onto which the eggs fell, then to be poured through a sieve and funnel into a glass tube. Most tubes were shipped in Styrofoam coolers with ice packs, although some were packed in bubble wrap and sent in an envelope by overnight mail. Veterinarians also filled out questionnaires on the history of each pet from which eggs were gathered.

The researchers exposed 2,200 of the samples, or isolates, to the insecticide. “A minimum number of eggs tested would be 40 per isolate for one test,” says Rust, but he adds that it is probable that “more like 250,000 to 350,000 eggs were tested.”

When an isolate was tested, some of the eggs were left untreated as a control. Eggs hatching proved the isolate was viable. Of the 2,200 tested, 1,837 were considered valid. Isolates that showed the potential of resistance were placed on cats to rear eggs for testing on varied concentrations of imidacloprid to develop a curve describing response to different doses. Results of testing proved to the satisfaction of researchers that the all of the isolates remained susceptible to the insecticide.

“I think it is pretty remarkable that the study showed no reduced susceptibility to imidacloropid over a 17-year period,” says Rust. “I think we have developed a procedure by which other insecticides used to control cat fleas can be monitored.”

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.