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.)