Spot-on products have dramatically changed flea control in small animals. Development of resistance in flea populations is a valid concern, as indicated by previous experience with carbamates, organophosphates, pyrethroids, and pyrethrins. Resistance to neonicotinoid compounds (eg, imidacloprid) has been shown in some agricultural pests. This study reports on a multinational effort to monitor susceptibility of Ctenocephalides felis to imidacloprid. Flea samples from veterinary clinics in Australia, Germany, France, the UK, and 29 U.S. states were collected from 2002 to 2009. Participating clinics collected flea eggs from infested cats and dogs according to a standard protocol and express-shipped them to a diagnostic laboratory that used an established diagnostic dose protocol to determine susceptibility. Flea eggs were placed on larval flea–rearing medium containing imidacloprid (3 ppm) and incubated appropriately, and adult fleas were counted. Of 72,489 eggs in the 882 isolates tested, only 372 individual fleas from 22 isolates survived to adulthood. The bioassay was repeated for isolates showing > 5% survival. None of these subsequently exceeded this 5% survival threshold, suggesting that survival was a chance effect rather than a systematic shift in susceptibility.

Commentary: I remember when organophosphates and lindane were used for flea control! The development of spot-on treatments has increased the quality of life for pets and people worldwide. Vigilant monitoring of susceptibility is important because experience has shown that development of resistance to insecticides is a real concern. To date there have been only sporadic reports of resistance to fipronil and imidacloprid. Molecular analysis of flea isolates has identified alleles conferring cyclodiene resistance and potential cross-resistance to fipronil. The bioassay used in this near decade-long study is of particular interest because the method for collection of field samples is relatively simple. If a cluster of reports of resistance develops, fleas can be collected from the source and shipped to a designated laboratory so a bioassay can be conducted.—Karen A. Moriello, DVM, Diplomate ACVD

Large-scale monitoring of imidacloprid susceptibility in the cat flea, Ctenocephalides felis. Rust MK, Denholm I, Dryden MW, et al. MED VET ENTOMOL 25:1-6, 2011.