In vitro activity of fluralaner and commonly used acaricides against Dermanyssus gallinae isolates from Europe and Brazil.
Background: The poultry red mite Dermanyssus gallinae negatively impacts bird welfare and health, and interferes with egg production and quality, while emerging acaricide resistance limits control options. Fluralaner, a novel miticide for administration in drinking water, is approved for control of D. gallinae infestations. Mite sensitivity testing is relevant to gauge field isolate susceptibility to available treatments. Methods: Thirteen D. gallinae isolates collected during 2014 through 2016 from farms in Germany, France, Spain and Brazil, and a 2001 laboratory-maintained isolate were used for acaricide contact sensitivity testing. Tested compounds were cypermethrin, deltamethrin, phoxim, propoxur, and the recently available acaricides, spinosad and fluralaner. In each study, at least one isolate was exposed to increasing concentrations of at least one acaricide. In one study, additional testing determined the sensitivity of the 2001 isolate to fluralaner using a mite-feeding test, and of fluralaner, phoxim and spinosad using an immersion test. At least two replicates were used for each dilution. Vehicle and untreated controls were also included. Results: Based on 90% mortality (LC90) values, the laboratory isolate was susceptible to fluralaner (15.6-62.5 parts per million, ppm), phoxim (<500 ppm), propoxur (<125 ppm), and deltamethrin (500-1000 ppm). All field isolates remained sensitive to fluralaner concentrations ≤125 ppm. Spinosad LC90 values for laboratory and field isolates ranged between 2000-4000 ppm. For phoxim, relative to the laboratory isolate, there was reduced sensitivity of two German isolates (LC90 up to 4000 ppm) and two French isolates (>4000 ppm). An isolate from Spain demonstrated reduced sensitivity to phoxim, propoxur and deltamethrin; an isolate from Brazil showed reduced sensitivity to propoxur and cypermethrin. Mite LC90 when exposed to fluralaner by blood feeding was <0.1 ppm. Conclusions: Contact sensitivity testing indicated apparent resistance to at least one of phoxim, deltamethrin, cypermethrin and propoxur in 13 field isolates from Europe and Brazil. All isolates were highly susceptible to fluralaner. Fluralaner was approximately 1000 times more active by feeding than by contact. Fluralaner's distinct mode of action and efficacy against isolates largely refractory to those acaricides, makes it a promising option for the control of D. gallinae infestations of poultry.