Resistance against macrocyclic lactones in Psoroptes ovis in cattle

Abstract

BackgroundPsoroptic mange is an important disease in beef cattle, and Belgian Blue cattle are particularly susceptible. Treatment failure of macrocyclic lactones against Psoroptes ovis has been reported, but clear evidence of in vivo resistance is still lacking. This study was conducted to investigate ML efficacy in 16 beef farms in Belgium and The Netherlands in vivo and in vitro. MethodsOn each farm a group of animals (n= 7-14) with clinical psoroptic mange was treated with two subcutaneous injections of a macrocyclic lactone with 7-10 days interval (15 farms) or a single injection with a long-acting macrocyclic lactone (1 farm). In vivo efficacy was assessed by the reduction in mite counts and clinical index (part of the body affected by lesions), the cure rate after the first treatment round and the number of treatment rounds needed to cure all animals. In vitro knock-down and mortality was evaluated in a contact test. ResultsCure rates after the first treatment round varied from 0-80%. All farms needed two or more rounds of treatments to obtain full efficacy. Clinical index had a high variation and only started to reduce after the second treatment round. Only three farms were categorized as susceptible with a mean mite count reduction>95% and a lower limit of the uncertainty interval>90%. One farm had a mean reduction>95%, but its lower limit of the confidence interval was <90%. All other farms had mean reductions<95% and lower limits of their uncertainty intervals<90%. No correlation was found between in vitro lethal dose 50 and knock-down dose 50 values and any parameter of in vivo efficacy. ConclusionUnambiguous treatment failure was detected on 12/16 beef farms, confirming the presence of macrocyclic lactone resistance in Belgian Blue beef farms. In vitro parameters could not discriminate the farms based on their in vivo susceptibility. The mean reduction in mite counts and the lower limit of the confidence interval stood out as most useful parameter to identify acaricide resistance.