Resistance against macrocyclic lactones in Psoroptes ovis in cattle

Abstract

Background Psoroptic 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 the efficacy of macrocyclic lactone products in 16 beef farms in Belgium and The Netherlands in vivo and in vitro . Methods On each farm a group of animals (n= 7-14) with psoroptic mange was treated with two subcutaneous injections of a macrocyclic lactone product 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 (estimated proportion of the body surface affected by lesions), the proportion of the animals with negative mite counts after the first treatment round and the number of treatment rounds needed to obtain zero mites counts in all animals. A mite population from a given farm was categorized as sensitive when the mite count reduction after the first treatment round >95% and the lower limit of the uncertainty interval >90%. Resistance was detected when both parameters were below the threshold. Resistance was suspected when only one was below its threshold. In vitro knock-down and mortality was evaluated in a contact test. Results The proportion of the animals with negative mite counts after the first treatment round varied from 0-80%. All farms needed two or more rounds of treatments to obtain zero mite counts on all animals. Clinical index had a high variation and only started to reduce after the second treatment round. Mite populations from three farms were categorized as sensitive, one as suspected resistant and the other 12 as resistant. No correlation was found between in vitro lethal dose 50 and knock-down dose 50 values and any parameter of in vivo efficacy. Conclusions Unambiguous treatment failure was detected on 12/16 beef farms, confirming the presence of macrocyclic lactone resistance on Belgian Blue beef farms. In vitro parameters could not discriminate the farms based on their in vivo sensitivity. The mean reduction in mite counts and the lower limit of the confidence interval are proposed as most useful parameters to identify acaricide resistance.

Disease surveillance in England and Wales, March 2021

“…Control is challenging as macrocyclic lactones (ML) are the only licensed acaricide for Psoroptes species in the UK, but the response to treatment is often poor and they are even less effective if applied topically. Resistance of P ovis to MLs was reported by Wouter van Mol and others in Belgian blue cattle 2 …”