Resistance to macrocyclic lactones in the parasitic mite Psoroptes ovis (Acari: Psoroptidae)

Abstract

Psoroptes ovis (Hering, 1858) is an astigmatid mite that causes the highly contagious and debilitating disease known as sheep scab. This ectoparasitic infection has high economic and welfare costs for British sheep farming.
The condition is augmented through a host hyper-immune response that instigates the production of an exudate, antigenic serum containing immunoglobulins in response to the mite recombinant protein, Pso o 2. This reaction causes intense pruritus and epidermal inflammation, which rapidly leads to severe deterioration in host condition. The most commonly used therapeutic treatment is topically or intramuscularly administered macrocylic lactones (MLs). Over the past decade there have been widespread reports of scab outbreaks that do not appear to respond to this group of compounds. A study by Doherty et al. (2018) was the first to demonstrate evidence of P. ovis resistance to the ML, moxidectin, in non-responding populations, through a formulated bioassay. The data described in the present study supports the preceding evidence of moxidectin resistance in these non-responding populations, whilst also indicating that there are varying levels of intraspecific resistance to all three commonly used ML treatment compounds: ivermectin, doramectin and moxidectin. Outbreak samples that did not show evidence of resistance (despite being secondary outbreaks) and inviable samples received due to storage or transport method, highlights the widespread insufficient knowledge and understanding of P. ovis, its control and management, within the agricultural community. This further demonstrates the importance of strong biosecurity and stringent monitoring of regional flock health, necessary to maintain sustainable low disease prevalence within the United Kingdom.

Date of Award	25 Jun 2019
Original language	English
Awarding Institution	University of Bristol
Sponsors	Zoetis ltd
Supervisor	Richard Wall (Supervisor)