Sheep scab

: comparison of spatial and temporal patterns determined by clinical and serological diagnosis.

Abstract

The prevalence and spatial clustering of ovine psoroptic mange (sheep scab), in areas previously identified as at higher relative risk, are compared, using retrospective reporting of clinical outbreaks (2012-2020) and serological testing using the flock ELISA (2021/22). The study included 15 clusters of contiguous/connected farms (or with shared grazing) in three study regions in England: the North, Midlands and Southwest. A retrospective questionnaire was completed by 276 farmers; of these 254 completed subsequent ELISA blood testing.
A clinical scab outbreak was reported by 17.4% (CI ±4.6, n=48) of farms in 2020, while 25.6% (±5.5%, n=65) of farms tested positive with the ELISA. A small number of farms reported frequent clinical scab outbreaks (4 farms reported a scab outbreak every year surveyed (2012-2020), and these farms are likely to act as a persistent source of infection to contiguous farms.
Comparison of the clinical scab history and serologically determined outbreaks on individual farms identified a group of farms (n=52) that did not report clinical scab in 2020, or in some instances, the previous 8 years (n=20), but were subsequently found to be seropositive with the ELISA. Further analysis determined that farms in this group were predominately in the North (44%), and
most of those used common grazing. It is suggested that this pattern may have been observed because these flocks contain less clinically susceptible hill breeds and are handled less frequently, resulting in persistent scab outbreaks remaining undetected for long periods. Spatial clustering analysis found very little evidence to suggest localised clustering. It is suggested that the fine-scale movement and complexity of land use by farmers at an individual level may have disrupted any localised spatial clustering that would demonstrate farm-to-farm transmission.
It is concluded that the use of the ELISA blood test for scab diagnosis should be an important element of future management strategies.

Date of Award	6 Dec 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Richard Wall (Supervisor) & Emily J Nixon (Supervisor)