Sheep scab: comparison of spatial and temporal patterns determined by clinical diagnosis or ELISA

Background: Ovine psoroptic mange (sheep scab) is an infectious condition caused by an allergen-induced hypersensitivity response to the mite, Psoroptes ovis. Infestation results in clinical disease, economic loss and welfare issues in many sheep-producing countries. The study aim was to compare the prevalence and spatial pattern of sheep scab on contiguous farms, using both self-reported clinical outbreak history (2012-2020) and serological ELISA testing (2021/22).

Methods: Farms included in the study were in three regions of known high scab prevalence in North, Central and Southwest England; 254 farms completed both a questionnaire, giving their clinical scab history, and ELISA blood testing.
Results: A scab outbreak was reported by 17.4% (CI ±4.6, n=48) of farms in 2020 based on clinical diagnosis; scab was diagnosed by the ELISA on 25.6% (±5.5%, n=65) of farms in 2021/22. Comparison of self-reported clinical scab with the ELISA test results identified a group of farms (n=52) that did not report scab in 2020, or in some cases did not report having scab over the previous 8 years (n=20), but which were nevertheless seropositive in 2021/22. 
Conclusion: A small number of flocks, particularly those using common grazing in northern England, where handling is infrequent, often with less susceptible sheep breeds, may have persistent scab infestations that are generally undetected by clinical inspection. The data highlight the advantages of serological testing to identify exposure to scab in flocks where clinical signs are less easily detected.