Circulating microRNA profiles are associated with acute pain and stress in castrated and tail docked lambs

Maintaining animal welfare is an essential component of animal production systems. However, multiple measurements are required to inform an animal's welfare state as there are currently no universal measurement tools. Novel biomarkers are increasingly being explored as measures of stress, pain and disease status in livestock. Here we investigate host-encoded microRNAs (miRNAs) as biomarkers of stress and pain to determine the welfare of Australian Merino sheep (<i>Ovis aries</i>) following castration and tail-docking. This study used samples from a retrospective trial that compared the effectiveness of pain-relief given to lambs following castration and tail-docking. Plasma collected from lambs at 0 h and 30 mins that received no pain relief post treatment were investigated. Over 1100 novel miRNAs were identified from deep sequencing of small RNA isolated from serum samples. Altered expression of 18 miRNAs was observed in lambs post-castration and tail-docking, of which one miRNA (oar-miR-1-3p) was previously characterised. A supervised machine learning model identified a five-miRNA signature that classified post-castrated and tail-docked sheep from pre-treatment with 99% accuracy. This study describes the characterisation of circulating miRNAs in Merino sheep and demonstrates that miRNAs may have utility for objective measures of animal welfare status in relation to pain.