One flock, four lactations: a longitudinal study of intramammary infections in meat-sheep production system

The economic and welfare impacts of intramammary infections make mastitis among the most significant diseases of dairy farming worldwide. However, mastitis in meat-producing sheep systems remains comparatively understudied. In this study we aimed to investigate the prevalence, temporal distribution and causative agents of intramammary infections in a meat sheep production system common in the UK. We followed the mammary health of 247 age- and breed-matched Scotch Mule ewes over their first four lactation cycles. Over the four lactations a total of 671 milk samples were collected at weaning or on occurrence of clinical mastitis. One-hundred and twelve bacterial isolates were derived from the milk samples with coagulase-negative Staphylococci representing 53.6% of the total, followed by Staphylococcus aureus (33%), Mannheimia spp. (7.1%) and Streptococcus spp. (4.5%) and Escherichia coli (1.8%). The proportion of subclinical to clinical disease changed significantly over years 1-4 with clinical disease more prevalent in Year 1 compared with subsequent years (6.3%, 1.7%, 2.9%, 2.2% respectively). Subclinical infections remained at similar levels for three lactation cycles before a significant increase in Year 4 (9.7%, 8.6%, 9.2%, 19.6%). Five S. aureus Multi-Locus Sequence Types (MLST) were identified including a novel sequence type officially named ST9241. No temporal change in MLST was found, but two types (ST1640 and ST1958) were exclusively associated with subclinical infections. These findings provide new insight into the prevalence and epidemiology of bacterial agents involved in intramammary infections in meat sheep and provides an archive of bacterial isolates to support future studies into mastitis pathogens.