Dynamic circulation and strain diversity of Mycoplasma ovipneumoniae in a low-biosecurity sheep flock

Mycoplasma ovipneumoniae is an important respiratory pathogen in sheep, commonly implicated in chronic pneumonia. Although widely reported, data on its occurrence, strain diversity and the relationship between upper respiratory tract carriage and pulmonary infection remains limited. The study examined a conventional flock of 230 Merino x Berrichon du Cher ewes. Thirty-eight sheep underwent physical examination and nasal swabbing, including 15 animals with additional post-mortem lung sampling. Additionally, lungs from 15 further sheep were examined, yielding 30 lung samples for gross and histopathological assessment. Microbiological examination was performed using cultivation procedures and isolates were identified by applying MALDI-ToF mass spectrometry. Genetic diversity of M. ovipneumoniae strains was assessed using a newly established core genome MLST scheme. Clinical findings were consistent with respiratory disease, and lung lesions were compatible with M. ovipneumoniae-associated pneumonia. M. ovipneumoniae was cultivated from all 38 nasal swabs (100%) and from 27 of 30 lung samples (90%). Cultivation of lung samples identified three principal pathogens: M. ovipneumoniae, Pasteurella multocida, and Mannheimia haemolytica, with concurrent detection observed in a subset of cases. Core genome sequence-based typing indicated both high genetic diversity and the clonal spread of individual M. ovipneumoniae strains on the farm. This study documents M. ovipneumoniae infection in an Austrian sheep flock and highlights the frequent involvement of co-pathogens and the high genotypic diversity of circulating strains. These findings underscore the importance of targeted surveillance, molecular characterisation, and integrated control measures to mitigate respiratory disease. Further multi-farm and longitudinal studies are warranted to clarify transmission dynamics and the clinical significance of strain variation.