Stage-Specific Associations Between Respiratory Pathogens and Mortality in Sheep: A Multivariate Analysis

Ovine Respiratory Complex (ORC) is a major cause of lamb mortality in intensive and semi-intensive sheep production systems. Its multifactorial etiology, involving bacterial and viral pathogens together with host and environmental factors, complicates disease control and limits the effectiveness of conventional preventive measures. Although respiratory pathogens are commonly detected in healthy lambs, their contribution to mortality at different production stages remains poorly understood. This study evaluated the relationships between respiratory pathogens and lamb mortality using canonical multivariate analysis. A total of 200 bronchoalveolar lavage samples were collected from apparently healthy lambs aged 50-60 days during winter (n = 80) and summer (n = 120) on 10 commercial sheep farms in southwestern Spain. Samples were analyzed by real-time PCR for the detection of major bacterial and viral respiratory pathogens, and mortality records from farms and feedlots during 2022-2023 were incorporated into the analysis. A high herd-level prevalence was detected for Pasteurella multocida, Mannheimia haemolytica, Bibersteinia trehalosi, and Mesomycoplasma ovipneumoniae, whereas Histophilus somni, Orf virus, and parainfluenza virus type 3 showed intermediate prevalence. No significant seasonal differences were observed in pathogen prevalence. ORC-associated pathogens accounted for approximately 20% of the variance in mortality, suggesting that non-infectious factors also play an important role in lamb losses. Canonical analysis identified stage-specific multivariate patterns, with M. ovipneumoniae and P. multocida contributing most strongly to the canonical dimension characterized by on-farm mortality, whereas M. haemolytica, together with P. multocida and parainfluenza virus type 3 (PIV3), contributed most strongly to the dimension characterized by feedlot mortality.These findings improve our understanding of the epidemiology of ORC and support the development of targeted surveillance, prevention, effective vaccination of ewes against the serotypes circulating within farms, and management strategies to reduce lamb mortality, improve animal welfare, and enhance the sustainability of sheep production.