Molecular investigation and viral load analysis of bovine respiratory syncytial virus in cattle with bovine respiratory disease complex in Inner Mongolia, China.

Bovine respiratory syncytial virus (BRSV) is a major pathogen of the bovine respiratory disease complex (BRDC), causing significant morbidity and economic losses worldwide. However, its molecular epidemiology in Inner Mongolia, one of China's largest cattle-producing regions, remains poorly characterized, particularly among calves with clinical respiratory disease. In this study, we developed and performed a preliminary evaluation of a multiplex RT-qPCR assay targeting the F and N genes of BRSV, with ABL1 as an endogenous control, for rapid and reliable detection in symptomatic calves. A total of 909 clinical samples collected in 2023 from calves showing respiratory signs were tested, revealing an overall BRSV detection rate of 21.23% among clinically affected calves with respiratory disease. Lung tissues showed higher detection rates (35.43%) than nasal swabs (17.85%). The virus was more frequently detected in central and western regions, and its occurrence exhibited seasonal peaks in summer and winter. Intensive and large-scale farms tended to have higher infection rates and viral loads than pastoral households. Viral load also varied by sample type and season, being highest in lung tissues and during spring-summer. These findings provide baseline molecular epidemiological data on BRSV among clinically affected calves in Inner Mongolia and, although they are not representative of population-level prevalence, they highlight the burden of BRSV detection within this symptomatic diagnostic cohort and suggest that the developed multiplex RT-qPCR assay may serve as a useful tool for diagnostic screening and targeted monitoring in clinically affected herds or outbreak investigations.