Table 1_Molecular investigation and viral load analysis of bovine respiratory syncytial virus in cattle with bovine respiratory disease complex in Inner Mongolia, China.docx

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.

牛呼吸道合胞病毒（Bovine respiratory syncytial virus，BRSV）是引发牛呼吸道疾病综合征（bovine respiratory disease complex，BRDC）的主要病原之一，在全球范围内造成大量发病病例与显著经济损失。然而，作为中国最大的肉牛产区之一的内蒙古地区，其BRSV的分子流行病学特征仍未得到充分阐明，尤其是在出现临床呼吸道症状的犊牛群体中。本研究针对BRSV的F基因与N基因开发了多重RT-qPCR（multiplex RT-qPCR）检测方法，并以ABL1作为内参基因，对该方法开展了初步评估，以实现对症状性犊牛的快速、可靠检测。本研究共对2023年采集的909份表现出呼吸道症状的犊牛临床样本进行了检测，结果显示，在出现临床呼吸道疾病的犊牛中，BRSV总体检出率为21.23%。其中肺组织的检出率达35.43%，高于鼻拭子样本的17.85%。该病毒在内蒙古中西部地区的检出频率更高，且其流行呈现夏季与冬季两个季节高峰。集约化大型养殖场的感染率与病毒载量均高于牧区散养户。病毒载量也随样本类型与季节存在差异，其中肺组织样本以及春夏季节的病毒载量最高。本研究结果为内蒙古地区临床发病犊牛的BRSV分子流行病学特征提供了基础数据；尽管该数据无法代表群体水平的流行率，但仍凸显了该症状性诊断队列中BRSV的检测负担，同时表明本研究开发的多重RT-qPCR检测方法可作为临床发病牛群或暴发调查中诊断筛查与针对性监测的有效工具。