Abstract
Bovine respiratory disease complex (BRDC), a multifactorial syndrome driven by viral and bacterial co-infections, poses significant challenges to cattle health in northern China. We performed a large-scale epidemiological and phylogenetic investigation (2022-2024) to identify BRDC pathogens in the region. A total of 5052 samples from symptomatic Holstein calves (nasal swabs, sera, tissues) were analysed using virological, bacteriological and molecular methods. Viral pathogens-bovine viral diarrhoea virus (BVDV), bovine respiratory syncytial virus (BRSV), bovine parainfluenza virus-3 (BPIV-3) and bovine herpesvirus-1 (BHV-1)-dominated infections, with pronounced seasonal peaks in colder months. The prevalence of non-cytopathic BVDV highlighted clinical difficulties. Bacterial infections involved Mycoplasma bovis (M. bovis), Mannheimia haemolytica and Pasteurella multocida, with M. bovis prevalent in mixed infections. Phylogenetic analysis revealed biological connections between Chinese isolates and international strains (e.g., BVDV subgenotypes 1a/1d and BPIV-3c), showing global pathogen flow. Transmission electron microscopy revealed the presence of viral popular structure. Meanwhile, seasonality strongly influenced viral dynamics, while bacterial detection remained stable, involving environmental and management factors. Pathogen co-infections demonstrate the complexity of BRDC. This study provides the first comprehensive BRDC pathogen profile for northern China, emphasising the need for region-specific control strategies, including multipathogen vaccines and improved biosecurity.