Frequency of Bovine Respiratory Disease Complex Bacterial and Viral Agents Using Multiplex Real-Time qPCR in Quebec, Canada, from 2019 to 2023.

The bovine respiratory disease complex (BRD) is a multifactorial disease caused by various bacterial and viral pathogens. Using rapid pathogen detection techniques is helpful for tailoring therapeutic and preventive strategies in affected animals and herds. The objective of this study was to report the frequency of 10 pathogens by multiplex RT-qPCR on samples submitted for BRD diagnosis to a diagnostic laboratory (Biovet Inc., QC, Canada) in the Province of Quebec, Eastern Canada. From the 1st of January 2019 to the 31st of December 2023, a total of 1875 samples were analyzed. Most samples collected were individual samples (1547 of 1860 samples for which information was available (83.17%)), and the rest were from pooled samples of 2 (8.55%, = 159) or ≥3 specimens (8.28%, = 154). In 19.3% of the samples ( = 362), no pathogen was found, whereas 54.1% of samples had two or more different pathogens. Among the viruses, bovine coronavirus (BCV) was the most commonly found (27.5% of samples, = 516), followed by bovine respiratory syncytial virus (BRSV) (17.7%, = 332), whereas, for bacteria, (50.1%, = 940) and (26.9%, = 505) were the most common. The frequency of samples positive for , , influenza type D virus (IDV), bovine parainfluenza virus type 3 (BPI3V), bovine herpesvirus type 1 (BHV1), and bovine viral diarrhea virus (BVDV) was 22.6%, 22.4%, 4.6%, 4.3%, 2.7%, and 0.9%, respectively. In the multivariable Poisson regression model, the total number of pathogens increased with the number of animals in the pool, with an incidence risk ratio (IRR) of 1.15 (95% CI 1.02-1.29) and 1.32 (1.18-1.47) for 2 individuals in the pool and ≥3 individuals vs. individual samples, respectively. An increased number of pathogens were isolated in the winter season (IRR = 1.28 (95% CI 1.17-1.40)) compared to fall, and a lower number of pathogens were isolated in the summer compared to fall (IRR = 0.82 (95% CI 0.73-0.92)). These seasonal differences were mostly driven by the number of viruses isolated. This study gives interesting insights on the circulation of BRD pathogens in cattle from Eastern Canada.