Prevalence and genetic characterization of bovine coronavirus identified from diarrheic pre-weaned native Korean calves from 2019 to 2021.

Bovine coronavirus (BCoV) is associated with severe diarrhea in calves, winter dysentery in adult cattle, and respiratory diseases in cattle. However, there is currently limited information regarding its molecular characterization in the Republic of Korea (KOR). Therefore, this study investigated the prevalence of BCoV in diarrheic pre-weaned calves (aged ≤60 days) and compared BCoV genome sequences identified globally. A total of 846 fecal samples were collected from calves with diarrhea across 100 beef farms in the KOR. The samples were divided into three groups based on age as follows: 1-10 days (n = 490), 11-30 days (n = 277), and 31-60 days (n = 79). BCoV infection was detected in 50 calves by real-time RT-PCR analysis. The results showed that the prevalence of BCoV was associated with calf age (P = 0.028) and was significantly higher in calves aged 31-60 days (odds ratio: 2.69, 95% confidence interval: 1.24-5.85; P = 0.012) than in those aged 1-10 days. Our findings show that BCoV is an important etiological agent of diarrhea in calves aged 31-60 days. Fifteen full genome sequences (2019-2021 variants) of the spike, hemagglutinin/esterase, and nucleocapsid were obtained from the 50 BCoV-positive samples. Phylogenetic analysis of each gene revealed that BCoVs circulating worldwide might have no boundary between enteric and respiratory tropisms, demonstrating the presence of three BCoVs groups: the classical, Asia/USA, and European. Initially, Korean BCoVs were originated from the USA, but diverged since the 1980s and rapidly evolved independently, unlike in other Asian countries. In this study, Korean BCoVs are more recent BCoVs and present relatively high nucleotide substitution rates in all genes compared with other BCoVs. Our results showed that the 2019-2021 variants undergo continuous genetic evolution and that there are genetic differences among globally distributed BCoVs.