Isolation and Characterization of a Bovine Viral Diarrhea Virus Subgenotype 1b Strain from Diarrheal Cattle in Inner Mongolia, China

Bovine viral diarrhea virus (BVDV) is a major pathogen threatening the global cattle industry, causing severe economic losses. In China, BVDV is widely prevalent, with BVDV-1 being the main circulating genotype. Herein, a cytopathic (CP) BVDV strain, designated BVDV-HH03, was isolated and identified from diarrheic calves on a dairy farm in Hohhot, Inner Mongolia, via RT-PCR, Madin–Darby Bovine Kidney (MDBK) cell isolation, and neutralization assays. Subsequent whole-genome sequencing and phylogenetic analysis identified the isolate as belonging to the BVDV-1b subgenotype, sharing the highest nucleotide identity (>99%) with the BVDV-1 Nebraska strain isolated in the USA in 1990. Preliminary calf infection experiments indicated that BVDV-HH03 exhibited low pathogenicity under the experimental conditions. Two groups of 2-month-old calves received BVDV-HH03 inoculation by the combined intranasal–oral route; each group received inoculum with a titre of 106.5 or 107.5 TCID50/mL at an inoculation volume of 7 mL per calf. No obvious clinical signs or fever were observed in inoculated calves throughout the 14-day monitoring period. Nevertheless, all infected calves showed decreased peripheral white blood cell counts within the 8-dpi monitoring period, with a maximum reduction exceeding 40%, and viremia was detected in 3/5 and 4/5 animals from the low- and high-dose inoculation groups by RT-PCR, respectively. This study represents the first isolation of BVDV-1b in Inner Mongolia, providing valuable data for the prevention and control of BVDV, as well as for further research on the epidemiology and pathogenicity of BVDV-1b.