Molecular Characterization and Phylogenetic Analysis of Subgroup III Bovine Respiratory Syncytial Virus from a Dairy Outbreak in Thailand.

Bovine respiratory syncytial virus (BRSV) is a major viral pathogen associated with bovine respiratory disease (BRD), a leading cause of illness and economic loss in cattle worldwide. In June 2022, an acute respiratory outbreak occurred in a dairy herd in Photharam District, Ratchaburi Province, Thailand, affecting 25 of 103 cows (24.3%) and resulting in three deaths (2.9%). This study aimed to confirm BRSV as the etiological agent of the outbreak and to genetically and phylogenetically characterize Thai BRSV strains using partial G gene sequencing. Clinical signs included fever, nasal discharge, coughing, and subcutaneous emphysema. Nested reverse transcription-polymerase chain reaction (nested RT-PCR), a sensitive method for detecting viral RNA and targeting the F and G genes, confirmed BRSV in all samples. At the same time, bovine parainfluenza virus type 3 (BPIV-3), bovine viral diarrhea virus (BVDV), and bovine herpesvirus 1 (BoHV-1) were not detected. Phylogenetic analysis of partial G gene sequences showed that all Thai isolates clustered closely within subgroup III, with 100% nucleotide identity among themselves and 85.9-97.7% similarity to subgroup III strains from other countries. Amino acid alignment indicated conservation of key antigenic motifs, including the cysteine noose, with only minor substitutions compared to some foreign strains. This study provides the first genetic and phylogenetic characterization of BRSV in Thailand, highlighting the genetic stability of subgroup III and providing baseline molecular data to support regional surveillance, diagnostics, and vaccine strategies.