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

## TL;DR

This study identifies and genetically characterizes Subgroup III of BRSV from a dairy cattle outbreak in Thailand, providing insights for disease control.

## Contribution

The first molecular detection and phylogenetic analysis of BRSV in a Thai dairy outbreak.

## Key findings

- BRSV Subgroup III was confirmed as the cause of a respiratory disease outbreak in dairy cattle in Thailand.
- Thai BRSV isolates showed 100% nucleotide identity among themselves and clustered closely with subgroup III strains globally.
- Key antigenic motifs were conserved, indicating genetic stability of the subgroup III BRSV.

## Abstract

Bovine respiratory syncytial virus (BRSV) is a significant pathogen contributing to the bovine respiratory disease complex, a major cause of morbidity and economic losses in both dairy and beef cattle worldwide. In Thailand, previous research on BRSV has been mainly limited to seroprevalence studies. This report identifies Subgroup III of BRSV as the causative agent of a respiratory disease outbreak in a dairy farm in Photharam District, Ratchaburi Province. It is the first molecular detection of BRSV associated with a respiratory disease outbreak in dairy cattle in Thailand. Understanding the genetic and phylogenetic characterization of BRSV is crucial for developing effective biosecurity measures and vaccination strategies.

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.