# Outbreak of Respiratory Disease Due to Bovine Respiratory Syncytial Virus with Concomitant Infections by Histophilus somni and Pasteurella multocida in Adult Dairy Cows and Calves from Southern Brazil

## TL;DR

A respiratory disease outbreak in adult dairy cows and calves in Brazil was caused by BRSV along with other bacteria, and a new PCR test was used to detect the pathogens.

## Contribution

The first use of a multiplex PCR assay for diagnosing bovine respiratory disease in Brazil, highlighting BRSV and co-infections in adult cattle.

## Key findings

- BRSV, Histophilus somni, and Pasteurella multocida were detected in nasal and oral swabs from affected cows.
- Histopathology and IHC confirmed BRSV antigens in the lungs of a deceased cow.
- The multiplex PCR assay effectively detected and quantified BRD pathogens with higher nasal shedding of BRSV.

## Abstract

There are several reports of clinical outbreaks of Bovine Respiratory Syncytial Virus (BRSV) associated disease in calves but limited documented information in adult cattle. This report describes an outbreak of respiratory disease in adult dairy cows from Southern Brazil due to simultaneous infections by BRSV, Histophilus somni, and Pasteurella multocida. Nasal discharge, cough, and dyspnea were present in all categories of animals; two cows had clinical manifestations suggestive of subcutaneous emphysema. Paired oral and nasal swabs were used in quantitative multiplex PCR assays designed to identify pathogens of bovine respiratory disease (BRD). Organs from one cow that died was used in histopathological and immunohistochemical (IHC) analyses. Histopathology revealed interstitial pneumonia with intralesional tissues antigens of BRSV by IHC. The multiplex PCR assay detected elevated values of BRSV in most animals, with varying loads of H. somni and P. multocida. This PCR assay can be used for the fast and accurate detection of pathogens associated with BRD and represents the first utilization of multiplex PCR assay for the diagnosis of BRD in Brazil.

Although bovine respiratory syncytial virus (BRSV) is a key contributor to bovine respiratory disease (BRD) worldwide, there are few detailed reports of BRSV-related outbreaks in Brazil. This study describes the clinical, pathological, immunohistochemical (IHC), and molecular findings from a BRD outbreak in adult dairy cows from Southern Brazil. The affected cattle had dyspnea, nasal discharge, and coughing. One cow died, and samples were collected for diagnosis. Histopathology revealed interstitial pneumonia with multinucleated giant cells. IHC identified BRSV antigens in pulmonary tissue. A multiplex real-time PCR identified BRSV, Histophilus somni, and Pasteurella multocida in nasal and oral swabs, while only BRSV and H. somni were detected in the tissues of the cow that died. All animals had co-infections involving BRSV. The average cycle threshold (Cq) values for BRSV were 27.43 (nasal) and 32.68 (oral), with significant differences (p = 0.016), indicating higher nasal shedding. This qPCR assay was effective for detecting BRD pathogens, the quantification of viral and bacterial loads in animals with BRD and can be used for the rapid detection of respiratory pathogens. The elevated BRSV detection in oral samples suggests that this route may be an alternative for the collection of samples in cattle with profuse nasal discharge.