# Evaluation of Nasal Microbial Communities of Beef Calves During Pre-Weaning Outbreak of Bovine Respiratory Disease

## TL;DR

This study found that severe bovine respiratory disease in calves is linked to lower nasal bacterial diversity and overgrowth of harmful Mycoplasma, which may help improve disease prevention strategies.

## Contribution

The study reveals that reduced nasal microbial diversity and dominance of Mycoplasma species are associated with severe respiratory disease outbreaks in calves.

## Key findings

- Calves in severe outbreaks had lower nasal bacterial diversity compared to those in moderate outbreaks.
- Bacterial diversity increased after treatment, indicating recovery of microbial balance.
- Mycoplasma species, particularly Mycoplasma bovirhinis, dominated during disease outbreaks.

## Abstract

Bovine respiratory disease is one of the most costly and common health problems affecting young beef calves, causing sickness, death, and significant economic losses for cattle producers. While many bacteria live naturally in the nasal cavity of healthy calves, some can become harmful under stressful conditions and trigger disease outbreaks. This study examined the nasal microbiome of calves during two respiratory disease outbreaks before weaning, one moderate and one more severe, and then again one month after treatment. We found that calves in the more severe outbreak had less variety in their nasal bacteria compared to calves in the moderate outbreak. In both groups, bacterial diversity increased after treatment, suggesting recovery of a healthier microbial balance. During outbreaks, certain harmful Mycoplasma species were most prominent. These findings suggest that a loss of bacterial diversity and an overgrowth of harmful Mycoplasma are linked to respiratory disease in calves. Understanding how these microbial communities shift during disease may help develop better ways to prevent and manage respiratory disease in cattle, ultimately improving animal health and reducing economic losses.

Bovine respiratory disease complex (BRDC) is a leading cause of morbidity and mortality in pre-weaned calves, yet the role of commensal nasal microbiota in outbreak severity remains poorly understood. This study characterized nasal bacterial communities during two BRDC outbreaks of differing severity (moderate vs. severe) and at ~30 days post-treatment. Nasal swabs were collected from calves and analyzed using 16S rRNA gene sequencing (V1–V3 regions, Illumina MiSeq) and quantitative PCR targeting three major BRDC pathogens. Microbial community profiles differed between outbreak groups and across timepoints. Calves in the severe outbreak group exhibited lower microbial diversity compared to those in the moderate outbreak. In both groups, diversity significantly increased from outbreak to post-treatment. At the time of disease, nasal communities were dominated by the genera Mycoplasmopsis, Mesomycoplasma, and Caviibacter, with qPCR confirming Mycoplasma bovirhinis as the predominant species. These findings indicate that BRDC outbreaks in pre-weaned calves are associated with reduced microbial diversity and the dominance of pathogenic Mycoplasma species, with recovery characterized by greater bacterial diversity. Shifts in nasal microbiome composition between outbreak and post-treatment may reflect pathogen-driven disruption during disease and subsequent microbial community rebalancing.