# Cluster of Mycobacterium smegmatis mastitis cases in a dairy herd incorporating recycled manure solids bedding

## TL;DR

A study found a cluster of Mycobacterium smegmatis mastitis cases in a dairy herd linked to changes in barn management and bedding materials.

## Contribution

The study demonstrates the utility of whole genome sequencing for tracking non-tuberculous mycobacterial mastitis outbreaks.

## Key findings

- Mycobacterium smegmatis was isolated as the cause of mastitis in six cows over nine months.
- Whole genome sequencing revealed both genetic diversity and persistent strains in chronic infections.
- M. smegmatis mastitis cows tested positive for M. avium subspecies paratuberculosis antibodies, but no fecal evidence of the pathogen was found.

## Abstract

This study examined uncommon mastitis cases in a research barn incorporating recycled manure solids (RMS) as bedding. The cases occurred after barn renovation, including the conversion of the herringbone milking parlor to an automatic milking system (AMS) and the replacement of rubber mattress stalls with deep-bedded stalls maintained daily with RMS. Cows were milked at the herringbone milking parlor until the automatic milking system was available. Approximately 6 months after the implementation of AMS and deep-bedded stalls, the first two cows exhibited cow level somatic cell counts ≥ million cells/ml and palpable hardness in udder quarters manifested. However, commercial quantitative polymerase chain reaction testing, typically employed for mastitis diagnostics in Finland, did not identify pathogens in the quarter milk samples. Mycobacterium smegmatis was isolated through bacterial culturing. Within 9 months, five more M. smegmatis mastitis cases occurred in the dairy barn. Whole genome sequencing (WGS) of the isolates revealed considerable genetic diversity among strains. However, chronic infections in individual quarters caused by persistent strains were also detected. The WGS-based core-genome multilocus sequence typing approach demonstrated its efficacy as a robust tool for the molecular epidemiological exploration of bovine non-tuberculous mycobacterial mastitis. All lactating cows were tested for Mycobacterium avium subspecies paratuberculosis antibodies using an enzyme-linked immunosorbent assay test. Only the cows with M. smegmatis mastitis gave a positive reaction, although the causative agent of paratuberculosis was not detected in their fecal samples.