# Genetic Diversity of Nocardia cyriacigeorgica Isolated from Bovine Mastitis in Two Chinese Dairy Herds

## TL;DR

This study investigates the genetic diversity of Nocardia cyriacigeorgica in Chinese dairy herds, finding limited genetic variation and high environmental presence, suggesting the need for improved hygiene to control mastitis.

## Contribution

The study provides the first comprehensive characterization of Nocardia cyriacigeorgica's population structure and environmental distribution in Chinese dairy farms.

## Key findings

- N. cyriacigeorgica was detected in 21.9% of environmental samples, with highest rates on teat skin, nasal swabs, and milking equipment.
- Genetic analysis revealed limited diversity, with isolates clustering into a few closely related lineages.
- The findings suggest repeated cow exposure due to persistent environmental reservoirs of the pathogen.

## Abstract

Mastitis remains one of the most prevalent and economically important diseases in the dairy industry. Among the diverse pathogens associated with mastitis, Nocardia cyriacigeorgica has been increasingly recognized in several countries; however, its occurrence on farms and the extent of strain variation have not been well characterized. In the present study, two large Chinese dairy herds experiencing mastitis outbreaks were systematically investigated. Milk samples were collected from cows with clinical, subclinical, and healthy udders, alongside a wide range of environmental samples. N. cyriacigeorgica was detected in both milk and environmental sources, with the highest detection rates observed on teat skin, nasal swabs, and milking equipment. Analyses of genetic diversity showed that the isolates were highly similar, suggesting repeated exposure to contaminated environments. These findings provide new insight into the ecology of N. cyriacigeorgica in dairy herds and highlight the importance of environmental management and hygiene practices in reducing infection pressure and improving udder health.

Mastitis continues to impose considerable economic losses on the dairy industry; however, the epidemiological characteristics and genetic diversity of Nocardia cyriacigeorgica remain poorly characterized. In the present study, two large-scale Chinese dairy farms experiencing mastitis outbreaks were investigated, with a total of 666 milk and 288 environmental samples collected. From the milk samples, 85 isolates of N. cyriacigeorgica were identified, corresponding to detection rates of 22.7% in clinical mastitis, 11.2% in subclinical mastitis, and 28.0% in healthy milk. Whole genome sequencing combined with multilocus sequence analysis demonstrated markedly limited genetic diversity, with the majority of isolates clustering within a few closely related lineages (Simpson’s Index: 0.12 for clinical and 0.037 for subclinical samples). Furthermore, qPCR screening detected N. cyriacigeorgica in 21.9% of environmental samples, with the highest detection frequencies observed on nasal swabs (62.5%), teat skin (56.3%), interdigital spaces (56.3%), and milking equipment (45.8%). Taken together, these findings are consistent with shared environmental exposure within farms and suggest a role for persistent reservoirs in repeated cow exposure. This investigation provides a comprehensive characterization of the population structure and distribution of N. cyriacigeorgica in Chinese dairy herds and underscores the importance of environment-focused management strategies for reducing infection pressure and enhancing mastitis control.