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

Mastitis continues to impose considerable economic losses on the dairy industry; however, the epidemiological characteristics and genetic diversity of <i>Nocardia cyriacigeorgica</i> 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 <i>N. cyriacigeorgica</i> 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 <i>N. cyriacigeorgica</i> 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 <i>N. cyriacigeorgica</i> in Chinese dairy herds and underscores the importance of environment-focused management strategies for reducing infection pressure and enhancing mastitis control.