Prevalence of Potential Virulence Genes in spp. Isolated from Cows with Clinical Mastitis on Large Chinese Dairy Farms.

spp. is a common cause of clinical mastitis (CM) in dairy cows. However, relatively less information is available about distribution of virulence factors of spp. isolated from cows with CM. Objectives of this study were, therefore, to determine the prevalence of hypermucoviscosity (HMV) phenotype, capsule serotypes, and potential virulence genes in spp. from cows in China with CM. A total of 241 spp. isolates were recovered from cows with CM on 123 dairy farms (each had >500 lactating cows) located in 13 provinces of China. Of the isolates, 124 (51%) and 117 (49%) were identified as and , respectively. The prevalence of HMV was 16% for and 11% for ; B (78%), H1 (55%), (31%), and D (24%) were the prevalent virulence genes among , whereas B (50%), H1 (30%), and D (22%) were prevalent in . Prevalence of the gene was higher for (78%) than for (13%), whereas the gene was more prevalent in than in (12% and 1%, respectively). Fifty-six isolates were confirmed as K57, the most prevalent capsule serotype (45%). Twenty-one (18%), 20 (10%), and 9 (8%) of 117 isolates were positive for K57, K5, and K54 serotypes, respectively. As the predominant serotype, K57 isolates had a higher prevalence of the HMV phenotype and H1 than non-K57 . In conclusion, virulence factors were commonly detected for both and causing CM in Chinese dairy herds. HMV isolates were commonly identified, irrespective of species. In addition, as the predominant capsule in bovine , the K57 serotype may be better adapted to the udder environment; therefore, further studies targeting pathogenicity to mammary tissue should contribute new knowledge for vaccine development using this serotype.