Zoonotic transmission of asymptomatic carriage Staphylococcus aureus on dairy farms in Canterbury, New Zealand.

Zoonotic pathogen transmission is of growing concern globally, with agricultural intensification facilitating interactions between humans, livestock and wild animals. Staphylococcus aureus is a major human pathogen, but it also causes mastitis in dairy cattle, leading to an economic burden on the dairy industry. Here, we investigated transmission within and between cattle and humans, including potential zoonotic transmission of S. aureus isolated from cattle and humans from three dairy farms and an associated primary school in New Zealand. Nasal swabs (N=170) were taken from healthy humans. Inguinal and combined nasal/inguinal swabs were taken from healthy cattle (N=1163). Whole-genome sequencing was performed for 96 S. aureus isolates (44 human and 52 cattle). Multilocus sequence typing and assessments of antimicrobial resistance and virulence were carried out. Potential within- and across-species transmission events were determined based on single nucleotide polymorphisms (SNPs). Thirteen potential transmission clusters were detected, with 12 clusters restricted to within-species and one potential zoonotic transmission cluster (ST5). Potential transmission among cattle was mostly limited to single age groups, likely because different age groups are managed separately on farms. While the prevalence of antimicrobial resistance (AMR) was low among both bovine and human isolates, the discovery of an extended-spectrum beta-lactamase gene (bla TEM-116) in a bovine isolate was concerning. This study provides evidence around frequency and patterns of potential transmission of S. aureus on dairy farms and highlights the AMR and virulence profile of asymptomatic carriage S. aureus isolates.