STAPHYLOCOCCUS AUREUS ADLB GENE IS ASSOCIATED WITH HIGH INTRAMAMMARY INFECTION PREVALENCE IN DAIRY HERDS OF NORTHERN ITALY: A CROSS-SECTIONAL STUDY

Staphylococcus aureus is a major mastitis pathogen in dairy cattle worldwide, responsible for relevant economic losses. Environmental factors, milking routine and good maintenance of milking equipment were described as important factors to prevent intra-mammary infections (IMI). Staph. aureus IMI could be widespread within the farm or the infection could be limited to few animals. Several studies reported that Staph. aureus genotypes differ in their ability to spread within a herd. In particular, Staph. aureus belonging to Ribosomal Spacer PCR (RS-PCR) genotype B (GTB) / Clonal Complex 8 (CC8) was associated with high within-herd prevalence of IMI, while other genotypes were generally associated with individual cow disease. The adlb gene seems to be strictly related to Staph. aureus GTB/CC8, being a potential marker of contagiousness. We investigated Staph. aureus IMI prevalence in 60 herds in Northern Italy. In the same farms, we assessed specific indicators linked to milking management (e. g. teat condition score and udder hygiene score) and additional milking risk factors for IMI spread. RS-PCR and adlb-targeted PCR were performed on 262 Staph. aureus isolates, of which 77 underwent MLST. A predominant genotype was identified in most of the herds (90%), especially Staph. aureus CC8 (30%). In 19 out of 60 herds the predominant circulating Staph. aureus was adlb-positive and the observed IMI prevalence was relevant. Moreover, this gene was detected only in CC8 and CC97 genotypes. The statistical analysis showed a strong association between the prevalence of Staph. aureus IMI and both the specific CCs and the carriage of adlb gene, with the predominant circulating CC and the presence of the gene alone explaining the total variation. Interestingly, the difference in the odds ratio obtained in the models for CC8 and CC97 suggests that the circulation of these CCs does not lead to higher Staph. aureus within-herd prevalence per se, but by means of the adlb-gene they carry. In addition, the model showed that environmental and milking management factors had no or only a minimal impact on Staph. aureus IMI prevalence. In conclusion, we can state that the circulation of adlb-positive Staph. aureus strains within a herd has a strong impact on the prevalence of IMI. Thus, adlb can be proposed as a genetic marker of contagiousness for Staph. aureus IMI in cattle. However, further analyses using whole genome sequencing are required to understand the role of genes, ot