Dynamics and determinants of Staphylococcus aureus carriage in livestock veterinarians: a prospective cohort study.

BACKGROUND: Since 2003, a new clade of methicillin-resistant Staphylococcus aureus (MRSA) belonging to clonal complex (CC) 398 and associated with animal husbandry has emerged in the Netherlands. The purpose of this study was to determine the dynamics of carriage in persons with direct contact to livestock. METHODS: A 2-year prospective cohort study was performed in which the anterior nares and oropharynx of 137 livestock veterinarians were sampled for the presence of S. aureus every 4 months during the first year and again 1 year later. All S. aureus isolates were genotyped by staphylococcal protein A (spa) typing and with multilocus variable-number tandem repeat analysis (MLVA). RESULTS: The mean prevalence of MRSA CC398 carriage was 44% (range, 42%-46%), and for S. aureus the prevalence was 72% (range, 69%-75%). Thirty-two veterinarians (23%) were always carrying MRSA CC398 and 18 of those (56%, 13% of all veterinarians) had identical MLVA types at all sampling moments. CONCLUSIONS: A high proportion of veterinarians had persistent MRSA CC398 carriage during the 2-year study period, indicating that this variant may colonize humans for prolonged periods. Furthermore, prevalence of S. aureus carriage was extremely high, indicating that MRSA CC398 is not replacing the susceptible strains, but comes on top of it.