Research over 10 years has established the high risk of occupational exposure to MRSA in people working with pigs. However the nature of nasal colonization with livestock associated MRSA (LA-MRSA) and its health implications remain poorly understood. Multiple studies confirmed that detection rates in pigs, farm workers and veterinarians were higher than in people without animal exposure, but elevated risk of infection in these groups has not been clearly shown. We conducted a longitudinal study for 18 months to describe patterns of S.aureus (SA) colonization and infection in swine veterinarians. Overall, the monthly prevalence of SA and MRSA from nasal swabs of veterinarians was 58 to 82%, and 6 to15%, respectively. Predominant spa types were t034 (ST398, 50%), t002 (ST5, 25%) and t337 (ST9, 18%), a distribution similar to that found in a concurrent study in pigs in the USA. Despite this high level of exposure, and frequent occurrence of minor injuries in the cohort, no confirmed cases of infection were reported. Based on detection patterns, veterinarians were classified into three groups: Persistent carrier (PC, 38%), Intermittent carrier (IC, 61%) and Non-carrier (NC, 1%). Based on one-time quantitative testing of nasal swabs without enrichment, PC veterinarians carried significantly higher numbers of SA than IC. Among IC veterinarians, culture positivity was significantly associated with recent contact with pigs. Elevated prevalence of SA and MRSA in US veterinarians appears to be a consequence of exposure to pigs, however MRSA prevalence was much lower than seen in a similar study in Holland. Exposure did not lead to prolonged colonization in most subjects, and the higher numbers of SA in PC subjects suggests that unknown host factors may determine the likelihood of prolonged colonization by SA of livestock origin. Although the period of follow up was limited, the absence of clinical infections despite regular exposure suggests that major health risks due to livestock associated SA are unlikely. Introduction