Prevalence of colonization by methicillin-resistant Staphylococcus aureus ST398 in pigs and pig farm workers in an area of Catalonia, Spain

Background: A livestock-associated clonal lineage (ST398) of methicillin-resistant Staphylococcus aureus (MRSA) has been identified causing colonization or infection in farm workers. The aim of the study was to analyze the prevalence of MRSA-ST398 colonization in pigs and in pig farmers in an area with a high pig population (Osona, Barcelona province, Catalonia, Spain). Methods: We performed a cross-sectional prevalence study in Osona (Catalonia, Spain), from June 2014 to June 2015. All pig farm workers from 83 farms were studied. Twenty of these farms were randomly selected for the study of both pigs and farmers: 9 fattening and 11 farrow-to-finish farms. All workers over the age of 18 who agreed to participate were included. Samples were analyzed to identify MRSA-ST398 and their spa type. Results: Eighty-one of the 140 pig farm workers analyzed (57.9% (95% IC: 50.0-66.4%)) were MRSA-positive, all of them ST398. The mean number of years worked on farms was 17.5 +/- 12.6 (range: 1-50), without significant differences between positive and negative MRSA results (p = 0.763). Over 75% of MRSA-ST398 carriers worked on farms with more than 1250 pigs (p < 0.001). At least one worker tested positive for MRSA-ST398 on all 20 selected pig farms. Ninety-two (46.0% (95% IC: 39.0-53.0%)) of the nasal swabs from 200 pigs from these 20 farms were MRSA-positive, with 50.5% of sows and 41.4% of fattening pigs (p = 0.198) giving MRSA-positive results. All the isolates were tetracycline-resistant, and were identified as MRSA-ST398. The spa type identified most frequently was t011 (62%). Similar spa types and phenotypes of antibiotic resistance were identified in pigs and farmers of 19/20 tested farms. Conclusions: The prevalence of MRSA-ST398 among pig farm workers and pigs on farms in the studied region is very high, and the size of the farm seems to correlate with the frequency of colonization of farmers. The similar spa-types and phenotypes of resistance detected in pigs and workers in most of the farms studied suggest animal-to-human transmission.