Methicillin-Resistant Staphylococcus aureus CC398 in Humans and Pigs in Norway: A "One Health" Perspective on Introduction and Transmission

Background. Emerging livestock-associated methicillin-resistant Staphylococcus aureus (MRSA) persist in livestock populations and represent a reservoir for transmission to humans. Understanding the routes of introduction and further transmission is crucial to control this threat to human health. Methods. All reported cases of livestock-associated MRSA (CC398) in humans and pigs in Norway between 2008 and 2014 were included. Data were collected during an extensive outbreak investigation, including contact tracing and stringent surveillance. Whole-genome sequencing of isolates from all human cases and pig farms was performed to support and expand the epidemiological findings. The national strategy furthermore included a "search-and-destroy" policy at the pig farm level. Results. Three outbreak clusters were identified, including 26 pig farms, 2 slaughterhouses, and 36 humans. Primary introductions likely occurred by human transmission to 3 sow farms with secondary transmission to other pig farms, mainly through animal trade and to a lesser extent via humans or livestock trucks. All MRSA CC398 isolated from humans without an epidemiological link to the outbreaks were genetically distinct from isolates within the outbreak clusters indicating limited dissemination to the general population. Conclusions. This study identified preventable routes of MRSA CC398 introduction and transmission: human occupational exposure, trade of pigs and livestock transport vehicles. These findings are essential for keeping pig populations MRSA free and, from a "One Health" perspective, preventing pig farms from becoming reservoirs for MRSA transmission to humans.