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MRSA CC398 in Humans and Pigs in Norway: A “One Health” Perspective on Introduction and Transmission

Emerging livestock-associated methicillin-resistant Staphylococcus aureus (LA-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 notified cases of LA-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, two slaughterhouses and 36 humans. Primary introductions likely occurred by human transmission to three 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 to prevent pig farms from becoming reservoirs for MRSA transmission to humans.

Carl Andreas Grøntvedt, Petter Elstrøm, Marc Stegger, Robert Leo Skov, Paal Skytt Andersen, Kjersti Wik Larssen, Anne Margrete Urdahl, Øystein Angen, Jesper Larsen, Solfrid Åmdal, Siri Margrete Løtvedt, Marianne Sunde, and Jørgen Vildershøj Bjørnholta; MRSA CC398 in humans and pigs in Norway: A One Health perspective on introduction and transmission; Clin Infect Dis. first published online August 11, 2016 doi:10.1093/cid/ciw552