A-Z Index × Submit A-Z Index × Submit A-Z Index Search Dropdown × Submit Facebook Twitter LinkedIn Syndicate Emerging Infectious Disease journal ISSN: 1080-6059 Disclaimer: Early release articles are not considered as final versions. Any changes will be reflected in the online version in the month the article is officially released.
Livestock-associated Staphylococcus aureus caused 200 human bloodstream infections in Denmark during 2007–2023; two thirds were caused by methicillin-susceptible isolates. Risk for death was comparable to that associated with human S. aureus clones. Industrial pig population size correlated strongly with disease prevalence. Seventeen percent of isolates showed signs of human adaptation.
Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of difficult-to-treat infections, causing an estimated 130,000 deaths worldwide in 2021, and is classified as a high-priority pathogen by the World Health Organization ( 1 , 2 ). The emergence of livestock-associated (LA) methicillin-resistant S. aureus (MRSA) clonal complex 398 (CC398) (LA-MRSA CC398) in the early 2000s has led to increasing numbers of human infections in countries in Europe with industrial pig production, including Denmark, where LA-MRSA CC398 accounted for 12% (241/1,993) of all human MRSA infections in 2024 ( 3 , 4 ). LA S. aureus CC398 isolates are descendants of a human S. aureus CC398 variant but have lost the ΦSa3int prophage and the associated immune evasion cluster (IEC), which encodes immune modulators that protect human S. aureus clones from the human innate immune system ( 5 ). LA-MRSA CC398 is poorly adapted for human-to-human transmission ( 6 ) but nonetheless can spread from pigs to the community and healthcare facilities ( 7 ).
Previous efforts to assess the prevalence of LA S. aureus CC398 disease have focused almost exclusively on LA-MRSA CC398, including a study in Denmark ( 8 ), but overlooked that pigs also constitute a reservoir of methicillin-susceptible S. aureus (MSSA) CC398 ( 9 ) and that most S. aureus bloodstream infections are caused by MSSA isolates ( 10 ). For example, the incidence rate of MSSA bacteremia is ≈10 times higher than that of MRSA bacteremia in countries of the European Union and European Economic Area (37.9 vs. 4.43 cases/100,000 population) ( 10 ).
We conducted a nationwide study of LA S . aureus CC398 bacteremia in Denmark during January 1, 2007–December 31, 2023 ( Appendix ). The objectives were to determine the relative prevalence of LA-MRSA CC398 and LA-MSSA CC398 bacteremia in Denmark, estimate the relative risk for death in patients with bacteremia caused by LA S. aureus CC398 and human S. aureus clones, evaluate whether the number of LA S. aureus CC398 bloodstream infections correlates with the size of the industrial pig population across regions of Denmark, and investigate whether IEC is associated with increased risk for death among patients with LA S. aureus CC398 bacteremia.
Figure . Temporal changes in livestock-associated Staphylococcus aureus clonal complex 398 bacteremia, Denmark, 2007–2023. CC, clonal complex; LA, livestock-associated; MRSA, methicillin-resistant S . aureus ; MSSA, methicillin-susceptible ...
LA S. aureus CC398 caused 200 human bloodstream infections in Denmark during 2007–2023 ( Figure ). We summarized results by strain characteristics and patient demographic information ( Table 1 ). In brief, 36% of patients were female and 64% male, median age was 69 ye…