Abstract
Livestock constitutes a potential reservoir of meticillin-resistant Staphylococcus aureus isolates belonging to a recently derived lineage within clonal complex 398 (MRSA CC398-IIa). Since its discovery in the early 2000s, this lineage has become a major cause of human disease in Europe, posing a serious public health challenge in countries with intensive livestock production. To retrace the history of human colonisation and infection with MRSA CC398-IIa in Denmark, we conducted a nationwide, retrospective study of MRSA isolates collected from 1999 to 2011. Among 7,429 MRSA isolates screened, we identified 416 MRSA CC398-IIa isolates. Of these, 148 were from people with infections, including 51 from patients reporting no livestock exposure. The first cases of MRSA CC398-IIa infection in Denmark occurred in 2004. Subsequently, the incidence of MRSA CC398-IIa infection showed a linear annual increase of 66% from 2004 to 2011 (from 0.09 to 1.1 per 100,000 person-years). There were clear temporal and spatial relationships between MRSA CC398-IIa-infected patients with and without livestock exposure. These findings suggest substantial dissemination of MRSA CC398-IIa from livestock or livestock workers into the Danish community and underscore the need for strategies to control its spread both on and off the farm.
Highlights
In 2005, two studies, from France and the Netherlands, provided the first evidence of a reservoir of meticillinresistant Staphylococcusaureus (MRSA) in livestock, with transmission to humans [1,2]
MRSA clonal complex398 (CC398) has unique genetic characteristics compared with other MRSA strain types: it is nontypeable by SmaI-pulsed-field gel electrophoresis (PFGE) [4], it comprises a distinct set of spa types [5], and it contains a novel Sau1 type I restriction-modification system [6]
A total of 420 isolates were identified as MRSA CC398, of which 416 putatively belonged to CC398-IIa based on the presence/absence of tet(M) and scn (n = 389) or detection of canonical single nucleotide polymorphisms (canSNPs) (n = 27) (Table 1)
Summary
In 2005, two studies, from France and the Netherlands, provided the first evidence of a reservoir of meticillinresistant Staphylococcusaureus (MRSA) in livestock, with transmission to humans [1,2]. MRSA CC398 has been isolated from cattle, horses, chickens and turkeys, but currently pigs appear to be its primary host [3]. MRSA CC398 has unique genetic characteristics compared with other MRSA strain types: it is nontypeable by SmaI-pulsed-field gel electrophoresis (PFGE) [4], it comprises a distinct set of spa types [5], and it contains a novel Sau type I restriction-modification system [6]. These features challenged early genotyping efforts, which have been aided more recently by whole-genome sequencing
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