An emerging subtype of methicillin-resistant Staphylococcus aureus (MRSA), clonal complex (CC) 398, is associated with animals, particularly pigs. We conducted a matched case-control and a case-case study comparing 21 CC398 case-patients with 2 controls randomly selected from the Danish Civil Registry and 2 case-patients infected with MRSA other than CC398. On farms of case-patients, animals were examined for MRSA. Thirteen case-patients reported pig exposure. Living or working on farms with animals was an independent risk factor for CC398 in the case-control (matched odds ratio [MOR] 35.4, 95% confidence interval [CI] 2.7-469.8) and the case-case study (MOR 14.5, 95%CI 2.7-76.7). History of hospitalization was associated with an increased risk only in the case-control study (MOR 11.4, 95% CI 1.4-94.8). A total of 23 of 50 pigs on 4 of 5 farms were positive for CC398. Our results, corroborated by microbiologic testing, demonstrate that pigs are a source of CC398 in Denmark.
Methicillin-resistant Stapylococcus aureus (MRSA) is becoming increasingly recognized among persons in the community without established risk factors (1,2). MRSA primarily causes human disease and animals have not, until now, been considered a source of infection.
It has recently become apparent that animals, particularly pigs, can constitute a separate MRSA reservoir and be a source of a novel and rapidly emerging type of MRSA in humans; namely MRSA clonal complex (CC)398 by multilocus sequence typing (MLST) (3). MRSA CC398 consists of 8 MLST types, the predominant type being sequence type (ST)398, and a range of closely related protein A (spa) gene types (i.e., t011, t034, t108, and t1793) (4,5) .
Although transmission appears to be primarily between animals, indistinguishable isolates have been found in their human contacts, particularly those with occupational exposure (3-7). MRSA CC398 (ST398) was first detected in 4 pigs and 1 healthy pig farmer in France (3,8). Clinical infection was described in the daughter of a pig farmer in the Netherlands in 2004 (7). That study showed that 23% of pig farmers in a small survey in the same region were seropositive for MRSA CC398.
Denmark has a low incidence of MRSA. In 2006, only 706 new MRSA patients (colonization only or infection) were reported, which corresponds to 13/100,000 population (9). To maintain this status, Denmark has adopted a strict "Search and Destroy" policy, which includes active screening of at-risk persons at admission to a hospital (10). Prompted by reports from the Netherlands, we identified a small (<1% of all MRSA patients) but increasing number of MRSA CC398 human patients after 2003. Furthermore, CC398 was detected in a pig in Denmark in 2006 (11). With an annual production of 25 million slaughter pigs, Denmark has a large potential reservoir for MRSA CC398.
We report results of an analytical study of MRSA CC398, in parallel with systematic farm and microbiologic investigations, to identify risk factors for MRSA CC398 acquisition in persons in Denmark. Although farm and pig exposure have been postulated as risk factors after interviews with MRSA CC398 patients in other studies (4,6,7,12), no previous study has included interviews with control populations to determine if these exposures were higher than would be expected in the general population.
Lewis HC, Mølbak K, Reese C, Aarestrup FM, Selchau M, Sørum M, et al. Pigs as source of methicillin-resistant Staphylococcus aureus CC398 infections in humans, Denmark, Emerg Infect Dis. 2008 Sep; [Epub ahead of print]