Research Article: The first nosocomial outbreak of methicillin-resistant Staphylococcus aureus in horses in Sweden

Date Published: February 8, 2012

Publisher: BioMed Central

Author(s): Karin Bergström, Anna Aspan, Annica Landén, Christopher Johnston, Ulrika Grönlund-Andersson.


Methicillin-resistant Staphylococcus aureus (MRSA) in animals is a rare finding in Sweden. In horses, MRSA was first detected in a screening survey in 2007. In 2008, six clinical cases occurred in an equine hospital, indicating an outbreak.

All MRSA isolates detected, 11 spa-type t011 and one t064 (n = 12), in infected horses (n = 10) and screening of horses (n = 2) in Sweden from December 2007 to March 2010 were retrospectively analysed with pulsed-field gel electrophoresis (PFGE) using Cfr9I and ApaI restriction enzymes, to study relationship between the isolates. Medical records of infected horses and outbreak investigation notes were scrutinised to monitor the clinical outcome and other aspects of the outbreak.

Eight of the 10 infected horses were linked to one equine hospital and two to another hospital in the same region. The six horses infected with MRSA in 2008 underwent surgery during the period 22 May-7 July in one of the hospitals. Four more infections linked to the two hospitals were notified between 2009 and March 2010.

Association in time and space of six horses infected with an identical MRSA strain of spa-type t011 confirmed an outbreak. Two isolates found in 2009 and 2010 in the outbreak hospital were closely related to the outbreak strain, indicating one circulating strain. Both spa-type t011 and t064 have been reported in horses in Europe prior to these findings. The observation that the infections cleared although antimicrobials were not used is encouraging for future prudent use of antimicrobials. The time from surgery to bacteriological diagnosis was not acceptable in most cases, as contagious spread was a risk. Sampling when symptoms of infection are noticed and accurate analysis are thus important.

Partial Text

Staphylococcus (S.) aureus is a potentially pathogenic bacterium causing pyogenic infections in both man and animals. Nosocomial and community-acquired infections with therapeutic limitations due to methicillin-resistant S. aureus (MRSA) have become a worldwide problem in humans [1-5]. However, according to the European Antimicrobial Resistance Surveillance Network annual report for 2010, a significant decrease in human MRSA rates has occurred in seven European countries in recent years, which might be the result of successful preventive measures [6].

This study confirmed the first nosocomial outbreak of MRSA in a Swedish equine hospital. This was also the first MRSA infections notified in horses in Sweden, although one nasal carrier of spa-type t011 had been found in a screening survey of horses about six month earlier, in December 2007 [38]. As the screening was anonymous, it could not be determined whether there were connections between that finding and the subsequent outbreak. The outbreak strain belonged to the MRSA variant ST398, spa-type t011 [38,39]. This variant has been associated with livestock in Europe [12,19] and with horses [15,17,18,28,29], but had only been found in horses and a few humans [48] in Sweden.

MRSA was shown to be present in the Swedish equine population, although it appeared only regionally. The restriction enzymes Cfr9I and ApaI worked well for MRSA ST398 and demonstrated close genetic relatedness between most isolates from the horses tested, representing one strain. This and the correlation in time and space between the cases verified the occurrence of an outbreak. Later sporadic infections with the same strain indicated one circulating strain, but the primary source of the outbreak was never identified.

The authors declare that they have no competing interests.

KB initiated and designed the study together with UGA. KB interpreted the results and drew conclusions in discussion with the other authors. AL and AA set up the PFGE analyses and helped with the writing of that part. CJ was the contact at the hospital and provided information and records on the horses. All authors have read and approved the final manuscript.