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

Abstract

Background: 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.

Method: 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.

Results: 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.Nine of the 11 spa-type t011 isolates had identical Cfr9I and ApaI PFGE pattern. All six infected horses from 2008 presented with this MRSA. Two t011 isolates differed in one and two bands, respectively, in PFGE.Nine horses suffered from surgical site infections (SSI). No antimicrobials were used following the MRSA diagnosis and the infections cleared. The time from surgery to MRSA diagnosis differed greatly between the horses (range 15-52 days).

Conclusions: 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.

Figure 1

Red dot indicate approximate location of MRSA-infected horses in Sweden, June 2008 to March 2010.

Figure 2

PFGE pattern of representative MRSA isolates, spa-type t011 and t064. Restrictions enzyme ApaI and Cfr9I. ApaI: Lane 1 - demonstrate the outbreak strain named pulsotype A. Lane 2 - pulsotype A1 and A2 isolates lacking one band. Cfr9I: Lane 5 - pulsotype A and lanes 7 + 8 - pulsotype A1, identical in Cfr9I gel. Lane 6 - pulsotype A2, one extra band. Lanes 11 and 12 show difference between spa-type t011 (pulsotype A) and spa-type t064 (pulsotype B). C = control: S. aureus NCTC 8325 L = lambda ladder: Size range of 50-1,000 kb (PFG Marker, New England Biolabs Inc.) http://www.neb.com/nebecomm/products/productN0340.asp.