Outbreaks of a Methicillin-Resistant Clone ST398-t011 in a Hungarian Equine Clinic: Emergence of Rifampicin and Chloramphenicol Resistance After Treatment with These Antibiotics.

Between July 2011 and May 2016, a total of 40 strains originating from 36 horses were confirmed as methicillin resistant (methicillin-resistant [MRSA]) in a university equine clinic. An additional 10 MRSA strains from 36 samples of clinic workers were obtained in October 2017. The first equine isolate represented the sequence type ST398, -type t011, and SCC IV. This isolate was resistant to a wide spectrum of antimicrobial agents. MRSA strains with the same genotype and with very similar resistance profiles were isolated on 21 more occasions from September 2013 to September 2014. A second outbreak occurred from May 2015 until May 2016. The first isolate in this second outbreak shared the same genotype, but was additionally resistant to chloramphenicol. The second isolate from August 2015 also showed resistance to rifampicin. The clone was isolated 18 times. Most of the human isolates shared the same genotype as the isolates from horses and their resistance patterns showed only slight differences. We can conclude that the MRSA-related cases at the Department and Clinic of Equine Medicine were all nosocomial infections caused by the same clonal lineage belonging to the clonal complex 398. The clonal complex 398 of equine origin is reported for the first time in Hungary. In addition, our observation of the emergence of new resistance to antimicrobial agents within the clonal lineage after treatment with antibiotics is of concern. Strict hygiene regulations have been introduced to lower the incidence of MRSA isolation and the related clinical disease.