High occurrence of methicillin-resistant Staphylococcus aureus ST398 in equine nasal samples

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Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) infections do occur in equine patients. Little is known, however, about their origin and the general equine MRSA colonization status. In West European horses in particular, neither the colonization rate nor the present strains or their antimicrobial susceptibility patterns are known.

In the present study, a sample of 110 (Belgian, French, Dutch and Luxemburg) horses presented at a Belgian equine clinic was screened for nasal MRSA carriage. An indirect culturing protocol using a 0.001% colistin and nalidixic acid containing broth was compared to a direct agar method. Phenotypic identification following growth on a chromogenic MRSA screening agar (ChromID™ MRSA) was combined with genotypic analysis (PCR, PFGE, SCCmec, spa, and MLST typing). Antimicrobial susceptibility was tested through disk diffusion.

Twelve (10.9%) horses carried MRSA, with the enrichment protocol resulting in a significantly higher isolation rate. None of the isolated strains were typeable through SmaI PFGE. They all harboured SCCmec type IVa or V and belonged to spa type t011 or t1451 of the ST398 lineage. All isolates were tetracycline resistant and sulfonamide and enrofloxacin susceptible. Macrolide, lincosamide, trimethoprim and aminoglycoside susceptibility varied and in total five different antimicrobial resistance patterns were distinguished.

These results show that ST398 is certainly present in West European horses. Due to its known interspecies transmission and the structure of the equine industry, the presence of this clone in horses poses a substantial health hazard for both animals and humans.

Introduction

In recent years, methicillin-resistant Staphylococcus aureus (MRSA) has been isolated from several animal species (Lee, 2003, Goñi et al., 2004, Rich and Roberts, 2006, de Neeling et al., 2007) and the question of potential interspecies and zoonotic transmission has been raised (Weese et al., 2006a, Monecke et al., 2007). In horses it is known to cause varying types of infection in both colonized and non-colonized individuals (Shimizu et al., 1997, Weese et al., 2006b, Hermans et al., 2008).

The importance of MRSA colonization in the equine population is indeed twofold: colonized equids are at a higher risk of developing infections themselves and they can contaminate their environment, contact animals and humans (Weese et al., 2004, Weese et al., 2006a, Weese et al., 2006b, Weese and Rousseau, 2005). Hence, equine MRSA colonization has important implications for both horse and man (Weese et al., 2006a). A high colonization and thus infection risk, incurred by large-animal personnel (colonization prevalence as high as 15.6%), has been reported (Weese et al., 2005, Hanselman et al., 2006). Vice versa, personnel and especially veterinarians may play an important role in the dissemination of the pathogen among animals (Anderson and Weese, 2006).

Up till now, most of the equine research has focused on American and Canadian horses and points to Canadian epidemic MRSA-5 (CMRSA-5) as the predominant equine strain (Seguin et al., 1999, Weese et al., 2005, Weese et al., 2006b). Little is known however about the European equine colonization status.

Due to the occurrence of a cluster of clinical MRSA infections at the institution of the first author (Hermans et al., 2008) questions were raised as to the source of patient contamination. Therefore, the aim of this study was to screen the arriving patient population to determine its colonization rate and the strain types involved. This is a first requirement for the epidemiologically and legally important differentiation between hospital-acquired versus hospital-expressed infections (Weese et al., 2006b). Determination of the isolates’ antimicrobial susceptibility patterns could also provide a first treatment directive in case of later infections. Furthermore, a first, although rough, estimate of the MRSA colonization status of West European equids would be provided.

Section snippets

Study population

Nasal swabs were taken from 110 horses presented at the Department of Surgery and Anaesthesiology of Domestic Animals, Faculty of Veterinary Medicine, Ghent University, a tertiary referral centre receiving mostly Belgian, French, Dutch and Luxemburg equine patients. All horses arriving at the clinic between March and July 2007 were eligible for sampling. Four vets were charged with swabbing each patient within 1 h after arrival. Whenever immediate sampling was impossible or owner or patient

Results

Of the 850 horses arriving at the clinic during the study period 110 (12.9%) were swabbed. The obtained samples originated from 80 (72.7%) Belgian, 24 (21.8%) French (Departments Pas-de-Calais, Nord, Somme), 3 (2.7%) Dutch (province Zeeland) and 3 (2.7%) Luxemburg horses. Eighty-nine (80.9%) of them were first time visits, 21 (19.1%) had already been at the clinic. One horse that came from a different department in the same clinic was considered to be a readmission. For the others the time

Discussion

The aim of this study was to determine if and in what percentage MRSA could be isolated from the equine population arriving at a West European equine referral facility. In addition, each isolate was genetically typed and its antimicrobial susceptibility tested.

Compared to the 2.7% equine (CMRSA-5) carriers arriving at a Canadian clinic (Weese et al., 2006b), the proportion found in the present study (10.9%) is both astonishing and disturbing. Several factors, such as a high regional MRSA

Conclusion

In conclusion, we can state that the proposed protocol was successful in screening an equine population for MRSA colonization. It demonstrated that the MRSA clone ST398 has become a nasal opportunistic pathogen in West European horses with a plausible zoonotic potential. Finally, antimicrobial susceptibility testing can be an important tool from both a therapeutic and epidemiologic point of view.

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