Methicillin-resistant staphylococci isolated from animals

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Abstract

Staphylococci isolated from animals (n = 311) were screened for methicillin resistance by oxacillin agar screening. Oxacillin-resistant strains were tested for the presence of the mecA gene by PCR. Isolates were identified by standard techniques and 16S rDNA analysis, and their antimicrobial susceptibilities were tested using an agar diffusion method. MecA-positive strains were further analyzed using pulsed-field gel electrophoresis (PFGE). From 11 multidrug-resistant staphylococci, 6 were mecA-positive: 2 methicillin-resistant Staphylococcus aureus (MRSA) and 4 Staphylococcus haemolyticus. Screening of 300 staphylococci (100 S. aureus, 100 S. intermedius and 100 coagulase-negative staphylococci (CNS)) randomly chosen from the strain collection of the Veterinary Microbiological Diagnostic Center yielded five oxacillin-resistant coagulase-negative staphylococci, four of which were mecA-positive.

PFGE showed that all mecA-positive staphylococci isolated from animals had distinct patterns. However, one MRSA isolated from a flank fistula of a dog showed homology to a human epidemic MRSA cluster, suggesting that transfer of MRSA between humans and dogs might occur.

Introduction

Methicillin-resistant Staphylococcus aureus (MRSA) is an important cause of human nosocomial infections worldwide. In The Netherlands, however, the prevalence of MRSA is low among humans due to a national ‘search-and-destroy’ policy and the restrictive usage of antimicrobial drugs. Patients with a higher risk of MRSA infections are screened for MRSA carriage and are kept in isolation until they are proven culture negative (Vandenbroucke-Grauls, 1996). Therefore, in The Netherlands less than 1% of clinical S. aureus strains are MRSA and only 0.03% of non-risk patients admitted to hospitals are nasal MRSA carriers (Wertheim et al., 2003). To date, there are few reports on infections of animals with MRSA. In Korea, Japan, Belgium and the United States of America infections with MRSA have been reported in horses (Hartmann et al., 1997, Shimizu et al., 1997; and Seguin et al., 1999), dogs (Gortel et al., 1999, Pak et al., 1999; and Tomlin et al., 1999) and cattle (Devrieze and Hommez, 1975). Methicillin resistance in staphylococci is mediated by the mecA gene, encoding the penicillin-binding protein 2a (PBP2a), which has a reduced affinity for the penicillinase-resistant penicillins like methicillin and oxacillin and for all other beta-lactam antibiotics. The mecA gene has also been detected in coagulase-negative staphylococci (CNS) isolated from healthy horses in Japan (Yasuda et al., 2000) and from Staphylococcus intermedius isolated from dogs (Gortel et al., 1999). Phenotypic methicillin resistance has been reported from S. intermedius isolated from canine skin lesions in Spain (Piriz et al., 1996). In 2003, we reported the first isolation of MRSA from animal origin in The Netherlands (van Duijkeren et al., 2003). This MRSA was cultured from an infected wound in a Dutch dog that underwent surgery abroad. Penicillinase-resistant penicillins are not used in veterinary medicine in The Netherlands except for cloxacillin that is used in products for intramammary administration in cattle. Therefore, susceptibility testing to these antimicrobial drugs is not a routine procedure and infections with MRSA in animals may go undetected.

The objective of the present study was to gain insight into the presence of methicillin-resistant staphylococci (MRS) among staphylococci isolated from clinical infections in animals in The Netherlands. The MRS found were characterized by PFGE in order to gain more insight in their epidemiology. In addition, we compared MRS isolated from animals with those isolated from humans in order to investigate whether these strains are related and to examine their zoonotic and anthroponotic potential.

Section snippets

Bacterial isolates

Eleven multidrug-resistant staphylococci (intermediate susceptible or resistant to four or more antimicrobials) were selected from the strain collection of the Veterinary Microbiological Diagnostic Center (VMDC). This strain collection contains staphylococci (n = 1630) isolated at the VMDC from clinical infections of dogs, cats, horses, cattle and many other animal species. The staphylococci in the strain collection were cultured from samples submitted by the animal hospitals of Utrecht

Results

Among 311 staphylococci isolated from animals we found 10 mecA-positive isolates (Table 1). Of 11 multidrug-resistant staphylococci selected from the strain collection of the VMDC, 6 were phenotypically resistant to oxacillin and all 6 were mecA-positive. (Table 1; nos. 1–6). Two isolates were identified as S. aureus, and four CNS as S. haemolyticus, as confirmed by conventional methods and 16S rDNA partial sequence analysis. PFGE showed that both S. aureus and the four S. haemolyticus isolates

Discussion

Until recently it was assumed that methicillin resistance is absent among staphylococci isolated from animals in The Netherlands because MRSA has never been found in surveillance programs. Our study shows that MRSA do occur, although with low prevalence, among staphylococci isolated from animals in The Netherlands. Both MRSA were cultured from dogs indicating that MRSA is more prevalent among dogs than among other animal species. This is remarkable as more than 90% of canine infections with

Acknowledgements

We would like to thank Mr. R. Noomen (VMDC, NL) for technical assistance and Dr. F. Reubsaet (RIVM, NL) for performing the 16S rDNA partial sequence analysis. We also thank Prof. Dr. J.P. van Putten and Dr. D.J. Houwers for their critical reading of the manuscript.

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