The nasal vestibulum is the optimal sampling site for MRSA screening in hospitalised horses

The increased incidence of methicillin-resistant Staphylococcus aureus (MRSA) infection in equine hospitals highlights the need for infection control protocols based on optimal patient screening. In horses, the deep ventral meatus of the nasal cavity is the principal site sampled to detect MRSA. However, in humans, the anterior nares are the preferred sampling site. The objective of this study was to determine the optimal sampling location in the nasal chambers for MRSA in horses by comparing the results obtained from three different locations (the vestibulum, diverticulum and ventral meatus) in 240 hospitalised animals. Antimicrobial susceptibility testing and epidemiological typing were conducted on representative subsets of the isolates obtained. Compared to the more invasive ventral meatus sampling (relative sensitivity 68.9%; isolation rate 37.9%), vestibulum (RS 81.1%; IR 44.6%, P=0.13) and diverticulum (RS 52.3%; IR 28.8%, P=0.03) sampling were more or less sensitive, respectively. In total, 132 horses (55%) were MRSA positive with the vast majority (98.5%) carrying genotyped isolates of the livestock-associated (LA)-MRSA clonal complex (CC) 398, and only a minority (1.5%) CC8. Of the 22 MLST typed isolates, five belonged to a novel ST2197 (t011, CC398). Although 93.9% of the isolates were multi-resistant (to β-lactam, tetracycline, trimethoprim, and gentamicin), <5% were resistant to virtually all antimicrobials commonly used in equine medicine. The study findings indicate that detection of MRSA in horses may be enhanced by replacing the traditional deep sampling of the ventral nasal meatus by the less invasive approach of sampling the nasal vestibulum.