Longitudinal study of horses for carriage of methicillin-resistant Staphylococcus aureus following wound infections

An outbreak of methicillin-resistant Staphylococcus aureus (MRSA) infections in horses in Sweden raised questions concerning the risk posed by horses to their surroundings following MRSA infections. This initiated a longitudinal study to investigate how long MRSA-infected horses remained positive and to test the sensitivity of different anatomical sampling sites for detection of MRSA. Between October 2008 and June 2010, 9 of 15 horses notified as having MRSA-infected wounds fitted the case criteria for the study. The cases were sampled at five anatomical sites (nostrils, corner of mouth, pastern, perineum, and previous infection site) on six to seven occasions or more during approximately 12–18 months. MRSA-specific broth and agar were used for culture. Verified MRSA isolates were spa -typed. The sensitivity of sampling sites was calculated. The most sensitive sampling site was the nostrils, with a sensitivity of 0.91 (95% CI: 0.59–1.00). The other test sites had a sensitivity of 0–0.09. Individual cases tested positive, but with time all tested negative. The observed carriage time ranged from 55 to 711 days (median = 143, IQR: 111–172 days), but these data should be interpreted with caution since only a small number of cases were studied.