Transmission of MRSA to human volunteers visiting a swine farm

Transmission of methicillin-resistant Staphylococcus aureus (MRSA) from animals to humans is of great concern due to the implications for human health and the health care system. The objectives were to investigate the frequency and duration of MRSA carriage in human volunteers after a short time exposure in a swine farm. The experimental study included 34 human volunteers staying one hour in an MRSA-positive swine farm in four trials. In two of the trials, the influence of farm work involving pig contact was studied using a cross-over design. The quantity of MRSA in nasal swabs, throat swabs, and air samples were measured at different time points and analyzed in relation to relevant covariates.This investigation showed that overall 94% of the volunteers acquired MRSA during the farm visit. Two hours after leaving the stable, the nasal MRSA count had declined to unquantifiable levels in 95% of the samples. After 48 hours, 94% of the volunteers were MRSA-negative. Nasal MRSA carriage was positively correlated to personal exposure to airborne MRSA and farm work involving pig contact and negatively correlated to smoking. No association was observed between MRSA carriage and face touching behavior, nasal MSSA carriage, age, and gender. The increase in human MRSA carriage among the volunteers with pig contact seems to be dependent on the increased concentration of airborne MRSA of the surrounding air and not directly on the physical contact. MRSA was not detected in any of the throat samples.Importance The experimental approach made it possible to elucidate the contributions of airborne MRSA levels and farm work on nasal MRSA carriage in a swine farm. Short-time exposure to airborne MRSA poses a substantial risk for farm visitors to become nasal carriers but the carriage is typically cleared within hours to a few days. The risk for short-time visitors to cause secondary transmissions of MRSA is most likely negligible due to the observed decline to unquantifiable levels in 95% of the nasal samples already after 2 hours. The MRSA load in the nose was highly correlated to the amount of MRSA in the air and interventions to reduce the level of airborne MRSA or the use of face masks might consequently reduce the nasal contamination.