Livestock-associated methicillin resistant Staphylococcus aureus in pigs - prevalence, risk factors and transmission dynamics

In 2004, an association between human carriage of methicillin resistant Staphylococcus aureus (MRSA) and contact with pigs was found. To assess the implications of this finding for veterinary and public health more insight into the prevalence, risk factors and transmission dynamics of this so-called livestock-associated (LA-)MRSA was needed. Therefore, field and experimental studies were conducted in pig and human populations of which the results are presented in this thesis. First, observational studies on pig farms were performed to estimate the prevalence of MRSA positive herds, and to identify factors associated with LA-MRSA in pig herds. It was shown that LA-MRSA was present in the majority, i.e. ~70%, of Dutch pig herds and that the prevalence increased over time. Larger herds were more often found LA-MRSA positive than smaller herds, and transmission was shown to occur by animal trade. From all this, it was concluded that LA-MRSA has become endemic in the Dutch pig population. Secondly, studies on LA-MRSA in pigs, the environment and personnel in pig slaughterhouses were performed. In pigs, a clear increase in LA-MRSA positive pigs from 0 to 60% was shown in the time period between loading at the farm and stunning at the slaughterhouse. This indicated a very rapid transmission of LA-MRSA between pigs through direct contact or through contact with a contaminated environment. An increase in LA-MRSA positive environmental samples taken in the slaughterhouse was found during the working day. In personnel, LA-MRSA prevalence was 6% and working with live pigs was the single most important factor for being positive; personnel not working with pigs or working only with dead pigs were all LA-MRSA negative. Thirdly, transmission of LA-MRSA within herds was studied longitudinally both in an experimental setting and also in 6 pig herds. Transmission rates and the factors affecting these rates were determined. The results of both studies indicated that LA-MRSA is able to spread easily and persist in pig populations, resulting in an endemic situation. Use of selective antimicrobials has a positive effect on the transmission rate of LA-MRSA, but transmission occurs even without use of antimicrobials. The key to limiting LA-MRSA transmission from pigs to humans is to eliminate the source, i.e. eradicate LA-MRSA from pig herds, and a combination of different intervention strategies controlling both within- and between-herd transmission will be needed to achieve this.