Livestock-associated MRSA in veal farming : risk factors for MRSA carriage in veal calves and humans

Abstract

Traditionally,Methicillin-resistant Staphylococcus aureus (MRSA) has been considered as a hospital-associated pathogen (HA-MRSA). However, since 2004, MRSA has been found to be emerging in livestock (LA-MRSA), particularly pigs and veal calves. Animals have the capacity to act as reservoirs of MRSA, and potentially transmit this bacterium to humans in close contact with MRSA-colonized animals. This thesis focuses on MRSA in veal calf farming. The main aim of this thesis was to investigate associations between determinants and ST398 MRSA carriage in both humans and veal calves and their inter-relationship. Furthermore the persistence and dynamics of MRSA carriage in both human and veal calves were quantified. Calves were more often MRSA carrier when treated with antibiotics. However, several age-related- and farm management factors seemed to be correlated with use of antimicrobials in calves. On the other hand, farm hygiene was associated with a lower prevalence of MRSA in calves. Longitudinal data showed that MRSA prevalence and MRSA air loads in stables rapidly increased during the production cycle, especially after releasing calves from their individual houses, but not simultaneously with or directly after treatment with antimicrobials. A direct relationship between human and animal MRSA carriage was demonstrated; the number of MRSA positive calves on the farm and human carriage was positively associated. Human MRSA carriage was strongly dependent on the duration of animal contact and exposure. Human persistence and dynamics of MRSA ST398 was investigated after both short- and long-term exposure to MRSA positive animals. LA-MRSA acquisition after short-term (up to 3 hours per day maximally) occupational exposure is frequent. However, the majority of people who acquire LA-MRSA during occupational exposure, test negative for MRSA again within 24 hours. In farmers, who are long-term exposed to MRSA positive animals, the presence of LA-MRSA is strongly animal-exposure related. During absence of animal contact, MRSA prevalence decreased rapidly, which suggests that LA-MRSA is a poor persistent colonizer in humans. The findings in this thesis provide important insights that add to our understanding of LA-MRSA carriage in animals and humans. Antimicrobial use in calves is associated with higher MRSA prevalence. However, future research should be focussed on the quantitative contribution of specific antibiotic classes or dosages to MRSA occurrence in livestock environments. In addition, to reduce MRSA occurrence in veal farming, optimizing the complex structure of the veal calf production chain needs specific attention. Our data point out that exposure plays a major role in MRSA carriage in humans but it seemed that MRSA of ST398 is not a persistent colonizer in humans. This is in line with other studies in which a low nosocomial transmission rate of ST398 MRSA was demonstrated. These findings implicated that control measures as described in the Search and Destroy policy could be less stringent. However, the exact ST398-related disease risk for farmers remains unclear and needs further exploration. The ongoing evolution and development of ST398 MRSA suggests that adaptation to the human host might be happening and therefore close monitoring of its evolution and surveillance over time will be required.