Methicillin-resistant Staphylococcus aureus in Danish production animals

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) is recognized as one of the most common multiresistant bacteria found in in both human and veterinary medicine, and the newly emerged livestockassociated MRSA clonal complex 398 (LA-MRSA CC398) pose a risk of zoonotic infections in humans. An increase in the occurrence of MRSA CC398 positive carriers in Denmark has been observed in the general community, where a proportion of cases occur in people without livestock exposure. This development threatens the national low levels of MRSA in humans and the sustainability of the domestic ‘search-and-destroy’ control policies. To arrest this development it seems important to contain MRSA CC398 within pig farms, which are the primary reservoir of MRSA CC398, followed by attempts to lower the within farm level of MRSA CC398. Quantitative methods for MRSA CC398 within farms are needed in order to assess the effect of intervention strategies. However, S. aureus has on multiple occasions proven difficult to control and has illustrated its ability to adapt to new niches and disseminate with great success to new reservoirs. This PhD project presents an evaluation of the feasibility to measure MRSA CC398 within pig herds in a quantitative and reproducible manner using swab and air samples, and an assessment of MRSA CC398 loads within different units of the pig production. We found nasal swab samples to be more reproducible than skin swab samples. Further, nasal samples were found to be a better proxy for airborne MRSA CC398 compared to skin samples, however, the correlation was found insufficient to recommend that air samples would be enough for complete quantification of MRSA CC398 within production units. The highest levels of MRSA CC398 within pig farms were seen in the farrowing and weaning unit which means, that the highest risk for farmers to become colonized with MRSA CC398 occurs when working within these two units. Unfortunately, these two units are where the most intense human-to-pig contact occurs which is worrisome, as human carriage are known to play a role in dissemination of MRSA CC398 into naive animal productions. Focus was consequently turned to investigate the emergence of possible new animal reservoirs in Denmark. We investigated if Danish veal and dairy herds constitute a reservoir of MRSA CC398. Both production lines was found positive for MRSA CC398 in low prevalence, however veal calf was thought merely to be contaminated, whereas indications of dairy herds as a persistent reservoir was observed. The Danish mink production has gained our attention for some time throughout the present study period, as human cases of MRSA CC398 in people with mink contact, have been seen since 2009 and infections in mink caused by MRSA CC398 was first observed in 2013. Consequently, we wanted to determine the degree of MRSA CC398 positive farms and to identify where the bacterium most often could be detected. A total of one third of the screened farms were found positive for MRSA CC398 with the bacterium most often found on the paws and in the pharynx, which poses a human health hazard to farmers, who risk getting bites and scratches when handling the animals. Based on results from phylogenetic analysis of isolates from both the cattle and mink production, we suggested a spillover from the pig production to be the primary source of introduction. In mink, results pointed to an introduction via contaminated feed. The introduction into mink has subsequently led to increased carriage and infections in people with contact to mink, observed since 2011. The results obtained as part of this PhD project, emphasize the importance of lowering the levels of MRSA CC398 within farms to reduce dissemination and emergence of new reservoirs. Further, the results illustrated the need for continued screening of low prevalence MRSA CC398 positive productions and possibly unknown positive productions. This is needed in order to try and take control of the development, emergence and spread of MRSA CC398, with the subsequent goal of preventing the emergence of reservoirs with possible relevance to human health.