Dynamics of MRSA carriage in veal calves: a longitudinal field study.

Colonization of Methicillin-Resistant Staphylococcus aureus (MRSA) in food producing animals has public health implications, but intervention targets have not yet been identified. In this field study occurrence and dynamics of MRSA in veal calves were investigated longitudinally on three farms. Determinants generally associated with MRSA carriage, such as environmental exposure and antimicrobial use, were explored. In addition, the reliability and reproducibility of MRSA detection in nasal samples from veal calves were investigated as well as the additional value of rectal samples to establish MRSA status of an individual animal. On these three farms, 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. These observations constitute the hypothesis that antimicrobial use may not necessarily be the only condition for MRSA transmission in veal calves, but indicate that other factors may contribute to transmission as well. MRSA in calves was present both nasally and rectally. The reproducibility and repeatability of the nasal samples were moderate. The results of this study give a better understanding of the dynamics of MRSA in a field situation.

Evaluation of isolation procedures and chromogenic agar media for the detection of MRSA in nasal swabs from pigs and veal calves.

Since the emergence of MRSA in livestock, screening of animals for the detection of MRSA is widely practised. Different procedures are published for animal samples but a systematic comparison of methods has not been performed. The objective of this study was to compare three available commonly used procedures and three chromogenic agars for detecting MRSA in nasal swabs from pigs (n=70) and veal calves (n=100). Procedures 1 and 2 used a pre-enrichment comprising Mueller Hinton broth with 6.5% NaCl followed by selective enrichment with 4 microg/ml oxacillin+75 microg/ml aztreonam (procedure 1) and 5 microg/ml ceftizoxime+75 microg/ml aztreonam (procedure 2) respectively. Procedure 3 used a selective enrichment broth only, containing 4% NaCl, 5 microg/ml ceftizoxime+50 microg/ml aztreonam. After selective enrichment, media were streaked on to three different chromogenic agars. Significantly more MRSA were found for pig as well as for veal calf samples with procedures 1 and 2. No significant differences were found between procedures 1 and 2. For nasal swabs from pigs significantly more MRSA-positive samples were found when MRSA Screen (Oxoid) or MRSASelect (Bio-Rad) agars were used compared to MSRA ID (bioMérieux). For calf samples no significant differences between the different agars were found. In conclusion, the results of this study show that procedures 1 and 2, both using additional high salt pre-enrichment are superior and should be recommended for MRSA detection in nasal swabs from pigs and veal calves. The preferred choice of chromogenic agar depends on the sample matrix.