Investigating the Spread of Antimicrobial Drug-Resistant Microorganisms in Dairy Sheep Farms: A Follow-Up Study

Abstract

Recently, we investigated the spread of antimicrobial drug-resistant (AMR) microorganisms in five dairy sheep farms in Greece to identify indicators for improved field surveillance. The high percentage of samples of feeds and milk positive to Escherichia coli and Staphylococci, including AMR isolates, recorded in this previous study generated the interest to further investigate the parameters likely to affect positivity of these certain indicators in the same farms. For this reason, 76 samples were collected, comprising milk collected from the raw milk tank (n = 5), swabs from milking shells (n = 48), feeds (n = 6), and home-grown feeds (n = 17). Samples were processed for the detection of the pathogens mentioned above and the assessment of AMR using conventional microbiology and the polymerase chain reaction. The overall percentage of positive samples was 71.1%. The percentage of isolates that were characterised as AMR of those detected was 27.8% (30% Staphylococci, 21.4% E. coli). The results indicate that AMR testing in milking shell swabs is advisable for improving AMR stewardship and should be regarded as complimentary to testing samples from the raw milk tank, because these may not depict the microbial burden of milking shells. Furthermore, the spread and antimicrobial resistance of tested bacteria in feeds are characterised by considerable variability and should therefore be assessed longitudinally.