Profile and evolution of antimicrobial resistance of ovine mastitis pathogens (1995–2004)

Antibiograms of selected mastitis pathogens, performed during the decade 1995–2004, were retrospectively analysed in order to evaluate antimicrobial resistance and determine whether resistance changed over time. Results of 2763 strains of Staphylococcus aureus, coagulase negative staphylococci, Streptococcus uberis and Escherichia coli, against penicillin, ampicillin, oxytetracycline, kanamycin and streptomycin are discussed. Strains were isolated in clinical milk samples from sheep suspected of mastitis. The evolution over time of resistance was evaluated by means of logistic regression analysis. The resistance of staphylococci to penicillin appeared to be lower than those usually reported (4.1% of resistant strains in S. aureus; 15.3% for CNS). Higher rates of resistance were observed for aminoglycosides, relevant for S. uberis (84.5% for kanamycin and 92.5% for streptomycin) and S. aureus (14.6% for kanamycin and 63.3% for streptomycin). Overall resistance appeared to confirm the lower resistance in ovine pathogens than in bovine ones. Logistic regression highlighted no trends to increase for resistance over time.