Distribution and Antimicrobial Resistance of Clinical and Subclinical Mastitis Pathogens in Dairy Cows in Rhone-Alpes, France

The goal of this study was to estimate the distribution of pathogens, as well as their antimicrobial resistance pattern, in cows affected by clinical or subclinical mastitis in the Rhone-Alpes region of France. A total of 1770 samples were taken between January 2007 and March 2008, leading to the identification of 1631 bacterial isolates. Streptococcus uberis (22.1%), Escherichia coli (16%), and coagulase-positive staphylococci (15.8%) were identified as the major causative agents of clinical mastitis, whereas coagulase-positive staphylococci (30.2%), coagulase-negative staphylococci (13.7%), and Streptococcus dysgalactiae (9.3%) were predominantly implicated in subclinical mastitis. Yet, in both types of mastitis, about 20% of all cases were due to a large number of different bacterial species that were isolated at a low frequency (<5%), which cannot be considered as minor (e. g., Klebsiella spp.) or noncontagious (e. g., Corynebacterium spp.). The overall proportion of antibiotic resistance was low, except for penicillin G in staphylococci, as well as for macrolides and tetracycline in streptococci. Yet, these resistance proportions were much lower than those reported in human medicine. Besides providing up-to-date information on mastitis in France, this survey also indicates the prudent use of antibiotics by veterinarians. As a result, this study suggests that the risk of transmission of resistant bacteria from milk or milk products to human is very limited, even in case of consumption of raw milk. However, it also confirms the fact that attention must be maintained to avoid any emergence of such resistant bacteria.