Antimicrobial resistance patterns of bovine mastitis pathogens isolated in the Northeast of Italy between 2010 and 2014

Bovine mastitis is one of the main health problems in dairy cattle and represents a major cause of economic losses in dairy production. As well as the financial implication of this disease, the importance of mastitis in public health should not be overlooked. The extensive use of antibiotics in the treatment and control of mastitis has possible implications for human health through an increased risk of antibiotic resistant strains that may enter the food chain. The aim of this study was the assessment of the main antimicrobial-resistance (AMR) patterns of bovine mastitis pathogens isolated in Northeast of Italy from 2010 to 2014 during the normal diagnostic activity performed in the local veterinary state laboratory (IZSVe). All the isolates tested were identified from individual quarter milk samples, which were collected from animals with clinical and subclinical mastitis. Milk culture was carried out according to the National Mastitis Council guidelines and the in vitro antimicrobial susceptibility was evaluated using the agar disk diffusion method as described by NCLSI guide Vet01-A4. All the isolates were tested with the following antimicrobials: penicillin, ampicillin, oxacillin, clavulanate-amoxicillin, cephalothin, cefquinome, cefoperazone, tylosin, spiramycin, tetracycline, enrofloxacin, kanamycin, sulfisoxazole, trimethoprim + sulphametoxazole, and rifaximine. In our study, out of 16095 strains isolated from bovine milk, 29.2% were Coagulase-positive Staphylococci (CPS), 28.3% Streptococcus uberis and Enterococcus spp., 19.3% Coagulase-negative Staphylococci (CNS), 9.1% E. coli, 7.6% Streptococcus dysgalactiae, 5.6% Streptococcus agalactiae, 0.5% Klebsiella spp., 0.5% Serratia spp.. Overall, more than 90% are Gram-positive bacteria with significantly (p<0.05) higher average sensitivity percentages than Gram-negative strains (79.8% vs 72.5%). For CPS, the lowest sensitivity weighted average percentages were reported towards betalactams (84.6%), tetracycline (88.7%) and macrolides (89.7%). Within betalactams, the sensitivity range was wide (57.8-99%): penicillin G (57.8%) and ampicillin (65.9%) had the lowest susceptibility values, whereas oxacillin (95.8%), clavulanate-amoxicillin (92.7%), and cephalothin (99%) showed higher susceptibility. For CNS the least effective antibiotics were tetracycline (80.6%), betalactams (82.2%), macrolides (88.8%) and quinolones (90%). Betalactams showed a wide range of sensitivity: penicillin (53%), ampicillin (60,9%), clavulanate-amoxicillin (96.8%), oxacillin (78.1%), cephalothin (97.6%), cefoperazone (95.6%), cefquinome (97.9%). In CNS high in vitro susceptibility was found also towards sulphonamides (95.9%), and aminoglycosides (95.5%). The lowest sensitivity percentages for Streptococcus uberis and Enterococcus spp. were found towards aminoglycosides (19.9%), quinolones (35.5%) and tetracycline (37.2%). Higher percentage were found for macrolides (65.7%) and sulphonamides (76.6%), while the sensitivity patterns towards betalactams members were as it follows: oxacillin (51.6%), penicillin G (86.3%), ampicillin (82.2%), amoxicillin-clavulanate (98.5%), cefalotin (90.3%), cefoperazone (91.7%) and cefquinome (93.8%). For E. coli the lowest susceptibility levels were shown towards tetracycline (70.5%), ampicillin (56%), clavulanate-amoxicillin 66.7%, while the most efficient antimicrobials in vitro were quinolones (89.6%), sulphonamides (87.3%) and cefquinome (94.6%). In conclusion, the assessment of routine laboratory data can provide useful and costless information in order to improve therapeutic protocol and represents an important surveillance tool to investigate the level of antimicrobial resistance in dairy cows population.