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Mastitis is the most costly disease in the dairy industry. Selecting the proper antibiotic treatment is beneficial for economic and avoids the emergence of antimicrobial resistance. The objective of the present study was to investigate the prevalence of methicillin and vancomycin resistant isolates of mastitis-causing Staphylococcus aureus and Enterococcus faecalis as a probable source of transferable vancomycin resistance to staphylococci. A total of sixty-one Staphylococcus aureus and eight Enterococcusfaecalis isolates were investigated for genotypic and phenotypic antimicrobial resistance.Presence of the mecA, vanA and vanBgenes were surveyed by PCR. The MIC (Minimum Inhibitory Concentration) of vancomycin was determined bybroth microdilution test for all the isolates.Moreover, the antibiotic resistance patterns of the isolates to the most common classes of antibiotics used in dairy cattle such as β-lactam, macrolides and tetracyclines were determinedusing the disk diffusion method. Among Staphylococcus aureus isolates, one MRSA (methicillin-resistant Staphylococcus aureus) isolate was detected while 47.5% of isolates were detected as multidrug-resistant. Furthermore, no phenotypic and genotypicvancomycin-resistance Staphylococcus aureus was found. Most of the Enterococcus faecalis isolates (6/8) showed high MIC for vancomycin (in the range of 128- 1024 μg/ml) and one vanA-type Enterococcus faecalis was observed. This study indicates thatsince the source of transferable resistance to vancomycin exists in dairy farms, there is a potential for emerging and spreading VRSA (vancomycin-resistant Staphylococcus aureus) in dairy cattle which is a risk to animal and human health.

Background: This field trial was conducted to describe and compare the efficacy of two treatment regimens comprising of pre-calving systemic administrations of marbofloxacin and tylosin in combination with dry cow therapy. For more precise detection of Staphylococcus aureus IMI, PCR (nucA gene) and RAPD-PCR genotyping have been applied. A total of 841 quarters from 212 dry cows within two herds were assigned to three groups TYLO, MARB and CONT. At 21±3 days prior to parturition, tylosin was injected for three days (10 mg/kg, SC) to cows from TYLO and at the same time, single injection of marbofloxacin (8 mg/kg, SC) was administered to cows from MARB. Cows assigned to CONT served as an untreated control. All quarters allocated to the groups received intra-mammary infusion of Kanaclox DC® at drying off. Milk samples from all quarters at one week, before last milking of dry off, 3 and 7 DIM were obtained for bacteriological and oxidative analyses. Results: Despite the fact that no significant differences in total cure rate within the groups was demonstrated, the S. aureus cure rate achieved in TYLO and MARB were 74 and 73.5% respectively both of which being significantly higher than in CONT (58.1%). No significant differences in total new IMI were observed in TYLO (40.9%) and MARB (38.5%) groups compared with CONT group (42.5%). Furthermore the rate of new S. aureus IMI was higher in TYLO (24.5%) and MARB (24.1%) than in CONT (13.8%) group. Clinical mastitis rate in cows that received systemic injections (TYLO=3.8%, MARB=5.8%) was significantly lower than those detected in control cows (CONT=11.3%). Paired S. aureus isolates from dry off and post-calving have been clustered into 9 different RAPD types (A-I). 8 paired strains collected at dry off were identical to those at post-calving and 35 strains had more than 60% of dissimilarity. Conclusion: The study demonstrates that dealing with herds in which S. aureus IMIs are prevalent, using close up broad-spectrum systemic antimicrobial agents can be unrewarding for preventing new S. aureus IMIs. However close up administration of systemic antimicrobials have the potential to reduce the incidence of clinical mastitis during lactation.