Pattern of antibiotic resistant mastitis in dairy cows

Aim : To study the prevalence of drug resistant mastitis and their pattern of antibiotic resistance in dairy cows from Tamil Nadu. Materials and Methods: Isolation and identification of resistant pathogens were performed from acute clinical mastitis samples. Based on culture, isolation and sensitivity tests, cows with resistant mastitis were grouped as; Group I: Escherichia coli (n=119), Group II: Staphylococcus aureus (n=104) and Group III: Methicillin-resistant Staphylococcal aureus (MRSA) (n=12). The isolates were tested using agar disc diffusion method for their antimicrobial susceptibility and modified resazurin assay microdilution technique for minimum inhibitory concentration (MIC) to 8 antimicrobial drugs. The organisms were also confirmed for their identity by performing PCR on the bacterial pellet targeting the specific genes such as 16s-23s rRNA, inecA and blaZ respectively for the resistant pathogens and also confirmed by sequencing. Results: Antibiotic resistant mastitis was detected in 235 out of 401 cows accounting to 56.1%, The predominant resistant causative pathogen was E. coli (50.64%) followed by S. aureus (44.25%) and MRSA (5.11%). In vitro antibiotic sensitivity test and MIC breakpoints, E. coli 5. aureus and MRSA organisms showed more sensitivity to enrofloxacin, amoxicillin + sulbactam, gentamicin and ceftriaxone and had highest resistant to penicillin followed by amoxicillin, oxytetracyclinc and methicillin, E. colt and S. aureus isolates were found to be resistant to 1 or 2 antimicrobials, whereas most of the MRSA isolates were found to he multi-drug resistant i.e resistance to 3 or more of antimicrobials. Out of 235 milk samples, the specific target gene 16s-23sr RNA (E. coli), 16s-23s rRNA (S. aureus) and MRSA (inecA and blaZ) could he amplified from 119, 104 and 12 isolates with a percentage positivity of 50.64 (119/235), 89.64 (104/116) and 10.34 (12/116) respectively. Conclusion: Prevalence of antimicrobial resistance (AMR) in bovine mastitis pathogens was high. Most MRSA pathogens were multidrug resistant. E. coil and S. aureus isolates were resistant to few antimicrobials.