Prevalence, characterization, and antibiotic susceptibility profiles of bovine mastitic bacteria in Nyeri County, Kenya

Bovine mastitis remains a significant burden in the dairy industry, resulting in substantial economic losses.The present study determined the prevalence, bacterial diversity, and antibiotic resistance profiles of the bacteria isolated from bovine mastitis cases in Nyeri County, Kenya.A total of 1600 milk samples from 400 lactating dairy cows were collected and screened for mastitis.Clinical and subclinical mastitis was screened by physical examination and the California Mastitis Test (CMT).No clinical mastitis cases were detected; however, 200 CMT+ (positive) milk samples were sampled and cultured for bacteriology.The isolates were confirmed using Matrix-Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-TOF) and polymerase chain reaction (PCR).The antibiotic susceptibility profile for the isolates was determined using the Kirby-Bauer disc diffusion method.Based on the CMT and culture method, the prevalence of subclinical mastitis (SCM) was 12.5% and 88% at the cow level, respectively, indicating that CMT is a poor screening test for bovine mastitis.The most predominant pathogens were Coagulase-negative Staphylococcus (CoNS) at 70% (96/136) and Staphylococcus aureus (23%).Among 136 isolates evaluated for antibiotic susceptibility testing, S. aureus and CoNS showed the highest resistance to tetracycline (71.9% and 66.0%, respectively) and ampicillin (53.1% and 47.9%, respectively).Furthermore, both species were more susceptible to norfloxacin (90.6%) and gentamicin (93.8%).Escherichia coli isolates showed significant resistance to ampicillin (66.7%) and cefoxitin (50%).Notably, Acinetobacter baumannii and Corynebacterium bovis isolates were 100% sensitive to all the antibiotics screened.However, 26.5% (36/136) of the isolates demonstrated multiple antibiotic resistance (MAR) to commonly used antibiotics in mastitis treatment.In this study, Methicillin-resistant Staphylococcus aureus (MRSA) was identified in 15.6% (5/32) of the samples.Genotyping of S. aureus revealed the presence of blaZ genes in 33.3% of the isolates and tetK genes in 16.7%.In conclusion, a high prevalence of SCM was documented in the study, with CoNS being the dominant pathogen causing mastitis.Therefore, the high prevalence of SCM and high multiple antibiotic resistance point to a significant public health concern, particularly in relation to milk as a direct food product.There is a need for further studies to establish the extent of antibiotic resistance transmission into the food chain, with a view to mitigating this public health challenge.