Prevalence of subclinical mastitis in a selected group of cows in Matara District of Sri Lanka: Risk factors, pathogens and antimicrobial resistance

This study aimed to determine the prevalence of subclinical mastitis (SCM), identify associated risk factors, characterize major bacterial pathogens, and evaluate their antimicrobial resistance in dairy cows in the Matara District, Sri Lanka. The study was conducted from October 2024 to March 2025, involving 220 lactating cows from 60 randomly selected groups. SCM was diagnosed using the California Mastitis Test (CMT), and positive milk samples were subjected to bacteriological identification. Antimicrobial susceptibility testing was performed following Clinical and Laboratory Standards Institute (CLSI) guidelines. Farm- and animal-level data were collected using a structured questionnaire and analyzed using descriptive statistics, chi-square tests, and binary logistic regression using IBM SPSS (version 25) Statistics. The overall cow-level prevalence of SCM was 30.0%, with a quarter-level prevalence of 11.5%. Logistic regression analysis identified older age (>5 years), higher parity, early lactation stage, and high milk yield as significant animal-level predictors of SCM (p < 0.05). Friesian crossbred cows showed comparatively higher susceptibility than other breeds. At the farm- level, larger herd size, once-daily milking, poor udder hygiene, poor housing cleanliness, and absence of supplementary feeding were significantly associated with increased SCM risk. Staphylococcus aureus (54.6%) was the predominant isolate, followed by Escherichia coli (19.7%) and Streptococcus spp. (13.6%). Antimicrobial susceptibility testing revealed high resistance of S. aureus to amoxicillin, while E. coli showed high resistance to enrofloxacin and amoxicillin. However, gentamicin and neomycin remained highly effective against E. coli, and enrofloxacin showed complete efficacy against Streptococcus spp. In conclusion, SCM remains a significant constraint to dairy productivity in southern Sri Lanka. The findings highlight the need for improved milking hygiene, routine bacteriological screening, and prudent antimicrobial use to mitigate antimicrobial resistance and improve udder health within a One Health framework.