ASSOCIATION OF COW-LEVEL FACTORS WITH MASTITIS OCCURRENCE IN DAIRY CATTLE IN SAMDRUPJONGKHAR DISTRICT

Abstract

Mastitis continues to be a major concern impacting dairy production and herd productivity in Bhutan, and evidence on cow-level factors associated with mastitis is limited. Routine surveillance data offer an opportunity to explore host-related associations under field conditions, to assess the association between selected cow-level factors (age, parity, and breed) and mastitis occurrence among dairy cattle. A cross-sectional study was conducted using routine mastitis surveillance data from 355 lactating cows screened across six sub-districts under Samdrupjongkhar District. Mastitis status was determined using the California Mastitis Test (CMT), with cows classified as positive if at least one udder quarter tested positive. Age and parity were categorized into biologically relevant groups, and breed was classified as Pure Jersey or Jersey cross on phenotypic characteristics. Firth penalized logistic regression was applied to address sparse outcome events and unequal subgroup sizes. Odds ratios (OR) with 95% confidence intervals (CI) were estimated, and model fit was assessed using the likelihood ratio test. The overall mastitis prevalence was 12.1% (43/355). Unadjusted prevalence was higher among Pure Jersey cows (42.1%) compared with Jersey cross cows (10.4%). In the multivariable Firth regression model, breed was the only factor significantly associated with mastitis occurrence. Jersey cross cows had markedly lower odds of mastitis compared to Pure Jersey cows (OR = 0.15, 95% CI: 0.06-0.42, p< 0.001). Age and parity were not significantly associated with mastitis. The overall model was statistically significant (LR χ² = 19.55, df = 6, p = 0.0033). Mastitis occurrence in the study population was relatively low, and breed was the only cow-level factor significantly associated with mastitis. Due to the use of routine surveillance data and a limited number of positive cases, the findings should be interpreted cautiously. Further studies incorporating larger sample sizes and management-level risk factors are recommended to better characterize mastitis epidemiology in Bhutan.