An observational study investigating the association of teat condition with clinical mastitis risk

The objectives were to investigate the association between machine milking-induced teat tissue changes and the occurrence of clinical mastitis in Holstein dairy cows. We conducted 2 separate prospective cohort studies at 1 commercial dairy farm in New York State to study the association of machine milking-induced short-term changes (STC, trial 1) and machine milking-induced long-term changes (LTC, trial 2) with clinical mastitis occurrence. Each trial consisted of four 3-wk periods. On d 1 of each period, STC and LTC were assessed through visual inspection and manual palpation. Short-term teat tissue changes were assessed based on teat discoloration, the condition of the teat base, and the consistency of the teat end and categorized into a binary variable (STC present or absent). Long-term teat tissue changes were categorized into 4 groups as follows: no callosity present (score 1), callosity ring present with no or only mild roughness (score 2); callosity ring and roughness present with keratin fronds extending 1 to 3 mm from the teat orifice (score 3); and callosity ring present with excessive keratin fronds extending ≥4 mm from the teat orifice (score 4). Data on clinical mastitis occurrence was extracted from the herd management software. We used the first clinical mastitis event of a quarter for each 3-wk trial period and applied a 14-d gap to define a new mastitis case within a quarter between 2 trial periods. In trial 1, STC was documented in 6,110 of 66,702 cases (9.2%) over the four 3-wk periods, The frequency distribution of LTC in trial 2 was as follows: score 1 in 7,348 of 61,624 cases (11.9%), score 2 in 48,553 cases (78.8%), score 3 in 5,514 cases (8.9%), and score 4 in 209 cases (0.3%). We documented 944/66,702 (1.4%) clinical mastitis cases at the quarter level during trial 1 and 866/61,624 (1.4%) cases during trial 2. Generalized linear mixed models with a logit link and a binomial distribution were used to test if machine milking-induced teat tissue changes were associated with clinical mastitis occurrence. In trial 1, we analyzed 66,702 quarter-level observations from 5,094 cows. We found no significant association between STC and clinical mastitis occurrence. Compared with a quarter with a teat without STC, the odds (95% CI) of clinical mastitis were 1.22 (0.95-1.59). In trial 2, we used 61,624 quarter-level observations from 4,998 cows. We found no significant association between LTC and clinical mastitis occurrence. Compared with a quarter with a teat with a smooth end (score 1), the odds of a clinical mastitis case were 1.02 (0.77-1.36) for score 2, 1.19 (0.84-1.69) for score 3, and 2.14 (0.80-5.71) for score 4. We concluded that in the study cohort presented here, machine milking-induced teat tissue changes were not associated with the occurrence of clinical mastitis.