Risk factors for repeated cases of clinical mastitis during the same lactation

Abstract Text: The objective of this nested case-control study was to identify factors associated with the occurrence of repeated clinical mastitis (CM) cases during the same lactation. Between July 2013 and January 2014, a 400-cow herd of Holstein cows was visited monthly for data collection. Quarter milk samples were collected from all CM cases. A repeated case was defined when a cow experienced the second case of CM (regardless of the quarter) within the same lactation. For each repeated case that occurred, three control cows matched by days in milk (DIM) were randomly selected. On the visit day, teat and udder measurements were performed on cases and controls, such as position of the udder floor in relation to the hock, udder depth, teat length and diameter, teat end hyperqueratosis score, ease of milking score, and presence of udder abnormalities (lesions or unbalance). Milk production, parity and the occurrence of further cases of CM were also recorded. The odds of a repeated clinical case were estimated as a function of the variables studied. Data from 68 repeated cases and 171 controls were available. The median time to the first case of mastitis and interval between the first and second cases were 136 DIM and 58 days, respectively. Of the 68 case-cows, 24% had the same pathogen isolated from both the first and second cases, and 57% experienced CM in the same quarter. Parity was associated with the occurrence of a repeated case only in the univariate analysis and was forced into the final model. Cows of first parity were 2.5 more likely to experience a repeated case when compared to cows of parity > 2 (P = 0.04). Ease of milking and udder position in relation to the hock remained in the final model. Cows whose udder was below the hock and cows easy to milk were 6.5 (P < 0.01) and 6.8 (P< 0.01) times more likely to experience a second case of CM when compared to those whose udder was above the hock or difficult to milk, respectively. Results of this study suggest that ease of milking is an important risk factor for repeated CM that should be carefully considered in genetic improvement programs.

Keywords: milk quality, epidemiology, clinical mastitis