Cow factors affecting the risk of clinical mastitis

The objective of the present study was to identify cow risk factors associated with development of clinical mastitis (CM) in subsequent stages of lactation. A total of 3,309 lactations from spring-calving Holstein-Friesian cows were included in the analysis; parity number ranged from one to three, inclusive. A generalised estimating equations approach with a logit link function was used to account for the binary nature of the data and the unequal number of repeated records per cow. The depend-ent variable was the probability of developing CM in the subsequent stage of lactation. Independent variables included in the model were chosen using stepwise selection; herd, year of birth, month of calving, parity, period of lactation and previous CM his-tory significantly affected the probability of CM. Two-way interactions between parity and period of lactation and between parity and incidence of CM in the previous lac-tation were also included in the model. A greater probability of developing CM is expected in cows that experienced CM in the previous lactation and/or previously within the same lactation. The probability of CM occurring in cows that experienced at least one case of CM in the previous lactation was 0.92 to 3.75 times that of a cow that experienced no CM in the previous lactation. It is possible to predict the probability of an animal developing CM in the subsequent stage of lactation when information is available on the parity and month of calving of the animal and its pre-vious history of CM