Relationship of early first lactation somatic cell count with risk of subsequent first clinical mastitis

The relationship between initial Somatic Cell Count (SCC) and time to first clinical mastitis was estimated from data including 20,422 Holstein heifers, without clinical signs of mastitis during the first month of lactation and with a first test day SCC lower than 400,000 cells /ml. Number of days from 35 days after first calving to first clinical mastitis event in first or second lactation was studied using survival analysis methodology, allowing for censored data. The model included the effects of SCC and milk yield at first test, herd-year, calving month, and lactation stage. Separate analyses were also performed for subsets of herds with low or high mastitis frequency and SCC. The risk of first clinical mastitis was highest around the second calving, in lactations starting in summer, and for high-yielding cows. The probability of clinical mastitis occurring increased continuously as initial SCC increased. The same pattern was observed in herds with low or high SCC level. In herds with the lowest mastitis frequencies, relationship between initial SCC and mastitis occurrence was weakest. But in all situations, results indicated that cows with the lowest initial SCC had the lowest risk for first clinical mastitis, without any intermediate optimum.  2000 Elsevier Science B.V. All rights reserved.