Fluctuations, particularly in small herds tested monthly can be marked. Categorizing the pattern of ES mastitis as either high prevalence or high incidence must be done early in the herd investigation. Ultimately it will determine whether recommendations emphasize shortening the duration of infection, in high prevalence herds, or reducing the new infection rate, in high incidence herds. Individual cow SCCs and clinical case records are used to identify affected animals. The frequency of cases among animals of different parity and different stages of lactation is tabulated.
Case counts by lactation number help to identify management practises that differ among age cohorts.
For example, precalving environments, nutritional programs and therapy usually differ between heifers and cows and may influence the risk of mastitis. Other age differences also occur. Older cows, because of increased size, and longer lifetime exposure to the lactating cow environment, may be more prone to teat injury and milk leakage. It has been suggested that cows leaking milk at dry off time have a four times greater risk of a clinical case of mastitis during the dry period. Different mastitis rates in different parity groups also can reflect historic differences in housing, bedding, therapy or milking systems.