Epidemiologic Considerations in Reporting Herd Somatic Cell Counts

Abstract

The reporting of herd summaries of individual cow somatic cell counts differs between the nine Dairy Records Processing Centers. The objectives of this paper are 1) to review the present reporting; 2) to compare the methods used to calculate herd somatic cell counts; and 3) to discuss the epidemiologic implications of these methods.Estimates of central tendency and frequency distributions are the most commonly used herd summary statistics for somatic cell counts. Analyses of the frequency distributions are less widely available.Potential uses of herd somatic cell counts are 1) to measure one aspect of herd milk quality; 2) to estimate the prevalence of mastitis; 3) to analyze mastitis patterns within the herd; and 4) to investigate new areas for mastitis control research. Presently, the best available methods for these applications appear to be 1) bulk tank somatic cell counts or weighted arithmetic means for gauging milk quality; 2) frequency distributions for estimating mastitis prevalence; 3) analyses of frequency distributions or mean linear score by lactation and stage of lactation for identifying mastitis patterns within a herd; and 4) frequency distributions or estimated incidence rates, adjusted for differences in age and stage of lactation, for investigating new areas for mastitis control research.