Influence of Parity and Stage of Lactation on the Somatic Cell Count in Bacteriologically Negative Dairy Cows

This study examines the influence of parity, stage of lactation, and single isolations (i.e., the isolation of a microorganism that could not be reisolated in the same quarter in the next sampling) of staphylococci other than Staphylococcus aureus (coagulase-negative staphylococci), Corynebacterium bovis, or esculin-positive cocci other than Streptococcus uberis (referred to as esculin-positive cocci throughout) on the monthly log_e-transformed somatic cell count (SCC) for 180 first, second, and third parity cows that were observed over a whole lactation. Repeated measures ANOVA was used to analyze the data. No significant effect was found for the infection variable. However, the results indicated that even single isolations of coagulase-negative staphylocci, C. bovis, or esculin-positive cocci resulted in a numerical or statistically significant increase in SCC. Least squares mean SCC (log_e-transformed) for bacteriologically negative cows and cows with single isolations of coagulase-negative staphylococci, C. bovis, or esculin-positive cocci were 3.90, 3.97, 4.08, and 4.17, respectively. Significant effects of parity, stage of lactation, and the interaction of parity and stage of lactation could not be found when only bacteriologically negative cows were considered. Least squares mean SCC for first, second, and third parity cows were 3.80, 3.93, and 3.97, respectively. However, the effects of parity, stage of lactation, and the interaction of parity and stage of lactation were significant when all 180 cows were included. Therefore, these effects must be due to factors that were present in the infected groups.