Predicting the probability of quarter infection (by major pathogens) from somatic cell concentration.

The herd factor did not significantly affect distributions of somatic cell counts of 6,560 milk samples (collected in 3 herds) within an infection status. Minor-infections distribution for a herd was closer to that of healthy quarters than that of major infections. About 35% of the quarters infected by major pathogens had counts less than 400,000 cells/ml. Statistical analysis by conditional probability indicated that herd major-infection level should be taken into account to predict the probability of quarter infection by major pathogens. Discrimination with a good reliability between presence and absence of pathogens, as estimated by a single somatic cell count, is possible only in herds with high major-infection level.