Effect on outcome of intramammary challenge exposure with Staphylococcus aureus of somatic cell concentration and presence of an intramammary device.

Results of experimental Staphylococcus aureus intramammary challenge of all quarters of 6 cows, each fitted with an intramammary device (IMD) in 2 quarters, and of 10 quarters of 3 cows not fitted with an IMD were reported. Infection was established in all 34 quarters, regardless of presence or absence of an IMD. Neither the course nor severity of early S aureus intramammary infection were influenced by the presence of an IMD or by differences in milk somatic cell (MSC) concentration in the gland at the time of bacterial challenge infusion, up to a MSC concentration of nearly 1 million/ml. Cumulative success of experimental infection in this and a previous study from our laboratory was nearly 100% in glands in which the MSC concentration was less than 1 million/ml and about 17% when the MSC concentration exceeded 1 million/ml.