The use of induced mammary infections for evaluating dry cow treatment products. I. Development of a method.

Experimental infections were induced by infusing one of three different numbers of colony forming units of Staphylococcus aureus, strain 305, into 74 quarters of 17 cows, at 30, 15 or five days before drying off. The remaining 26 served as controls. A total of 73% of infused quarters were infected both at drying off and calving, although some changes in status occurred during the dry period. All control quarters were negative at drying off, but two, one in each of two lactations of the same cow, became infected during the dry period. There was no change in infection status during the dry period in 86.5% and 88.5% respectively of infused and control quarters. Disregarding dose size, 18/27, 19/24 and 17/23 quarters became infected following infusion at five, ten and 30 days before drying off. Disregarding time, significantly more infections (88%) followed infusion of a 10(-4) dilution (mean count 3215 +/- 182 S.E.) than with 10(-5) and 10(-6) dilutions combined (65%). It was concluded that a dose of S. aureus, strain 305, consisting of 0.2 mL of a 10(-4) dilution in sterile milk of a six hour milk culture would provide optimum infection levels if no antibiotic treatment were given. Wether infusion took place 30, 15 or five days before drying off appeared immaterial.