EFFECT OF MILK SAMPLE COLLECTION STRATEGY ON THE SENSITIVITY AND SPECIFICITY OF BACTERIOLOGICAL CULTURE AND SOMATIC-CELL COUNT FOR DETECTION OF STAPHYLOCOCCUS-AUREUS INTRAMAMMARY INFECTION IN DAIRY-CATTLE

Four milk sample collection strategies for bacteriologic culture and i dentification of bovine intramammary infection due to Staphylococcus a ureus were evaluated, Milk samples were collected at 24 h intervals fr om 245 lactating mammary quarters of 62 cows from one commercial dairy herd on 6 successive days. A total of 1470 quarter milk samples were available for study, Based on the bacteriologic culture results of all six quarter milk samples, each quarter was classified as infected wit h or free of S. aureus. The case definition used to establish the 'gol d standard' was the isolation of two or more colonies of S. aureus on two or more occasions from the six quarter milk samples obtained from a given mammary quarter, The probability of a false-negative classific ation of a mammary quarter using all six culture results was estimated to be less than 0.0046, while the probability of a false-positive cla ssification was less than 0.0004. Twenty-two quarters from 16 of 62 co ws had S. aureus intramammary infection. Inocula (0.1 ml) for bacterio logic culture were prepared in the laboratory from quarter milk sample s to represent alternative strategies for milk sample collection on fa rms. Sensitivity and specificity of detection of S. aureus-infected ma mmary quarters and/or cows was then determined, The accuracy of somati c cell counts for the same purpose was also determined for several cut -off values. The range of sensitivity values for bacteriologic culture and SCC were 91-100% and 54-95%, respectively, The range of specifici ty values for each test method ranged from 97.6 to 100% and from 81 to 83%, respectively. Bacteriologic culture, using any of the sampling s trategies examined, had high specificity ( > 98%) and relatively high sensitivity ( > 91%) for identifying S. aureus intra mammary infection (IMI). However, there was a great difference in the number of culture attempts necessary to achieve this accuracy which would influence a d airy farm manager's choice of which type of milk sample collection str ategy to use.