A MODEL TO DETERMINE SAMPLING STRATEGIES AND MILK INOCULUM VOLUME FORDETECTION OF INTRAMAMMARY STAPHYLOCOCCUS-AUREUS INFECTIONS IN DAIRY-CATTLE BY BACTERIOLOGICAL CULTURE

A model was developed to evaluate the effects that methods of obtainin g milk samples and culture inoculum volumes had on the sensitivity of microbiological culture to detect Staphylococcus aureus intramammary i nfections (IMI). An assumption was made that milk from mammary quarter s infected with S. aureus only contains bacteria intermittently, A mod ified sine wave function was used to model this intermittent shedding pattern. Specifications for the components of the shedding cycle used in this function were based on quantitative culture results from 54 ex perimentally infected S. aureus quarters, sampled daily for a period o f 30-49 days. The components of the shedding cycle were length in days , peak number of CFU shed per milliliter of milk, and length of time i n the cycle when no shedding occurred. These components were used to e stimate the model's predicted distribution of S. aureus CFU ml(-1) mil k when individual quarter milk samples were cultured for S. aureus. Th e sensitivity of culture for several sampling methods was then calcula ted, The model predicted that culture of a single quarter milk sample had a sensitivity ranging from 60 to 87% for detection of S. aureus IM I depending on inoculum volume. Quarter milk samples taken on day 1 an d repeated either on day 3 or day 4, and cultured separately using 0.1 ml of milk for culture inoculum, were predicted to have sensitivities of 90-95% and 94-99%, respectively, Other milk-sampling strategies ex amined included culture of a composite milk sample (equal-volume mixtu re of milk from four separate mammary quarters) and pooled milk sample s in which samples from different milkings (either quarter or composit e samples) were mixed together and then cultured. The range of predict ed sensitivities of these other sampling strategies was 30-97%. Factor s having the greatest impact on the sensitivity of culture, in order o f importance were: the type of milk sample, the volume of milk culture d, and the time interval between repeated milk sample collection strat egies.