Variation in Wisconsin Mastitis Test Scores of bucket milk samples and relationship to bacterial infections.

Sources of variation in the Wisconsin Mastitis Test were studied in 4739 bucket milk samples from 111 cows in one herd during 103 days. Quarters of cows were classified infected or uninfected through bacteriologic assays of quarter milk samples. Quarters were uninfected in 70.4% of the cow-quarter-days and infected with Staphylococcus aureus and Streptococcus agalactiae in 1.4 and .9% of the quarter samples. Scores of all cow-days averaged 8.5. Wisconsin Mastitis Test scores of cow-days having no pathogens averaged 5.8. Correlation of duplicate samples was .97. Repeatability of monthly tests was .67. Days within month of lactation, month within age group, cow within age, and age differed. Older cows averaged higher Wisconsin Mastitis Test scores. Scores for uninfected cow-days did not differ with age. Scores were higher by 5.5 units in cows with infected quarters than in those with no infected quarters. The increase was greater with S. aureus or S. agalactiae present than with less virulent pathogens. In a cost-utility analysis, Wisconsin Mastitis Test was inefficient in detecting infected cows with a single test.