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Relationships Between Wisconsin Mastitis Test Scores and Cell Counts in Milk

Department of Clinical Pathology, University of California, Davis

ABSTRACT

Wisconsin Mastitis Test (WMT) scores and cell content were determined on milk samples of quarter and tank origin over a period of five days. Wisconsin Mastitis Test scores utilized for analysis ranged from 4 to 34 and cell content of the samples from 87,000 to approximately 4 million per ml.

The relationship between WMT score and cell content was the same for milk of quarter or tank origin when the samples were of similar age, except possibly for tank milk collected on a 48-hr pickup schedule.

A direct relationship between WMT score and cell content was obtained when the cell content, divided by 10⁵, was converted to its common logarithm and then plotted against the WMT score. The conversion of WMT score to cell content as originally suggested was found to underestimate the cell content of the sample by approximately 38%. Our best estimate of the line of regression for WMT scores on cell content of milk 0–24 hr old is Y_WMT = 0.352 + 21.17 log₁₀ (cells x 10^–5).

Wisconsin Mastitis Test score and cell content of milk did not decrease proportionately with age of milk and scores decreased faster than cell numbers. Therefore, the estimated line of regression for fresh milk cannot be used for milk older than 24 hr, if an estimate of the original cell content of the milk is desired. The regression line Y_WMT = –.231 + 19.51 log¹⁰ (cells x 10^–5) is our best estimate for converting WMT scores of milk between 25 and 48 hr old to cell content at time of milking.

Additional regression lines are presented for older milk and for comparison between laboratories. Implications of the changing relationship between WMT score and cell content of milk samples are discussed.