Ability of hematologic and serum biochemical variables to differentiate gram-negative and gram-positive mastitis in dairy cows

Medical records. of 142 dairy cows with clinical mastitis were examined to determine whether hematologic or serum biochemical results could be used to distinguish between mastitis episodes caused by gram-negative bacteria (n = 78) from those caused by gram-positive bacteria (n = 64). Signalment, his toric information, hematologic and serum biochemical results, milk culture results, and outcome (discharged from hospital or died) were obtained from the medical records. Cows with gram-negative mastitis had significantly (P < .01) lower blood leukocyte, segmented neutrophil, monocyte, and lymphocyt e counts and had higher blood hemoglobin concentrations and hematocrits:tha n did cows with gram-positive mastitis. Serum urea nitrogen was the only se rum biochemical result associated with pathogen type, and it was higher in cows with gram-negative mastitis than in those with grampositive mastitis. Mortality rate (25% overall) did not differ between groups. Logistic regres sion indicated that routine hematologic analysis (segmented neutrophil coun t, monocyte count, and hemoglobin concentration) was an accurate predictor of gram-negative mastitis, with a sensitivity of .93, a specificity of .89, and an overall accuracy of 91%. The values for sensitivity and specificity were higher than those previously reported for clinical tests differentiat ing mastitis episodes caused by gram-negative bacteria from those caused by gram-positive bacteria. Our results indicate that routine hematologic anal ysis is useful for predicting pathogen type in dairy cows with clinical mas titis, thereby facilitating treatment decisions.