Relationships among somatic cell count, California mastitis test, impedance and bacteriological status of milk in goats and sheep in early lactation

The objectives of this trial were to evaluate and compare the test characte ristics of a number of indirect tests of bacteriological status of the milk from goats and sheep and to assess the affect of varying levels of prevale nce of infection on the performance of those tests. The somatic cell count, California mastitis test (CMT) score, electrical im pedance and the bacteriological status of 220 and 262 milk samples from the glands of lactating dairy goats and sheep, respectively, were determined. The sensitivity and specificity of indirect tests in predicting the bacteri ological status were compared by analyzing the areas under the receiver ope rating characteristic curve and calculating the likelihood ratio at various cut-off values. Bacteria were isolated from 17.7 and 4.6% of glands from goats and sheep, r espectively. Somatic cell count was a better predictor of bacteriological s tatus than either the CMT score or impedance in both goats and sheep. Howev er, knowledge of the CMT score in both sheep and gears and the impedance in goats increases the likelihood of predicting the presence of a bacterial p athogen compared to no testing at all (P < 0.05). Knowledge of the age and days postpartum of the animals at the time of testing did not increase the probability that infection status could be predicted. Variation in cut-off levels to predict infection status reported in the literature may be due to variation in the prevalence of infection in the populations studied. Use o f Likelihood ratio allows comparison of tests over a range of prevalence's as likelihood ratios are independent of prevalence. The prevalence of infec tion within a herd should be considered when selecting a cut-off value for indirect testing for the presence of bacteria in milk Of Small ruminants. ( C) 2001 Elsevier Science B.V. All rights reserved.