Bulk milk samples of 227 at random selected herds and quarter milk samples of 10336 cows ofthese herds were collected once. Bulk milk somatic cell counts (BMSCCs), which are estimatedevery four weeks, being part of the Dutch Milk Quality Control Programme, were used to calculatethree derivative cell counts: the C3 count, the Cl3 count and the Ci8 count. The correlationof these counts with the prevalence of mastitis in herds was determined. Correlation coefficientsvaried from 0.42 to 0.62. BMSCC and the C3 were neither highly sensitive (maximum sensitivity 75%) nor predictive (maximum predictive value 70 %) in identifying herds with a high prevalenceof mastitis. At the cell count threshold C = 500, 55-76 % herds with a high prevalence ofsubclinical mastitis were not detected by the C3. Thirty-six percent of the herds with a C3 exceedingthe threshold of C = 500 and 42 % of the herds with a C3 exceeding the threshold of c =400 could not be classified as mastitis problem herds. Both BMSCC and C3 had higher sensitivitybut lower specificity and predictive value at a threshold of C = 400 than at the threshold of c= 500. BMSCC and the C3 could reasonably specifically (84-99 %) diagnose herds with a lowprevalence of mastitis. Because of their low sensitivity and predictive value it would not be justifiedto assume a high prevalence of subclinical mastitis on farms based on BMSCC and the C3,without further evidence. Direct bulk milk cell counts and their derivatives appeared unsuitablein determining the prevalence of mastitis in individual herds.
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