California mastitis test scores as indicators of subclinical intra-mammary infections at the end of lactation in dairy cows

Intramammary infections (IMI) during the dry period can be reduced through the use of dry cow therapy (DCT); in the future, its blanket use is likely to be questioned in the light of public concern regarding the routine use of antibiotics in food producing animals. One possible alternative is to limit DCT to cows with IMI just before drying off, which would require a quick, simple identification of sub-clinical IMI. In the present study we examined quarter milk samples obtained from 240 cows one week before and on the day of drying off, using the California mastitis test (CMT) and for IMI by bacteriological culture. The results indicated that high CMT scores at drying off may be good indicators of IMI: there was a significant association between the frequency of isolation of major pathogens and the CMT score in milk samples obtained one week before (Pearson’s χ2 = 27.04, df = 4, p < 0.001) and those at drying off (Pearson’s χ2 = 25.87, df = 4, p < 0.001).