A total of 757 milk samples were taken from 57 farms throughout Wales as part of a mastitis investigation. Problem cows were selected using the most recent milk recording or, if these data were unavailable, the whole herd was assessed using the California mastitis test (CMT) and scored on a scale of 0 to 3. The affected quarter was identified using CMT. The results of the bacteriology were grouped into negative and positive culture. All contaminated samples were excluded. The CMT recording at the time of sampling, the infection status of the cow (new, first, chronic or repeat) as defined by national milk records and the number of quarters infected at the time of sampling were identified for negative and positive samples. Selecting quarters with a higher CMT score increased the likelihood of positive culture. Culturing from a sample with a CMT score of 3 was over three times more likely to yield a positive culture than a score of 1 (odds ratio [OR] 3.74, 95 per cent confidence interval [CI] 1.41 to 9.97) and 1.7 times more likely to yield a positive culture than a score of 2 (OR 1.70, 95 per cent CI 1.16 to 2.50). Culturing from a score of 2 was not statistically different from a score of 1 in terms of the likelihood that a positive culture would be identified (OR 2.20, 95 per cent CI 0.83 to 5.93). There was no statistically significant difference in the proportion of negative samples when comparing chronic, repeat, new and first infections or number of quarters infected.
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