A 12-month prospective study of clinical mastitis was conducted in 482 British dairy herds with a bulk milk somatic cell count (BMSCC) of less than 150, 000 cells/ml. The mean proportion of cows in the herd with clinical mastitis was 23.1 per cent (range 0 to 80), with a mean of 1.50 quarter cases per cow. The mean incidence rate of clinical mastitis was 36.7 quarter cases per 100 cow years (range 0 to 208.2). Twelve per cent of the quarter cases showed systemic signs, including inappetence and severe lethargy. Over 22 per cent of quarter cases occured in the first seven days of lactation and over 50 per cent occured in the first 90 days of lactation. Cluster analysis indicated that the main difference between herds with a low and average incidence of mastitis was the proportion of clinical cases occuring in the first seven days of lactation, 14 per cent compared with 44 per cent, respectively. The risk of severe clinical mastitis compared with mild clinical mastitis decreased significantly as the individual cow somatic cell count (SCC) in the month before clinical mastitis was diagnosed increased.
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