Incidence of clinical mastitis on farms with low somatic cell counts in bulk milk

A total of 1140 clinical cases of mastitis, with at least one inflamed quarter, were reported on 125 farms with somatic cell counts in bulk milk less than 150,000/ml. The average annual incidence was 17.9 cases per 100 cows and ranged from none to 80 cases per 100 cows. The microorganisms most frequently isolated were Escherichia coli (16.2 per cent), coagulase negative staphylococci (13.0 per cent), Staphylococcus aureus (9.6 per cent) and Streptococcus uberis (8.0 per cent). Only two cases of Streptococcus agalactiae were found. As the incidence of clinical mastitis increased, the proportion of S aureus also increased, while the proportions of E coli, S uberis and Streptococcus dysgalactiae remained about the same. Most of the clinical cases of mastitis occurred in early lactation, in November, December and January. However, after correction for the number of calvings per month, the incidence of mastitis was highest in the early summer months.