Epidemiology of Streptococcus uberis intramammary infections in a dairy herd

From 1987 to 1991, almost 36 000 quarter samples of mammary secretion repre senting 1790 lactations of 510 dairy cows from a research herd were collect ed for bacteriological examination. The percentage of cows infected with St reprococcus uberis ranged from 12 to 16% of cows/year. S. riberis was isola ted from 14.2% of lactations over the 5-year period. The prevalence of S. u beris intramammary infection (IMI) was significantly higher in cows with gr eater than or equal to 4 lactations than in cows with 3 or fewer lactations . Regardless of lactation number, the prevalence of S, tuberis was highest before parturition, during early lactation and near drying off. The prevale nce of S: uberis infected quarters ranged from 1.3 to 2.3% of quarters/year ; the prevalence rate for the 5-year period was 2% of quarters. The quarter prevalence of S. uberis was lowest in cows with less than or equal to 3 la ctations, increased significantly with lactation number and was highest in cows with greater than or equal to 6 lactations. The percentage of quarters infected with S. tuberis varied significantly by year. The majority (95%) of S, uberis IMI were subclinical. The ratio of subclinical IMI to clinical IMI was lowest during early lactation, and increased with days in milk, an d with lactation age except for cows in their 5th and 6th lactations. Resul ts of this epidemiological investigation suggest that opportunities exist w here suitable control measures could be applied to reduce the impact of S. uberis infections in the dairy herd.