Herd prevalence and incidence of Streptococcus agalactiae in the dairy industry of Prince Edward Island.

Herd prevalence and incidence of mastitis caused by Streptococcus agalactiae was determined for dairy cattle on Prince Edward Island during December 1992 and June 1994. For each census, bulk tank milk samples from all dairy herds (n = 452) in the province were tested on two occasions, and the results were interpreted in parallel. The combined sensitivity of the testing protocol was estimated to be 91%. The confirmatory latex agglutination test had previously reported specificities approaching 100%. Therefore, the estimated specificity of the testing protocol was assumed to be 100%. The apparent prevalence of S. agalactiae in December 1992 and in June 1994 was 17.7 and 13.1%, respectively. Based on the characteristics of the test, the estimated true prevalence was 18.9% in December 1992 and 14.4% in June 1994. Infection with S. agalactiae was associated with elevated bulk tank somatic cell count (SCC) and elevated standard plate counts. Economic losses associated with S. agalactiae were attributed to production losses (associated with bulk tank SCC), milk quality penalties (associated with bulk tank SCC and standard plate count), and decreases in milk quality (associated with bulk tank SCC). For herds that had been negative for S. agalactiae in December 1992, evaluation in June 1994 yielded an incidence of new infections of 3.51 per 100 herds per year.