Duration of bovine intramammary infections in commercial dairy herds

Data on the infection status of cows on seven commercial dairy farms were collected over 492 full lactations. Foremilk samples were taken at an average interval of five weeks. A total of 249 streptococcal and 433 staphylococcal infections were diagnosed. Spontaneous elimination occurred in 49 per cent of all streptococcal infections and in 54 per cent of Staphylococcus aureus infections. The average duration of spontaneously eliminated infections was 10.8 weeks for Streptococcus agalactiae, 9.9 weeks for Strep dysgalactiae, 10.4 weeks for Strep uberis and 12.8 weeks for Staph aureus. The average duration of infections persisting until drying off was 19.3 weeks for Strep agalactiae, 18.7 weeks for Strep dysgalactiae, 18.5 weeks for Strep uberis and 25.2 weeks for Staph aureus. The method and rate of elimination of infection as found in this analysis are of value for estimating new infection rates and selecting quarters for dry cow therapy.