Dynamics of nonclinical bovine intramammary infections with major and minor pathogens.

Udder infections were recorded in 3 herds throughout a lactation period with bacteriologic examination being done at 3-week intervals. New infection rates and levels of infection at calving differed significantly (P less than 0.001) between herds, but persistence of infection did not. Of the 324 infections, 80.5% lasted up to drying off. Of quarters infected with major pathogens, 16.8% eliminated their infection (63% became reinfected); 20.8% of quarters infected with minor pathogens eliminated their infection (43% became reinfected). Significant differences in persistence of infections, quarter clearing rate (defined as the rate at which infected quarters become and remain noninfected) and ability to give way to and to take place of a preexistent infection, did not occur between Corynebacterium bovis and major pathogens. Though 66% of coagulase-negative staphylococci infections persisted up to drying off, they disappeared more frequently (P less than 0.01).