Effect of naturally occurring intramammary infections by minor pathogens on new infections by major pathogens in cattle

New mammary infections were recorded in 3 dairy herds during a lactation period by bacteriologic examination of milk samples at 3-week intervals. Influences of the infection status of quarters at the time of new infection and of microorganisms responsible for bacterial invasion were analyzed. The new infection rate in uninfected quarters was about 3 times the rate in quarters already harboring bacterial considered minor pathogens (coagulase-negative staphylococci and Corynebacterium bovis) or major pathogens (Staphylococcus aureus, streptococci). The frequency of new infections with major pathogens was almost halved by preexisting infections with minor pathogens (P = 0.05), mainly because of coagulase-negative staphylococci (P = 0.05) and, to a lesser extent, C bovis (P = 0.19). New infections by minor pathogens also were less frequent in quarters harboring a major pathogen (P less than 0.05), indicating that the competition or antagonism between mammary infections was a general phenomenon.