Effect of natural infection with minor pathogens on susceptibility to natural infection with major pathogens in the bovine mammary gland

Objective - To evaluate the effect of natural udder infection with minor pathogens on subsequent natural infection with major pathogens. Sample population - 7 dairy herds with low bulk milk somatic cell count. Procedure - During a 20-month prospective study, milk samples were collected from dairy cows at regular intervals and from quarters with clinical signs of mastitis. Incidence of intramammary infection was calculated in uninfected quarters and in quarters infected with minor pathogens. A within-cow, matched case-control analysis was used to evaluate the effect of minor pathogens on subsequent infection with major pathogens. Results - Quarters infected with minor pathogens had higher somatic cell count than did uninfected quarters. In quarters infected with Corynebacterium bovis, the rate of infection with major pathogens was lower, whereas in quarters infected with coagulase-negative Micrococcaceae, the rate of infection with major pathogens was higher than that in uninfected quarters. From the within-cow comparison, it appeared that in quarters infected with minor pathogens, infection with major pathogens was significantly lower than that in comparable control quarters not infected with minor pathogens. Conclusions - Minor pathogens have a protective effect against infection with major pathogens. The protective effect of C bovis against subsequent infection with major pathogens appears to be greater than the effect of coaqulase-neqative Micrococcaceae.