EFFECT OF NATURAL INFECTION WITH MINOR PATHOGENS ON SUSCEPTIBILITY TONATURAL 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. Sampl e population-7 dairy herds with low bulk milk somatic cell count. Proc edure-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 e ffect of minor pathogens on subsequent infection with major pathogens. Results-Quarters infected with minor pathogens had higher Somatic cel l count than did uninfected quarters. In quarters infected with Coryne bacterium 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 uni nfected quarters. From the within-cow comparison, it appeared that, in quarters infected with minor pathogens, infection with major pathogen s was significantly lower than that in comparable control quarters not infected with minor pathogens. Conclusions-Minor pathogens have a pro tective effect against infection with major pathogens. The protective effect of C bovis against subsequent infection with major pathogens ap pears to be greater than the effect of coagulase-negative Micrococcace ae.