Factors influencing the outcome of Escherichia coli mastitis in the dairy cow.

Eight dairy cows at various stages of lactation were infected in a single mammary quarter with approximately 5 X 10(2) colony-forming units of serum-resistant Escherichia coli. The severity of the mastitis which followed depended on the speed at which neutrophils were mobilised in the gland and the opsonic activity of the whey within the gland. Newly calved cows tended to be refractory to the presence of irritants in the udder and hence were slow to mobilise defence mechanisms following infection. A capsulated strain of E coli (B117) appeared to cause a more severe mastitis because it was more difficult to opsonise than a non-capsulated strain (P4).