Pathogenesis of experimental bovine mastitis following a small inoculum of Escherichia coli.

Mastitis was produced in four quarters of two lactating cows by the inoculation of 50 or 200 viable Escherichia coli. Changes were investigated by light microscopy and scanning electron microscopy. After 10 hours the changes were confined to the superficial layer of the epithelium of the teat and lactiferous sinuses; single cells or small groups of cells were damaged and were being extruded from the tissue. By 14 hours there was extensive necrosis and sloughing of the epithelial cells which did not extend beyond the basement membrane, and an intense inflammatory response associated with the epithelial damage. The somatic cells in the milk, mainly polymorphonuclear leucocytes, increased 40 to 250 times and strongly inhibited the survival of E coli. Epithelial damage appeared first in the ventral portions of the gland and then extended through the lactiferous sinus to the large ducts and secreting tissue. This pattern of damage together with the failure to observe bacteria attached to the epithelia was consistent with the production of diffusible toxins by the organisms.