The early pathogenesis of bovine mastitis due to Escherichia coli

The pathogenesis of coliform mastitis was studied after infusing each of ten lactating quarters of three dairy cows with a large dose (ca. 1 x 10(9) colony-forming units) of virulent Escherichia coli strain B117. This approach was adopted first to maximize the chance of observing microscopic lesions in the tissues of a gland and secondly to overwhelm the differences that might be shown between animals in their response to the infection. The infected glands were examined at intervals of up to 4 h after infection by scanning and transmission electron microscopy and by light microscopy. The earliest lesions were seen after 1 h and consisted of necrosis and sloughing of the epithelial cells of the teat and lactiferous sinuses. After 2 h this was more severe, and was followed by an intense neutrophil response. Neutrophils migrated through the epithelial lesions and at first remained attached to the epithelial surface, forming large mounds. This resulted in gross underestimation of the number of cells in the lumen of the gland when neutrophils in the secretion were counted. At no stage was there evidence of attachment of organisms to the epithelial cells. Tissue damage did not extent beyond the basement membrane, which helps to explain the rapid clinical resolution seen in most field cases of the disease. There was considerable variation in the degree of response shown by the three cows, and also within the infected glands, where the damage was most severe in the lactiferous and teat sinuses. It seems unlikely that all aspects of the disease could be attributed to endotoxin.