The leukocytes in milk may also release specific substances that attract more leukocytes to the area to fight the infection. Numbers of somatic cells remain in large concentrations after bacteria are eliminated until healing of the gland occurs. Clots formed by the aggregation of leukocytes and blood clotting factors may block small ducts and prevent complete milk removal. Damage to epithelial cells and blockage of small ducts can result in the formation of scar tissue in some cases, with a permanent loss of function of that portion of the gland. Return to Table of contents Mastitis Causing Bacteria Disease causing bacteria are often referred to as pathogens.
Environmental pathogens are often responsible for most of the clinical cases.
The incidence at calving is twice what it is at drying off. In New Zealand, most infections during the dry period and early lactation are due to Str. Subclinical infections are those in which no visible changes occur in the appearance of the milk or the udder, but milk production decreases, bacteria are present in the secretion, and composition is altered. Table 1 describes the negative relationship between SCC and milk yield in 30 Virginia dairy herds. As SCC increased, milk yield was depressed, with the impact greater in older lactation cows than first lactation heifers.