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. Many of the cows with SCC over 200,000 in Table 1 had subclinical mastitis. Even at European regulatory SCC of 400,000, a considerable number of cows in a herd could be expected to be infected.
Bacteria possess a wide array of defense mechanisms in an effort to avoid destruction.
Staphylococci produce a toxin that can impede migration of PMN towards chemoattractants. Also, as an infection persists and milk ducts remain clogged, secretory cells revert to non-producing state and alveoli begin to shrink. Substances released by PMN completely destroy the alveolar structure which are replaced by connective and scar tissue. Pockets of infection become walled off and they become difficult to reach with antibiotics. In addition, the clots formed by the aggregation of PMN and blood clotting factors may block small ducts and prevent complete milk removal.