Increases in SCC with Advancing Age and Lactation Tied to Infection History

From the NMC Newsletter "Udder Topics", April 1996

Somatic cell counts and mastitis incidence tend to increase as a cow gets older. Increases in average SCC with age may be due to increases in the proportion of cows infected and the average number of quarters infected per cow, and to chronic infections that produce increased tissue damage and cause a greater leukocyte response. Leukocyte numbers do not necessarily increase with age in cows with mastitis-free histories, suggesting that increased cell count is not due to age per se, but to prior exposure to udder pathogens.

Cell counts in milk from both infected and uninfected quarters are high at calving, but decline rapidly postpartum in uninfected quarters. Somatic cell counts do not increase, or increase only slightly, during late lactation in uninfected glands, but do increase when infection is present. Average cell counts tend to increase as lactation advances, but as with age, the increase is probably not due to stage of lactation, but to increased prevalence of subclinical mastitis over the lactation.

Source: "Current Concepts of Bovine Mastitis", National Mastitis Council, 1996

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