Immune competence of the mammary gland as affected by somatic cell and pathogenic bacteria in ewe with subclinical mastitis

ABSTRACT Immune competence of the ewe mammary gland was investigated by monitoring the leukocyte differential count, the cytokines pattern and the endogenous proteolytic enzymes in milk samples with different SCC and pathogenic bacteria. Furthermore, the leukocyte differential count and T lymphocyte populations were evaluated in ewe blood. A total of 1,500 individual milk samples were randomly selected from the pool of the samples collected during sampling and grouped into five classes, of 300 samples each, on the basis of SCC. Classes were: SC300 (< 300,000 cells/mL); SC500 (from 301,000 to 500,000 cells/mL); SC1000 (from 501,000 to 1,000,000 cells/mL); SC2000 (from 1,001,000 to 2,000,000 cells/mL) and SC>2000 (> 2,001,000 cells/mL). Microbiological analyses of ewe milk were conducted to detect mastitis related pathogens. Sheep whose udders were without clinical abnormalities, and whose milk was apparently normal but with at least 103 cfu/ml of the same pathogen were considered to have subclinical mastitis and therefore defined as infected. PMNL and macrophages increased with SCC whereas lymphocytes decreased. Milk samples with SCC > 1,000,000 cells/mL showed differences in leukocyte populations between uninfected and infected ewes with higher percentages of PMNL and macrophages and lower percentages of lymphocytes in infected animals. Non-viable PMNL levels were the highest in ewe milk samples with SCC < 300,000 cells/mL; starting from SCC > 500,000 cells/mL non-viable PMNL were higher in uninfected ewes than in infected ones. In infected animals giving milk with SCC > 1,000,000 cells/mL a higher CD4+/CD8+ ratio was observed suggesting that the presence of pathogens induced an activation of both CD4+ and CD8+. The level of TNF-α and IL-12 resulted higher in infected rather than uninfected ewes, irrespective of SCC. Plasmin activity increased along with SCC and was always higher in infected rather than uninfected animals; cathepsin D increased starting from SC2000 in milk samples from non-infected ewes and starting from SC500 in milk samples from infected animals. This research evidenced the utility to study the associations between somatic cells, cytokines, endogenous proteolytic enzymes and pathogenic bacteria to better understand the pathogenesis of subclinical mastitis in ewes and their effect on the immune response of ewe mammary gland.