Leukocyte Population in the Peripheral Blood and Mammary Secretions in the Field Cases of Subclinical Mastitis on the Day of Dry-off and Calving -

Mastitis is an inflammation of the mammary gland most commonly caused by bacterial infection, with the highest incidence soon after the calving. We hypothesized that some cases of the periparturient mastitis is due to a relapse of underlying mammary gland inflammation which is initiated prior to the dry period. In order to test our hypothesis, leukocyte population in the peripheral blood and milk on the day of dry-off as well as the day of calving were analyzed using cows with subclinical mastitis at the dry-off. Twenty-two cows were used, sixteen with subclinical mastitis (Mastitis Group) and six healthy cows (Control group). The leukocyte population in the peripheral blood and milk of the quarter that was infected with bacteria at dry-off point and calving day was assessed. Bacteria were isolated from milk samples in all mastitis quarters at dry-off, but no bacteria were detected in the colostrum. Counts and percentages of the peripheral CD4 + and CD8 + T-cells in the Mastitis Group were lower than the Control Group at both collection points, and significant differences were found at calving. Count of the peripheral CD21 + B-cell in the Mastitis group was lower at calving. In colostrum, the percentage of CD4 + T-cell and CD14 + cell in the Mastitis group were higher than the Control Group, but CD8 + T-cells were lower at both collection points. In the present study, decreased CD8 + and increased CD14 + cell were found regardless of the bacterial absence in the colostrum of the Mastitis Group. The reaction of the mammary immune cells of dairy cows may remain post-infection at drying-off throughout the dry period. the absence of bacteria at the calving of Mastitis group indicates that the stimulated immune reaction during the dry period was capable of clearing the existing intramammary bacterial infections. Periparturient immunosuppression caused by physiological changes results in an increased incidence of mastitis soon after the calving. However this study indicated that it is not the case in cows with SM at dry-off. The other immuno-suppressive factors, such as hypocalcemia, negative energy balance and bovine leukocyte virus infection during the periparturient period may heighten the susceptibility of cows to mastitis in addition to the normal physiological factors 5, 6, 8 . Without these additional immunosuppressive conditions, cows, which had SM at dry-off could go through the periparturient period with slightly stimulated immune status without developing diseases. Further investigation whether the immune response to intramammary pathogens vary the leukocyte function in the mammary gland is required.