Longitudinal evaluation of bovine mammary gland health status by somatic cell counting, flow cytometry, and cytology

Bovine mastitis phases induced by Staphylococcus aureus were assessed in 6 lactating cows before challenge and at 1, 4-8, and 9-14 days postinoculatio n (dpi). Milk lymphocytes, macrophages, and polymorphonuclear cells (PMN) w ere counted by conventional (manual) cytology, identified by CD3+ and CD11b + immunofluorescence and counted by flow cytometry (based on leukocyte forw ard and side light scatter values). Somatic cell counts (SCC) and recovery of bacteria were recorded at the same times. Preinoculation samples showed a lymphocyte-dominated composition. At I dpi, the percentage of PMN increas ed and that of lymphocytes decreased. At 4-8 dpi, PMN were predominant, but the percentage of mononuclear cells increased above that at I dpi and furt her increased by 9-14 dpi (when lymphocytes approached prechallenge values) . Based on leukocyte percentages, 3 indices were created from the data: 1) the PMN/lymphocyte percentage ratio (PMN/L), 2) the PMN/macrophage percenta ge ratio (PMN/M), and 3) the phagocyte (PMN and macrophage)/lymphocyte perc entage ratio (Phago/L). Significant correlations were found between cytolog ic and flow cytometric data in all of these indicators (all with P less tha n or equal to 0.01). These indices identified nonmastitic, early inflammato ry (1-8 dpi), and late inflammatory (9-14 dpi) animals. In contrast, SCC an d bacteriology did not. Although sensitivity of the SCC was similar to that of Phago/L, the specificity of SCC was almost half that of the Phago/L ind ex. Based on flow cytometry indicators, an algorithm for presumptive diagno sis of bovine mastitis was developed. Flow cytometry provides results as va lid as those obtained by conventional (manual) cytology, shows greater abil ity to identify mastitic cases than does SCC, and may identify 3 mammary gl and health-related conditions.