Identification of subpopulations of bovine mammary-gland phagocytes and evaluation of sensitivity and specificity of morphologic and functional indicators of bovine mastitis.

The number and function of bovine mammary-gland phagocytes were assessed in 8 lactating cows, each tested at least twice within an 8-mo period (total number of observations, 20). Macrophages and polymorphonuclear (PMN) cells were evaluated by conventional cytology, flow cytometry, fluorescent microscopy, and somatic-cell count (SCC). Phagocytosis was evaluated from the uptake of fluorescent beads and expressed as median fluorescence intensity (MFI). Two major subpopulations of phagocytes, of low or high MFI (LFI or HFI), were observed, and there were up to 4 sub-subpopulations within the HFI subpopulation of both macrophages and PMN cells. Fluorescent microscopy identified phagocytes containing up to 4 beads per cell. Cows showing < or = 72.3% phagocytes by cytology were regarded as non-mastitic (11 observations), and those showing > or = 80.7% phagocytes were considered to be mastitic (8 observations). Phagocyte MFI was negatively associated with mastitis; that is, the higher the MFI, the lower the SCC. The percentage of HFI PMN cells was the only indicator of mastitis with 100% sensitivity and specificity. Thus, bovine mammary-gland phagocytes consist of several subpopulations of different phagocytic ability, whose assessment more adequately predicts bovine mastitis than do morphologic indicators.