Group size and density effects on physical indices and cell-mediated immunity in two genetic lines of white leghorn layers.

Social stress resulting from varied group sizes with density in caging systems is a common problem that affects bird welfare in the poultry industry in the United States. To examine whether there is a genetic basis for variations in response to group size and density, 2 genetic strains of White Leghorn hens were used in this study: HGPS, a line selected for high group productivity and survivability in multiple-bird cages and DXL (Dekalb XL), a commercial line individually selected for production. Birds from each line were housed in either 4-bird cages (542 cm(2)/bird) or 10-bird cages (434 cm(2)/bird) from 17 to 60 wk of age. Blood samples, BW, and adrenal weight were collected from the birds at 30, 45, and 60 wk of age (n = 12, respectively) after euthanasia. Subsets of T cells (CD4(+) and CD8(+)) were measured by flow cytometry. Hematological parameters were collected from blood smears. Results showed that there were no significant differences for BW, the relative adrenal glands, and the heterophil:lymphocyte ratio between and across lines after the treatments (P > 0.05, respectively). Compared with DXL birds, the HGPS birds had a greater ratio of CD4(+):CD8(+) at 30 and 45 wk of age. There was an age-associated decrease in CD4(+) cells in all groups from 45 to 60 wk of age. However, an age-related decrease in CD8(+) cells from 45 to 60 wk was found in the DXL birds only. The unique changes in T-cell subpopulations and the ratio of CD4(+):CD8(+) may suggest that HGPS birds had a greater cell-mediated immunity than DXL birds. The genetic basis of variations in immunity may correlate with the line-unique ability of birds to cope in social environments and their survivability.