SUMMARY. In a 2 × 2 factorial study, a broiler starter ration was amended for vitamin A (control, C; deficient, A) and probiotic status (−, P) to investigate their modulatory effects on the host immune system. Birds were inoculated orally with Eimeria acervulina (EA) oocysts, and disease susceptibility was evaluated by assessment of fecal oocyst shedding. Humoral and local cellular mediated immunity were assessed by evaluation of antibody and cytokine (interferon-γ [IFN-γ] and interleukin-2 [IL-2]) levels in sera and intestinal secretions on a 3-day interval after inoculation. Fecal oocyst shedding was highest (P < 0.05) in A− birds, followed by AP, C−, and CP birds. Feeding the probiotic reduced shed oocysts by 20% in A fed birds and by 26% in C fed birds. Intestinal IFN-γ was relatively constant in all treatment groups except for A−, where it declined steadily and was lower (P < 0.05) from day 6 on. Serum IFN-γ levels fluctuated within each treatment and over time were not revealing. Intestinal IL-2 was highest in CP birds at 3 and 9 days postinfection (DPI) and lowest in A− birds at 3, 9, and 12 DPI (P < 0.05); no difference between treatments was found at 6 DPI (P > 0.05). Eimeria-specific intestinal antibody (Ab) level was constant (P > 0.05) in C− birds but increased with time (P < 0.05) in A−, AP, and CP birds. Serum Ab levels were also constant in A− and CP birds but increased (P < 0.05) in C− and AP birds after 6 DPI. The data demonstrate for the first time a probiotic-enhanced immunity in vitamin A deficient birds. This study is also the first to demonstrate the probiotic effect on local cell-mediated immunity of chickens, best manifested by apparent lower intestinal invasion and development by EA, on the basis of higher IL-2 secretion and lower EA oocyst production.
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Vol. 47 • No. 4