Avian coccidiosis. A review of acquired intestinal immunity and vaccination strategies.

The gut-associated lymphoid tissues contain B and T lymphocytes responsible for acquired immunity to avian coccidiosis. Intestinal B cells begin producing parasite-specific antibodies shortly after infection although their role in protecting against coccidiosis is debated. T-cell-mediated immunity, predominantly by intestinal intraepithelial lymphocytes and lamina propria lymphocytes, confers the main component of protective immunity to Eimeria. Many of these cells display the CD8 and gammadelta T-cell receptor surface antigens, phenotypic markers of cytotoxic T cells. Although their role in eliminating Eimeria infection remains to be completely elucidated, T cells have been implicated in parasite transport, and their activity is augmented by interferon-gamma and interleukin-2. Because of the importance of cell-mediated immunity, coccidiosis vaccines must be capable of stimulating intestinal T cells. Orally delivered, live parasite vaccines, either unattenuated or attenuated, are powerful stimulators of intestinal cell-mediated immunity, but antigenic variability between Eimeria species present in the vaccine and in the field may restrict their commercial application. The newer generations of recombinant DNA and subunit protein vaccines, particularly when used in conjunction with interferon-gamma and interleukin-2, have shown preliminary promise in controlling experimental infections but have yet to be commercially developed.