EIMERIA-ACERVULINA - INFLUENCE OF CORTICOSTERONE-INDUCED IMMUNOSUPPRESSION ON OOCYST SHEDDING AND PRODUCTION CHARACTERISTICS IN BROILERS, AND CORRELATION WITH A COMPUTER-SIMULATION MODEL

An experiment was conducted to investigate the effects of immune respo nsiveness on excretion of oocysts after E. acervulina infection and su bsequent effects on production characteristics of broilers (Gallus dom esticus). These effects were determined in broilers repeatedly infecte d with 2.85 x 10(3) oocysts of E. acervulina and treated with various dosages of corticosterone in the diet (0, 10, 20 and 30 p.p.m.). Corti costerone treatment did not have an effect on the peak oocyst excretio n, although it was administered from 4 days before initial infection. The number of oocysts excreted shortly after the peak and the length o f the excretion period were increased in corticosterone-treated groups . The absence of a difference in peak oocyst excretion was ascribed to the existence of a time-lag between first contact with the parasite a nd rate of development of protective immunity. In a recently developed computer simulation model this period was assumed to be 5 days. Assum ing that immunosuppression, through corticosterone, is only effective when protective immunity is in operation, the results indicate a time- lag of at least a few days, which supports the inclusion of such a tim e-lag in the computer simulation model. General immunosuppressive effe cts of the corticosterone treatment, monitored by antibodies and mitog en-induced lymphocyte stimulation confirmed that immunosuppression occ urred shortly after medication started. Infection did not have a signi ficant influence on production characteristics in animals without diet ary corticosterone. However. with increasing corticosterone levels the negative effects of infection on production also increased.