The effects of in ovo mibolerone treatment on the bursa of Fabricius and the humoral system of chickens: a dose-response study.

The effects of various doses of mibolerone, an analog of testosterone, in the development of the bursa of Fabricius and the humoral immune system were examined by injecting 0.01, 0.1, 1, 10 or 100 micrograms mibolerone into the chorio-allantoic cavity of 12-day-old chick embryos. Treatment with 0.01 or 0.1 micrograms of mibolerone caused slight or no bursal atrophy, 1 microgram caused substantial bursal atrophy, and 10 or 100 micrograms caused severe bursal atrophy at the time of hatching and at 6 weeks of age. When mibolerone-treated birds were challenged with sheep red blood cells and killed Brucella abortus at 4 and 5 weeks of age, birds treated with 10 or 100 micrograms of mibolerone had suppressed humoral responses to sheep erythrocytes and B. abortus whereas birds treated with 0.01 and 0.1 micrograms of mibolerone had normal humoral immune responses. Birds treated with 1 microgram mibolerone had normal humoral immune responses to sheep erythrocytes and B. abortus (except for low IgG responses to B. abortus) despite the fact that the birds had substantially atrophic bursae during their embryonic and post-embryonic development. These results are discussed in relation to an earlier claim that mibolerone given in microgram quantities in feed during the post-hatch period causes rapid development of the humoral immune system.