Studies on acquired immunity to coccidiosis in bursaless and thymectomized fowls.

Fowls hatched from embryos inoculated with testosterone between the sixth and ninth day of incubation hatched without a detectable bursa of Fabricius. These fowls failed to develop antibodies as the result of repeated infection with and levels of serum immune globulin were usually markedly reduced or undetectable. There were very few pyroninophilic cells in the caeca or spleen and secondary foci in the spleen and caecal lymphoid tissue were either not detected or were very reduced in number. The spleen and thymus weights were significantly reduced by testosterone treatment. Nevertheless, these fowls were successfully immunized, so that they resisted infection when challenged with viable oocysts of . Active infection of normal, susceptible, control fowls significantly reduced the thymus weight and increased the bursa weight. Severe haemorrhage into the caecal lumen of infected fowls resulted in lowered blood erythrocyte and lymphocyte counts and a reduction in the total serum protein. Antibodies, capable of lysing sporozoites, were detected in normal fowls after immunization and in normal susceptible fowls 5 days after initial infection. Complete surgical thymectomy was attempted within the first 1½ hours after hatching. However, this was only exceptionally complete and about 10 per cent of thymic tissue was detectable at subsequent post-mortem 56 days later. Thymectomized birds produced antibody, pyroninophilic cells and secondary spleen foci indistinguishable from normal control immunized chickens. However, there was a significant reduction in the number of small lymphocytes in the blood. Although thymectomized fowls were successfully immunized against infection, there was an indication, shown by daily oocyst discharge determinations that thymectomized fowls were less resistant during immunization than normal fowls. However, both groups of fowls, when challenged, were fully immune. The significance of partial thymectomy and the time of thymectomy in the fowl in relation to the acquisition of resistance is discussed. The results support our previous observations which have failed to demonstrate a significant role for humoral antibody in the mediation of resistance to .