Eighteen commercial hybrid broiler chickens were raised for a period of six weeks and assigned to either one of three treatment groups: treatment 1 was given a basal ration without additional vitamin E supplementation while treatments 2 and 3 were fed with the same basal rations supplemented with 50 IU and 150 IU vitamin E per kilogram of feed, respectively. All birds were vaccinated at day 14 against Newcastle's Disease (NCO) and subjected to two weeks of heat stress starting four weeks up to six weeks of age by exposure to temperature of 32-34°C using 100-watt incandescent bulbs from 11 in the morning to 4 in the afternoon. Serum samples were collected at days 21 and 35 to determine antibody titer against NCO while blood samples for total and absolute white blood cell (WBC) counts were collected on days 35 and 42.
The groups supplemented with vitamin E have a relatively high WBC, lymphocyte, heterophil, eosinophil, basophil, monocyte counts, and antibody titers compared to control but differences were not significant (P>0.05). Qualitative examination of the spleen's histology revealed no difference between groups in terms of follicular hyperplasia. Examination of the cloacal bursa histologically revealed a relatively high incidence of follicular regression and cellular deaths in the control group. Incidence of follicular regression in the unsupplemented group was significantly higher (P<0.05) compared to the group supplemented with 150 IU vitamin E.