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This study examined the sequential pathological changes in the lymphoid organs (bursa of Fabricius, thymus, spleen and caecal tonsils) of 7-week-old Harco pullet chicks that showed severe clinical disease and lesions during a natural infection with a virulent infectious bursal disease virus. Clinical signs were sleepiness, droopy appearance, greenish-whitish diarrhoea, anorexia and prostration followed by death. Mortality rate was 78% within 3 days of the infection followed by recovery. Gross lesions were marked haemorrhages in the pectoral and thigh muscles, mucosa of the proventriculus and gizzard junction, and caecal tonsils. Bursa of Fabricius, thymus, spleen and kidneys were initially enlarged; however, bursa of Fabricius and thymus were later atrophic. Histologic lesions showed marked oedema, infiltration of heterophils, hyperaemia, and lymphoid depletion and hyperplastic corticomedullary layer in the bursa of Fabricius, lymphoid necrosis in thymus, spleen, and caecal tonsils. Lymphocytic depletion was marked in the bursa of Fabricius as early as day 1 of the infection, and in the spleen, thymus and caecal tonsils on day 2 of the infection. However, there were fibroplasias in the bursa of Fabricius and thymus but repopulation of lymphocytes in the spleen and caecal tonsils of birds sacrificed on day 6 of the infection. Confirmation of IBD was carried out using agar gel immunodiffusion test. The above observations showed that marked depletion of lymphocytes in the lymphoid organs correlated with marked clinical IBD while repopulation of lymphocytes in the spleen and caecal tonsils correlated with the recovery phase in pullet chicks. The description of the pathological changes in lymphoid organs caused by the IBDV currently circulating in Nigeria will be useful in assessing the time and recognition of early diagnostic features of the disease. Keywords: Infectious bursal disease, Lymphoid organs, Pathology, Pullets

Evaluation of pathological changes of natural infectious bursal disease virus infection in the lymphoid organs of Black Harco pullets