This study investigated comparatively the genetic influence on the susceptibility of exotic cockerels, pullets and broilers to natural infectious bursal disease (IBD) virus in a flock of 150 seven-week-old exotic breed of chickens comprising of 50 Black Harco cockerels, 50 Black Harco pullets and 50 White Marshall broilers. Evaluation of possible genetic resistance was based on clinical signs, mortality and pathological changes in affected chicks. The virus was highly pathogenic for cockerels and pullets as clinical signs were evident on day 1 of the infection with hundred percent (100%) of the cockerels and pullets showing severe clinical disease on day 2 of the infection, while the broilers had the shortest timing as clinical signs were evident with 8% morbidity on day 2 of the infection only. The clinical signs were severe depression, diarrhoea, anorexia, prostration followed by death. Mortality was 92%, 78% and 6% for cockerels, pullets and broilers, respectively, within 3 days of the infection followed by rapid recovery. Severe haemorrhages were present in the skeletal muscles, bursa, proventriculus-gizzard junction and caecal tonsils of dead cockerels and pullets only, while dead broilers showed only swollen bursae. Histologic lesions showed marked oedema, congestion of blood vessels, haemorrhages and necroses in the skeletal muscles, kidney, liver, and thymus of cockerels and pullets. Lymphocytic necrosis and depletion were marked in the spleen and caecal tonsils of the cockerels and pullets. Marked lymphoid depletion, oedema and heterophilic infiltrations were observed at day 2 of the infection in the bursae of cockerels, pullets and broilers. Assessing the clinical signs and lesions observed from affected chickens revealed that broiler is the least susceptible. It also revealed that within the Black Harco breed, cockerels are more susceptible to clinical IBD than the pullets. The low morbidity and mortality, and differential lesions observed in broilers indicated probable genetic resistance to clinical IBD.
Key words: Infectious bursal disease, chickens, breeds, susceptibility disease pattern, pathology
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