A study of the epidemiology of infectious bursal disease in poultry in Queensland, 1976–1979

The epidemiology of infectious bursal disease (IBD) was studied by serology and sometimes by visual examination of the bursa of Fabricius in poultry flocks in Queensland during 1976–1979. Ten flocks, each of approximately 30,000 meat breeding chickens, were surveyed. All chickens had maternally‐derived antibody against IBD virus (IBDV) at hatching and active antibody was not detected while the chickens were brooded on rearing farms. When distributed to breeding farms, 7 of the flocks developed antibody when 11 to 25 weeks of age. The remaining 3 flocks were vaccinated by infection of 10% of the birds and within 4 weeks more than 80% of the chickens had developed precipitating antibody to IBDV. Blood samples of 20 to 30 broiler chickens were collected at slaughter (7 to 9 weeks of age) from each of 312 broiler flocks raised on 37 contract farms. While the samples from 21 flocks were without detectable antibody to IBDV, all serum samples for 263 flocks contained antibody. The ratio of bursal weight to bodyweight was significantly lower in birds from 144 flocks having antibody to IBDV than in birds from 10 flocks that were without detectable antibody. In sequential studies, IBDV antibody became demonstrable in 27 of 30 flocks when the chickens were one to 6 weeks of age and was accompanied by bursal atrophy. Serological investigation of 4 flocks of layer breeding chickens on a multi‐age farm at approximately monthly intervals resulted in antibody to IBDV being detected at every examination. Serological tests and bursal examinations were carried out weekly in 2 flocks each of 4000 layer chickens between one and 20 weeks of age. Serum antibody developed in one flock at 4 weeks of age and in the other at 17 weeks of age. In both flocks, bursal atrophy occurred concurrently with the development of antibody.