Abstract
Background: Infectious Bursal Disease (Gumborao disease) has become more severe than in early outbreaks in the 1980s. It is of significance to determine the epidemiology of the disease in the field and the best diagnostic method to provide data that can be used to develop effective control measures. The present research aims to study the epidemiology of IBD in Khartoum state and compare some commonly used laboratory techniques for diagnosis
Methods: We collected epidemiological data from thirty farms that showed signs suggestive of IBD, estimated the morbidity, the daily, and total mortality. We interviewed the owners about the type and the dose of the used vaccines. The bursas of Fabricius were collected aseptically in sterile bottles, and in neutral formalin for histopathology. The collected samples were processed and tested with the agar gel immunodiffusion (AGID) test, Counter-immuno-electrophoresis (CIEP), and RT-PCR. Samples positive in AGID were inoculated onto chicken embryo fibroblast (CEF) cell culture and chorioallantois membrane (CAM) and yolk sac of embryonated eggs. Twenty-two days old chicks were infected experimentally with three selected isolates and morbidity and mortality were compared between the three groups.
Results: The results showed that 70% of outbreaks occurred between 6-8 weeks of age, and the mean mortality rate was 51%. Epidemiologic, clinical, gross, and histopathological findings were characteristic of the severe disease caused by the very virulent IBDV (vvIBDV). The farms that used intermediate plus (or hot) vaccines had lowered mortality compared with the farms that used intermediate vaccines. The AGID was found more sensitive than CIEP since it detected 83.4% of the IBD antigen in the samples while the CIEP detected 66.7% of the samples. The RT-PCR was found to be rapid, specific, and was more sensitive detecting 100% of the tested samples. Virus isolation in embryonated eggs and cell culture was not successful.
Conclusions: A vvIBDV is responsible for the recent outbreaks of the disease in Sudan, resulting in a mean high mortality rate of 51%, even in vaccinated flocks. The RT-PCR and AGID are the best methods for laboratory confirmation. It is recommended to revise the current IBD vaccination programs in Sudan, taking into consideration the results of this study.

Key words: infectious bursal disease; Sudan; epidemiology; laboratory techniques