An infectious bursal disease virus strain obtained from an pathogenic strain that was attenuated in em-bryonated eggs, is produced in a primary culture of chicken embryo fibroblast, (CEF). This virus has been passaged 10 times further in CEF, and is now intended for use in the active immunisation of chickens against IBD. The vaccinę virus replicates well in CEF giving a titer of up to107.5 TCID50 per ml. In spite of its pathogenicity for CEF, the vaccine has no pathogenicity for chickens as shown by the absence of gross and histopathological lesions in the bursa of Fabricius (BF) of birds infected with it. Immunogenicity is retained and infact compares favourably with those of other IBD virus strains. The vaccine virus does not revert back to its original pathogenicity but can be adversely affected by storage at room temperature and at 37°C. It could however be stored at +2°C to +8°C or lower for up to six months without loss in potency, The vaccine can be administered by mouth or intramuscularly and as low as 50 TCID50 per bird guarantees full protection. However, as much as 125,000 times the guaranteed dose per bird has been administered without any observable changes in the BF of the affected birds. The field dosage is calculated so that at least one guaranteed dose (i.e. 50 TCID50 ) is still available to each bird even after incubation at 37°C for 7 days. The vaccine did not depress the immune response of chickens to ND vaccine intraocular when administered concurrently with it. The vaccine was tested for safety and immunogenicity in a population of two isolated flocks totalling 8504 birds. The immune status of a flock tested was significantly enhanced as a result of the vaccination (Table 6). More than 10.8 million doses have been issued to the field from 1979 to 1982 and the demand is increasing. Every batch of vaccine produced is tested for viability in CEF, sterility in bacterial culture media and for safety and potency in chickens.
DEVELOPMENT AND PRODUCTION OF INFECTIOUS BURSAL DISEASE (GUMBORO) VACCINE IN NIGERIA
