Abstract
In order to produce an attenuated vaccine strain of infectious bursal disease (IBD) virus, we attempted to isolate the variants by plaque-cloning from chick embryo fibroblast (CEF) cultureadapted RF-1 strain of IBD virus.Two types of attenuated plaque variants (clones Lp and Sp) were established. The Lp clone formed large plaques with a uniform diameter of 5mm in CEF cultures, whereas the Sp clone produced only small plaques with a uniform diameter of 1mm. Distinct differences between the two clones were also observed in terms of pathogenicity for chick embryos and growth capability in CEFcultures. s Neither of the clones could be distinguished from the parent virus by a cross-neutralization test, and the clones possessed enough immunogenicity to protect chickens when challenged with virulent wild-type IBD virus. The minimum effective immunizing doses of Lp and Sp clones necessary for the protection were 104 and 105 TCID50, respectively. When Lp and Sp clones were tested for their pathogenic properties in one-day-and 4-week-old chickens with respect to clinical symptoms and lesions in the bursa of Fabri-cius, the two clones were found to be more attenuated than the parent virus. These clones had safety and immunogenic potency in accordance with the current “Minimum Requirement of Live Infectious Bursal Disease Virus Vaccine for Chicks”. The Lp clone had more immunogenic potency and higher growth capability in CEF cultures than the Sp clone, suggesting that the Lp clone promises to be useful as a live vaccine strain.
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