Loss of virulence in a small plaque mutant of the infectious bursal disease virus.

A variant strain Cu-1 M was established from the infectious bursal disease virus strain Cu-1. This variant strain forms smaller plaques than the wild type; it has a retarded growth rate, does not lead to an overt disease in chickens and causes only minor lesions in the bursa of Fabricius. Infection with this strain induces solid protective immunity against infection with the virulent virus.