History of infectious bursal disease in the U.S.A.--the first two decades.

Infectious bursal disease (IBD) emerged in 1957 as a clinical entity responsible for acute morbidity and mortality in broilers on the Delmarva peninsula. The condition spread rapidly and was recognized throughout the U.S. broiler and commercial egg production areas by 1965. Early attempts to isolate the etiologic agent were impeded by a lack of specific-pathogen-free (SPF) eggs and by deficiencies in viral and serologic techniques. By 1967, the highly infectious nature of the agent was recognized. Reliable methods were developed to isolate the virus in embryonated eggs and to adapt it to tissue culture. The agent was characterized as a virus belonging to a new taxonomic group in 1976. The immunosuppressive property of IBD virus was first recognized in 1970 and was confirmed in structured trials in 1976. An early method of control involved planned infection of chickens. This technique lowered IBD mortality but often resulted in immunosuppression and further dissemination of field virus. A live attenuated vaccine was then developed, based on mild field isolates passaged in SPF eggs. This vaccine was federally licensed as the first of its kind for interstate use in 1968. It remains widely used today in breeders as a primer and in the control of very virulent IBD in many countries. The first two decades following emergence of IBD were characterized by close cooperation among scientists in academia, the biologics industry, and the USDA. By 1976, mortality caused by IBD was effectively controlled by vaccination. However, the more subtle effects of immunosuppression and the tremendous economic impact of the disease were just starting to be appreciated. Recognition of Delaware variants in the mid-1980s and emergence of very virulent forms of the condition in Europe and Asia beginning in 1989 attest to the continuing importance of IBD.