Classied in the genus Avibirnavirus, family Birnaviridae, IBDV is a double-stranded RNA (dsRNA) virus with a bisegmented genome. Of the two distinct serotypes identied, serotype 1 is further divided into three major subtypes that cause disease in poultry, whereas serotype 2 strains are infectious for poultry, turkey, and other free-living birds but do not cause clinical illness and thus are not targeted for immunization.