Molecular epidemiology of IBDV in Italy: emergence of Northwestern European reassortants (genotype A3B1) and co-circulation with local strains

Infectious bursal disease virus (IBDV) is a highly contagious, economically relevantimmunosuppressive pathogen of chickens. Despite belonging to a single serotype, virulentIBDVs display a remarkable heterogeneity in genetic and functional features. Traditionally,strains are categorized into classical, variant and very virulent viruses, but many atypicalIBDVs have been recently identified. A primary example is the emergence of North-WesternEuropean (NW EU) reassortants (genotype A3B1), first identified in 2017, which rapidlybecame predominant in Western Europe. A notable exception was Italy, where circulatingIBDVs belonged to a separate A3B1 clade related to Russian and Middle Eastern strains, butthe present study describes the entry of NW EU reassortants into this country as well.During a 2-year-long survey (March 2023-January 2025), 256 bursal pools were collectedfrom broiler farms and subjected to molecular analyses: 97 samples tested positive for fieldstrains (38%), 43 for vaccine strains (17%), and 116 were negative (45%). Notably, 46 fieldstrains were NW EU reassortants (47%), whereas 50 belonged to the Italian Russian-MiddleEastern (ITA/RUS/ME) clade (52%). The remaining strain showed VP2 features consistent withthe ITA/RUS/ME clade but clustered with classical attenuated IBDVs at VP1 level, either dueto reassortment or coinfection between field and vaccine viruses. Following their firstdetection, NW EU reassortants reached all major poultry producing regions and seeminglybecame prevalent over the ITA/RUS/ME clade. Such a swift epidemiological shift emphasizesthe threat posed by atypical IBDVs and the subsequent need for enhanced surveillance andcontrol measures.