Continuous circulation of an antigenically modified very virulent infectious bursal disease virus for fifteen years in Egypt

Infectious bursal disease virus (IBDV), the agent of an immunosuppressive and sometimes lethal disease in chickens, is causing recurrent outbreaks in broiler chickens in Egypt. In particular, an antigenically modified isolate of very virulent IBDV (vvIBDV) called 99323 was detected in Egypt nearly twenty years ago; this isolate was shown to be experimentally controlled by an antigenically classical live vaccine. However, acute IBD is still reported, even in vaccinated flocks, and little is known about the genetic and antigenic properties of viruses currently circulating in Egypt. In the present study, ten samples collected in Egyptian broiler farms in 2015 as well as five samples collected in 2001 were analyzed. Genetic analyses of partial VP2 sequences revealed that 8 isolates clustered with vvIBDV strains, and 5 with tissue culture adapted and vaccine strains. Similar results were observed for partial VP1 sequences with the exception of isolate 160019, for which VP2 clustered with the vaccine strain Bursine while VP1 clustered with vvIBDV, suggesting reassortment. For isolates genetically related to vvIBDV, antigenic profiling revealed two patterns: while some isolates exhibited typical European vvIBDV reactivity with lack of binding of mAbs 5, other revealed extensive antigenic modifications, with lack of binding of mAbs 3, 5, 6, 8 and 9, similar to isolate 99323. These different patterns were associated with a single amino acid mutation at position 321 of VP2 that is located within peak PHI. Full genome sequencing was performed for three isolates, among which two were representative of the two antigenic patterns observed for vvIBDV as well as the reassortant isolate 160019. This study highlights the co-circulation of both antigenically typical and modified vvIBDV during the last fifteen years in Egypt.