The magnitude of spleen-tissue interferon (IFN)-α, IFN-β, IFN-γ, interleukin (IL)-1β and IL-6 expression induced by infection with the R543K mutant was significantly lower than those of JXSP2-4, but higher than those of the JXSP-310RdRp-infected ducklings. This evidence concerns the gene IL6 and infection.