Pathogenicity of HK483, MON3, and single-gene reassortants in 2-week-old ducks. HK483, MON3, and a series of reassortants between HK483 and MON3 were generated by reverse genetics. Each of the viruses was inoculated into 2-week-old ducks and clinical symptoms of challenged ducks were observed over 14 dpi. White and gray boxes indicate the derivation of virus gene segments from HK483 and MON3, respectively. *: number of dead birds/number of examined ducks. **: mean death dpi.