Poultry exposure is a major risk factor for human H7N9 zoonotic infections of which the mode of transmission remains unclear. We studied the transmission of genetically related poultry and human H7N9 influenza viruses differed by four amino acids including the host determinant PB2 residue 627. A/Silkie Chicken/HK/1772/2014 (SCk1772) and A/HK/3263/14 (HK3263) replicated to comparable titers in chickens with superior oropharyngeal over cloacal shedding; both viruses transmitted efficiently among chickens via direct contact but inefficiently via airborne route. Interspecies transmission via the airborne route was observed for ferrets exposed to the SCk1772 or HK3263 infected chickens while low numbers of copies of influenza viral genome were detected in the air, predominantly at particle sizes larger than 4μm. In ferrets, the human isolate HK3263 replicated to higher titers and transmitted more efficiently via direct contact than SCk1772. We monitored "intra-host" and "inter-host" adaptive changes at PB2 residue 627 during infection and transmission of the SCK1772 that carried E627 and HK3263 that carried V/K/E polymorphism at 60%, 20%, and 20%, respectively. For SCk1772, positive selection for K627 over E627 was observed in ferrets during the chicken-to-ferret or ferret-to-ferret transmission. For HK3263 that contained V/K/E polymorphism, mixed V627 and E627 genotypes were transmitted among chickens while either V627 or K627 was transmitted from chicken-to-ferret or ferret-to-ferret with a narrow transmission bottleneck. Overall, our results suggest direct contact as the main mode for H7N9 transmission and identify the PB2-V627 genotype with uncompromised fitness and transmissibility in both avian and mammalian species.

IMPORTANCE:

We studied the modes of H7N9 transmission, as this information is crucial for developing effective control measures for prevention. Using chicken (SCk1772) and human (HK3263) H7N9 isolates that differed by four amino acids, including the host determinant PB2 residue 627, we observed both viruses transmitted efficiently among chickens via direct contact but inefficiently via airborne route. Chicken-to-ferret transmission via airborne route was observed along with the detection of viral genome in the air at low copies. In ferrets, HK3263 transmitted more efficiently than SCk1772 via direct contact. During the transmission of SCk1772 that contained E and HK3263 that contained V/K/E polymorphism at PB2 residue 627, positive selections of E627 and K627 were observed in chickens and ferrets, respectively. In addition, PB2-V627 was transmitted and stably maintained in both avian and mammalian species. Our results support applying intervention strategies that minimize direct and indirect contact at the poultry markets during epidemics.