The Emergence of the 2013 H7N9 and Related H7N7 Viruses in the Yangtze River Delta Region

Abstract

The novel H7N9 influenza A virus first detected in March 2013 has caused more than 130 cases of human infection in China, resulting in 39 deaths. This virus is a reassortant of H7, N9 and H9N2 avian influenza viruses and carries some amino acids linked to mammalian receptor binding, raising concerns of a new pandemic. However, neither the source populations of the H7N9 outbreak lineage nor the conditions for its genesis are fully understood. Through a combination of active surveillance, screening of virus archives, and evolutionary analyses, we found that H7 viruses have independently transferred from domestic ducks to chickens in China on at least two occasions. Subsequently they reassorted with enzootic H9N2 viruses to generate the H7N9 outbreak lineage, and a related but previously unrecognized H7N7 lineage. The H7N9 outbreak lineage has spread over a large geographic region and is prevalent in chickens at live poultry markets that appear to be the immediate source of human infections. Whether the H7N9 outbreak lineage will, or has, become enzootic in China needs further investigation. Like the H7N9 virus, the H7N7 virus was also mainly isolated from chickens at live poultry markets, and could efficiently infect ferrets, be shed via the nasal and rectal routes, and cause severe pneumonia. This indicates that H7 viruses pose a broader threat than the current H7N9 virus. Continued prevalence of this family of H7 viruses in poultry could lead to further sporadic human infections, with an ongoing risk that the virus might acquire efficient human-to-human transmissibility.