Abstract

Zoonotic and pandemic influenza continue to pose threats to global public health. Pandemics arise when novel influenza A viruses, derived in whole or in part from animal or avian influenza viruses, adapt to transmit efficiently in a human population that has little population immunity to contain its onward transmission. Viruses of previous pandemic concern, such as influenza A(H7N9), arose from influenza A(H9N2) viruses established in domestic poultry acquiring a hemagglutinin and neuraminidase from influenza A viruses of aquatic waterfowl. We report a novel influenza A(H3N8) virus in chicken that has emerged in a similar manner and that has been recently reported to cause zoonotic disease. Although they are H3 subtype, these avian viruses are antigenically distant from contemporary human influenza A(H3N2) viruses, and there is little cross-reactive immunity in the human population. It is essential to heighten surveillance for these avian A(H3N8) viruses in poultry and in humans.

Diverse influenza A viruses are found in aquatic waterfowl, poultry, swine, horses, aquatic mammals, bats, and domestic pets such as cats and dogs. Although there is a diversity of virus hemagglutinin (H1–H16) and neuraminidase (N1–N9) subtypes in aquatic birds, more restricted numbers of virus subtypes are established in other species, including chicken (1). The high mutation rates associated with an error-prone virus replication complex and the presence of a segmented genome enables genetic reassortment of gene segments of viruses of different species and interspecies transmission and adaptation to new hosts.

Influenza A virus subtypes H9 and H6 have formed established lineages in domestic chicken and game birds (quail, pheasant) farmed for consumption in Asia (2). The internal gene constellation of H9N2 viruses contains hemagglutinin (HA) and neuraminidase (NA) genes acquired from aquatic waterfowl to generate H5N1, H5N6, H7N9, and H10N8 viruses through genetic reassortment, and many of these viruses also became established in poultry, subsequently posing zoonotic and pandemic threats (3–5). A novel influenza A(H3N8) virus has been recently reported to cause zoonotic infection in Henan Province, China (6).

In this context, we report detection of novel H3N8 viruses recently identified in chicken in live poultry markets and chicken farms in Hong Kong, China, that are genetically similar to the zoonotic H3N8 viruses reported in mainland China (6). We also report that these recent H3N8 viruses have arisen in a manner akin to zoonotic H5N1, H7N9, and H10N8 viruses and that there is little cross-reactive immunity in the human population to these chicken H3N8 viruses.

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