H7N9

H7N9 is a subtype of influenza A virus, characterized as an enveloped negative-sense RNA virus with a segmented genome. This structural feature confers a relatively high mutation rate and facilitates genetic recombination through reassortment when hosts are co-infected with different influenza strains. The subtype causes influenza predominantly in avian species, though human infections have been documented following exposure to infected birds or contaminated environments.

Human infection with H7N9 presents with variable clinical manifestations ranging from mild respiratory symptoms to severe, potentially fatal disease. Documented symptoms include fever, cough, and gastrointestinal symptoms such as diarrhea. The severity of disease depends on both the viral strain and the host species affected. In poultry, low pathogenic strains typically cause mild or asymptomatic infection, while highly pathogenic strains produce significant respiratory distress, marked reductions in egg production, and elevated mortality in affected flocks.

H7N9 maintains enzootic circulation in numerous bird populations, with wild aquatic birds serving as natural asymptomatic carriers capable of disseminating viruses over extensive geographic ranges through migration. The first recognized human outbreak occurred in China in March 2013, affecting 18 individuals with six fatalities; epidemiological investigation linked all cases to exposure at live poultry markets. Prior to this event, the virus had been isolated only from birds during outbreaks in the Netherlands, Japan, and the United States. More recent detections include a highly pathogenic outbreak affecting approximately 152,000 birds in Terang, Australia in May 2024, and a March 2025 outbreak on a commercial broiler breeder operation in Mississippi, USA, involving nearly 48,000 chickens.

Transmission to humans requires prolonged close contact with infected birds or exposure to contaminated environments. Infected birds shed virus in saliva, mucus, and feces, while other infected animals may eliminate avian influenza viruses through respiratory secretions and additional body fluids. The virus spreads rapidly through poultry flocks and among wild bird populations, facilitating both enzootic maintenance and outbreak amplification.

Individuals with prolonged close contact with infected poultry or contaminated materials, including live bird market workers and those involved in poultry farming operations, face elevated exposure risk. The classification of H7N9 strains as low or high pathogenic refers specifically to virulence in domestic chickens and does not reliably predict severity of human disease manifestations.

Source-backed detail is not yet available.

Source-backed detail is not yet available.

1. 1 Wikidata. influenza A virus subtype H7N9.
2. 2 Wikipedia. Influenza A virus subtype H7N9 - Wikipedia.

ICD-10

J09

ICD-11

1E30.0

Total cases

1K

Total deaths

565

Peak month

2017-01

Coverage

2 reporting countries · 2013-11-01 → 2026-05-02