H7N9

H7N9 is an avian influenza A virus subtype, defined by the combination of its hemagglutinin type 7 (H7) and neuraminidase type 9 (N9). It was first reported to infect humans in 2013, with most early cases linked to exposure to infected poultry or live poultry markets in eastern China. Since then, the virus has caused several waves of human infections, but sustained, efficient human-to-human transmission has not been established.

Transmission and ecology

In humans, infections typically result from contact with infected birds, especially in settings with live poultry markets or poultry farms. Human-to-human transmission is limited and not considered sustained. In birds, H7N9 can circulate with relatively mild disease in poultry, which can hinder detection. Domestic poultry are the principal reservoir, though wild birds can contribute to geographical spread.

Virology and evolution

H7N9 is one of many avian influenza A subtypes that occasionally infect humans. The virus has undergone genetic reassortment, producing multiple genotypes and lineages. Some genetic changes have increased adaptation to poultry and created concerns about potential changes in virulence or transmissibility, but no sustained human transmission has been demonstrated.

Public health response

Control measures focus on reducing human exposure to infected birds and limiting viral spread in poultry. These include surveillance, temporary closures of live poultry markets, culling of infected flocks, and vaccination of poultry in some regions. There is no widely licensed vaccine for humans currently available, though vaccines for poultry have been used to reduce circulation. Health authorities monitor H7N9 closely due to its potential to adapt and the ongoing risk of outbreaks in regions with poultry contacts.

See also: avian influenza, influenza A subtypes, pandemic preparedness.