Avian influenza, or bird flu, is a contagious viral disease caused by influenza A viruses affecting wild birds and domestic poultry. In China, various strains have crossed to humans, making understanding their subtypes, transmission, and health implications important for public health.
Key Avian Influenza Strains in China
China has experienced human infections from several avian influenza A virus subtypes, with H5N1 and H7N9 being the most frequently identified and concerning. The H5N1 virus, highly contagious in birds, has been associated with severe illness and fatalities in humans since its widespread re-emergence in 2003, becoming endemic in poultry populations in some countries, including China. Human infections with H7N9 were first reported in China in 2013, with most cases linked to exposure to infected poultry or live bird markets.
Other strains have also caused sporadic human cases. Subtypes such as H3N8 have led to a small number of human infections in China, with some cases progressing to severe pneumonia. The H10N3 subtype, circulating among waterfowl and poultry, was first recorded in humans in Jiangsu, China, in April 2021, with additional cases following.
Transmission from Birds to Humans
The primary route for avian influenza transmission from birds to humans involves direct or close indirect contact with infected poultry. Humans become infected through exposure to infected birds’ feces, saliva, or respiratory secretions. Exposure often occurs in environments with live poultry.
Live poultry markets are recognized as high-risk environments for zoonotic spillover due to the close proximity between birds, their waste, and humans. Contaminated environments and surfaces where infected birds have been present also serve as potential sources of infection.
Human-to-human transmission of avian influenza viruses is extremely rare and has not been sustained. While limited instances of non-sustained spread have been reported for H5N1 and H7N9, these do not indicate widespread or efficient person-to-person transmission. Properly cooked poultry and eggs do not pose a risk for avian influenza infection, as the virus is destroyed by normal cooking temperatures.
Human Health Implications
Upon infection, human health implications from avian influenza can range from mild flu-like symptoms to severe, life-threatening conditions. Initial signs often mimic common influenza, including high fever, cough, sore throat, and muscle aches. However, infections with highly pathogenic strains like H5N1 and H7N9 can rapidly progress to severe outcomes.
Severe outcomes include aggressive pneumonia and acute respiratory distress syndrome (ARDS). The mortality rate associated with H5N1 infections has been high, with approximately 50% of reported human cases resulting in death globally since 1997. Similarly, H7N9 infections have led to severe respiratory illness and death in about 40% of reported cases.
Antiviral medications, such as oseltamivir, are available treatments for avian influenza, and their effectiveness is enhanced by early diagnosis and administration. Prompt medical attention is advised for individuals experiencing symptoms, especially if severe manifestations like breathing difficulties arise, particularly after potential exposure to infected birds. Laboratory testing is necessary for definitive diagnosis, as clinical manifestations alone are insufficient.
Control and Prevention Strategies in China
China has implemented a multi-faceted approach to control and prevent the spread of avian influenza, focusing on both animal and human health. Nationwide surveillance systems are in place to monitor avian influenza viruses in both poultry and humans. Surveillance helps detect new outbreaks and track circulating strains.
A strategy involves the culling of infected or exposed poultry flocks to contain outbreaks and reduce the viral load in affected areas. This measure prevents further spread within poultry and minimizes zoonotic transmission to humans. Widespread poultry vaccination programs have been adopted to build immunity in bird populations, reducing incidence and shedding of the virus.
To break transmission cycles, policies regulating or temporarily closing live poultry markets have been implemented, particularly during high-risk periods. These markets are recognized as interfaces where viruses can jump from birds to humans. Public awareness campaigns educate the public on safe practices around poultry and emphasize hygiene to reduce exposure risks.