The H5N1 Virus Strain

The highly pathogenic H5N1 avian influenza virus, commonly known as bird flu, is the strain that has raised major global concerns. Since it was first detected in Hong Kong poultry markets in 1997, this strain has spread across multiple countries in Asia, Africa, Europe, and the Near East. While most H5N1 outbreaks have been among domestic and wild birds, the virus has also led to human infections on rare occasions. So far, the World Health Organization (WHO) has confirmed 861 human cases and 454 deaths associated with H5N1 between 2003 and 2022. These mortality rates among infected humans have been as high as 60%.

Direct Bird-Human Transmission

The H5N1 Global Bird Flu virus mainly spreads from bird to bird through bodily discharges like saliva or feces. However, it can sometimes directly infect humans who have close contact with infected domestic or wild birds. Activities like slaughtering or butchering infected poultry, preparing infected meat, handling bird corpses or feathers, and direct contact with infected live birds have led to human cases. Many of the human victims have been farmers, workers in live poultry markets or people involved in the poultry industry.

Lack of Sustained Human Transmission

While some human infections and even clusters of cases have occurred, the virus has not adapted to transmit efficiently between humans so far. Most human cases have involved direct contact with infected birds. Sustained human-to-human transmission has not been documented, with a few ambiguous cases remaining exceptions. This is primarily because the virus prefers to bind to receptors deep in bird lungs instead of human upper airways. However, experts warn this could change if the virus accumulates adaptations for mammalian hosts through continuous spreading and evolution in birds.

Mutations and Reassortment

Influenza viruses constantly evolve to adapt to new hosts and evade immunity. The H5N1 strain circulating in birds since 1997 has now diversified into multiple genetic clades or subgroups. Several of these clades display enhanced pathogenicity and transmissibility in birds or mammals compared to earlier isolates. Furthermore, influenza viruses are adept at recombining or reassorting their gene segments should multiple viral strains co-infect the same host cell. Reassortment with human or other influenza viruses circulating in birds could generate novel strains capable of causing pandemics.

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