In News:

• Cambodia saw a resurgence of H5N1 avian influenza cases after over 10 years of no human infections.
• From February 2023 to August 2024, 16 human cases were reported, with 3 deaths caused by the A/H5 clade 2.3.2.1c virus.
• Notably, 14 of these cases were caused by a novel reassortant virus, involving a mixture of clade 2.3.2.1c and clade 2.3.4.4b gene segments.

Key Points:

• Reassortment of the Virus:
  • The reassortment between clades 2.3.2.1c (Southeast Asia) and 2.3.4.4b (global spread) has created a new strain.
  • This reassortant virus is responsible for the second wave of infections in humans, starting in October 2023.
• Zoonotic Transmission:
  • Investigations confirmed that direct contact with sick poultry or bird droppings was the primary source of human infections.
  • There have been no reported cases of human-to-human transmission.
  • The novel reassortant virus appears to have replaced the 2.3.2.1c strain in Cambodian poultry.
• Geographic Spread and Spillovers:
  • Clade 2.3.2.1c was first reported in Cambodian poultry in March 2014. It continued to circulate in both poultry and wild birds.
  • Clade 2.3.4.4b viruses began circulating in Cambodian live bird markets by 2021, co-existing with clade 2.3.2.1c.
  • There were two key spillovers to humans:
    • The first spillover in February 2023, associated with clade 2.3.2.1c, involved two related individuals, with one death.
    • The second spillover, beginning in October 2023, involved the novel reassortant virus.
• Genetic Analysis and Mutation Concerns:
  • Genetic sequencing showed significant changes in the hemagglutinin (HA) gene of viruses from human cases, indicating a shift from older local strains to newer sublineages.
  • The PB2 627K mutation in the novel reassortant is concerning, as it is linked to increased mammalian adaptation and the potential for airborne transmission, particularly in mammals like ferrets.
  • This mutation raises concerns about the virus’s ability to adapt to humans or other mammals.
• Environmental and Epidemiological Factors:
  • The reassortment is believed to have been facilitated by:
    • High-density poultry farming.
    • Wild bird migration.
    • Cross-border poultry trade in Southeast Asia.
  • These factors heighten the risk of zoonotic transmission, emphasizing the need for continued vigilance in the region.
• Surveillance and Response:
  • One Health investigations linked human cases to infected poultry, highlighting the importance of rapid response through whole genome sequencing.
  • The ongoing surveillance is critical, as the novel reassortant strain has already replaced clade 2.3.2.1c in Cambodian poultry.
• Public Health Recommendations:
  • There is an urgent need to strengthen sustained surveillance of avian influenza in both poultry and wild birds, particularly in Southeast Asia.
  • Public health strategies should focus on:
    • Reducing human exposure to infected poultry.
    • Promoting safe poultry handling practices.
    • Encouraging early healthcare-seeking behavior in individuals with potential symptoms.