Michael Coston
Editor, Senior Moderator
Referral: (NEJM) Resurgence of Zoonotic Highly Pathogenic Avian Influenza A(H5N1) Virus in Cambodia
#18,919
In early February of 2023 - after going 9 years without a reported case - Cambodia reported 2 H5N1 infections in a single household (father/daughter), with the 11-year-old daughter succumbing to the virus.
Eight months passed, and in Oct/Nov, a total of 4 more cases were reported. In 2024, 10 more cases emerged, and so far in 2025 we've seen 17 (see map above).
Unlike H5N1 cases reported in the United States - which are due to a milder clade 2.3.4.4b - recent Cambodian cases have been caused by a new reassortment of an older clade of the H5N1 virus (recently renamed 2.3.2.1e) - which appears to be spreading rapidly through both wild birds and local poultry.
One of our best sources of information on these outbreaks has been Dr. Erik Karlsson (@eakarlsson.bsky.social) from the Institut Pasteur du Cambodge, Phnom Penh, Cambodia, who - along with a long list of colleagues - has published a brief letter in the NEJM on first 16 cases.
Among them were 4 `clusters' (Father/Daughter, two neighbors, 2 siblings, and 2 cousins living in the same household), 6 fatalities, and 12 cases < 18 years of age.
While human-to-human transmission was considered, the available evidence suggests a `shared' exposure instead, with infected poultry the most likely source of infection.
Due to copyright, I've just posted the title and link, and I would urge you to go to the NEJM site to read it in its entirety.
Highly Recommended.
Resurgence of Zoonotic Highly Pathogenic Avian Influenza A(H5N1) Virus in Cambodia
Published October 22, 2025
N Engl J Med 2025;393:1650-1652
DOI: 10.1056/NEJMc2504302
VOL. 393 NO. 16 Copyright © 2025
As we've discussed often, Nature's laboratory remains open 24/7, and what we might be able to say about a specific virus today, could easily change tomorrow.
For some past blogs on this ongoing outbreak, you may wish to revisit:
#18,919
In early February of 2023 - after going 9 years without a reported case - Cambodia reported 2 H5N1 infections in a single household (father/daughter), with the 11-year-old daughter succumbing to the virus.
Eight months passed, and in Oct/Nov, a total of 4 more cases were reported. In 2024, 10 more cases emerged, and so far in 2025 we've seen 17 (see map above).
Unlike H5N1 cases reported in the United States - which are due to a milder clade 2.3.4.4b - recent Cambodian cases have been caused by a new reassortment of an older clade of the H5N1 virus (recently renamed 2.3.2.1e) - which appears to be spreading rapidly through both wild birds and local poultry.
One of our best sources of information on these outbreaks has been Dr. Erik Karlsson (@eakarlsson.bsky.social) from the Institut Pasteur du Cambodge, Phnom Penh, Cambodia, who - along with a long list of colleagues - has published a brief letter in the NEJM on first 16 cases.
Among them were 4 `clusters' (Father/Daughter, two neighbors, 2 siblings, and 2 cousins living in the same household), 6 fatalities, and 12 cases < 18 years of age.
While human-to-human transmission was considered, the available evidence suggests a `shared' exposure instead, with infected poultry the most likely source of infection.
Due to copyright, I've just posted the title and link, and I would urge you to go to the NEJM site to read it in its entirety.
Highly Recommended.
Resurgence of Zoonotic Highly Pathogenic Avian Influenza A(H5N1) Virus in Cambodia
Published October 22, 2025
N Engl J Med 2025;393:1650-1652
DOI: 10.1056/NEJMc2504302
VOL. 393 NO. 16 Copyright © 2025
As we've discussed often, Nature's laboratory remains open 24/7, and what we might be able to say about a specific virus today, could easily change tomorrow.
For some past blogs on this ongoing outbreak, you may wish to revisit:
Cambodia: Food Insecurity, Food Safety & H5N1
WHO: Influenza at the Human-Animal Interface Report - Identifies New H5 Clade 2.3.2.1e Infections in Cambodia & Vietnam
EID Journal: Antiviral Susceptibility of Influenza A(H5N1) Clade 2.3.2.1c and 2.3.4.4b Viruses from Humans, 2023–2024
https://afludiary.blogspot.com/2025/10/referral-nejm-resurgence-of-zoonotic.htmlWHO: Influenza at the Human-Animal Interface Report - Identifies New H5 Clade 2.3.2.1e Infections in Cambodia & Vietnam
EID Journal: Antiviral Susceptibility of Influenza A(H5N1) Clade 2.3.2.1c and 2.3.4.4b Viruses from Humans, 2023–2024