#19,227
Over the past 2 decades we've seen dozens of suspected ways that HPAI has transmitted to humans. While most have involved direct contact with an infected bird or animal, some have come from (direct or indirect) environmental exposures, and a few appear to have come from close contact with an infected human.
Due to the risks involved, the CDC has issued PPE guidance for both commercial and backyard raisers of poultry (and dairy workers), but it isn't clear how many are following this advice.
We've seen at least 2 fatalities linked to raising backyard birds in the United States (see H5N5 in Washington State and H5N1 in Louisiana, and there are a number of cases where we simply don't know the exposure.
Yet live bird markets flourish around the world, including here in the United States (see NJ & RI Both Report H5N1 in Live Markets), despite repeated calls for their closure (see Interventions in live poultry markets for the control of avian influenza: A systematic review).From 1 to 7 May 2026, one new case of human infection with avian influenza A(H5N6) virus was reported to WHO in the Western Pacific Region. The case is a 55-year-old female from Chongqing Municipality, China, with symptom onset on 16 April 2026. She developed severe pneumonia, was hospitalised on 23 April, and died on 3 May. She had purchased, slaughtered, and consumed poultry. Samples collected from a cutting board tested positive for influenza A (H5). All close contacts tested negative and developed no symptoms.
In 2022 we looked at Zoonoses & Public Health: Aerosol Exposure of Live Bird Market Workers to Viable Influenza A/H5N1 and A/H9N2 Viruses, Cambodia, and we've seen cases whose likely exposures were cited as simply living near, or walking past an LBM (see J. Infection: Aerosolized H5N6 At A Chinese LBM (Live Bird Market)).
These risks go far beyond just human exposure, of course, as live markets also bring together different types of birds which may also silently carry LPAI viruses like H3N2, H9N2, and H6N1, which could reassort with HPAI H5.
To that end we have a lengthy scoping review on the modes of transmission of the H5 virus to humans, one that includes a small - but significant - number of suspected human-to-human transmissions.
Due to its length, I've only posted the Abstract. Follow the link to read it in its entirety. I'll have a bit more after the break.
Mapping reported modes of transmission of highly pathogenic avian Influenza A (H5N1) to humans: A scoping review
Nicole Billias a b c, Victoria D’Alessandro a b, Dimitra V. Pouliopoulou a b, Jessica J. Wong a b h, Erin Miller a, Jessica P. Hopkins d e f, Eleni C. Boutsikari g, Lauren Cipriano h, Tiago da Veiga Pereira i, Jennie Johnstone j, Saverio Stranges k l m n, J. Scott Weese o, Joy C. MacDermid a b c, Kieran L. Quinn d, David Fisman d, Pavlos Bobos a b k
https://doi.org/10.1016/j.onehlt.2026.101492
Abstract
Background
Highly Pathogenic Avian Influenza A (subtype H5N1) poses a threat to human health, and its pandemic potential emphasizes the need to better understand detailed reported transmission pathways to humans. Existing literature is outdated or lacks detailed, comprehensive analysis of the range of transmission routes and how the virus may enter the human body.
Objective
To comprehensively map all reported H5N1 transmission pathways to humans, as well as viral entry routes.
Methods
CINAHL, Embase, MEDLINE, Scopus, PubMed, grey literature, and reference lists (of included studies) were searched up to October 29th, 2025, with no language restrictions. Observational studies and grey literature reporting H5N1 transmission evidence to humans were included. Two reviewers conducted duplicate screening independently. One reviewer completed data extraction, which was cross-verified for accuracy by a second. Findings were summarized narratively.
Results
120 sources met inclusion criteria (70 studies, 50 grey literature). Reported H5N1 transmission pathways were classified into animal-to-human (111 of 120 sources, 92.5%; including poultry-to-human in 102 sources [85.0%] and cattle-to-human in nine sources [7.5%]), environment-to-human (37 of 120 sources, 30.8%), and human-to-human (14 of 120 sources, 11.7%).
Reported transmission pathways were further classified as direct or indirect contact, synthesized, and linked to suspected routes of human entry, including mucosal entry (eyes, nose, mouth), inhalation of aerosols or droplets, ingestion, and percutaneous exposure. Entry routes are biologically plausible and do not imply relative likelihood or causal attribution.
Conclusions
There are multiple reported pathways of H5N1 exposure, and a single pathway may involve multiple ways to infect humans. Further research is needed to determine causal mechanisms, identify specific risk factors and measures of association, and strengthen evidence-based prevention strategies.
Two years ago the WHO published Interim Guidance to Reduce the Risk of Infection in People Exposed to Avian Influenza Viruses, which lists a number of `risk factors', including:
- keep live poultry in their backyards or homes, or who purchase live birds at markets;
- slaughter, de-feather and/or butcher poultry or other animals at home;
- handle and prepare raw poultry for further cooking and consumption;
Given the potential public health threat, it makes sense to try to limit high-risk exposures - particularly in backyard poultry, live bird markets, and commercial poultry and dairy operations - through better biosecurity and consistent use of PPEs.
But as we discussed five weeks ago in MMWR: Knowledge, Attitudes, and Practices Regarding Avian Influenza Among Owners of Backyard Flocks, many backyard poultry producers still have limited knowledge of avian flu symptoms and risks, and their biosecurity & PPE measures often fall short of recommendations.
Commercial poultry operations and dairy farms have been similarly reluctant or slow to fully implement biosecurity and PPE recommendations, and surveillance and reporting remains passive and incomplete.
And as for the public, most remain unconvinced of the seriousness of the threat (see Two Surveys (UK & U.S.) Illustrating The Public's Lack of Concern Over Avian Flu).
While HPAI H5 many never gain the ability to spread efficiently from human-to-human, we do seem intent on giving it every opportunity to do so.
