H5N1 Avian Bird Flu is not Human-to-Human Transmissible - Yet (12/5/24)
Some groups are reporting h2h transmission in humans is low - this is wrong (it has not been confirmed).
I have seen several sources (Wisconsin Dept. of Health, to name one) reporting that H5N1 transmission between humans is low - which is a false statement. As far as we know, it is none. There is no confirmed report of human-to-human transmission of H5N1 avian bird flu, although genetic evidence is mounting that the virus may require very few mutations (as little as one) to gain human transmissibility.
It’s one thing to say that the RISK of transmission is low. But to say the actual transmission is low is misinforming to say the least, disinforming to say the worst. My concern is this statement is being used to downplay the very real risk and avoid more broad scale monitoring of the virus across the US and North America.
With that out of the way, let’s talk about what’s going on in the US in terms of reported H5N1 infections and their source.
Mammal/Poultry Outbreak & Broad Signs of Looming Epidemic Potential
Thanks to excellent reporting by Helen Branswell of statnews.com, we are able to see a better picture of what’s going on in the country. With 15 states reporting infected herds, with many still in quarantine with active infections, California alone accounts for 70% of infected herds. This suggests potential for interactions between human and infected cattle is currently still very high, making cattle-to-human transmission a likely avenue.
In backyard & commercial poultry flocks, the spread is much more expansive.
I wrote a detailed Bluesky thread suggesting that the likely mechanism of H5N1 dispersal throughout the US is human-mediated. Meaning the crossover between wild birds, poultry stocks, and cattle livestock are all at the intersections of human travel, likely between dairy farms. Viruses that tend to have extremely high dispersal rates, including previous avian influenzas H5N1 and H3N1, also tend to have some human mediated component. If this is true, its unlikely that the outbreak will slow or stop anytime soon. Meaning we are likely in the exponential phase.
Possible Molecular Mechanisms of Human-to-Human Transmission of H5N1
We are riding a thin line between bird-to-human and cattle-to-human being the only transmission mechanisms. In a report today (Dec. 5th), researchers suggest a single amino acid change, glutamic acid to leucine, at site 226 of H5N1 hemagglutinin could cause the shift of binding specificity from avian to human. Hemagglutinins are a class of surface glycoprotein present on the viral capsid of avian flus. They are one of the key proteins responsible for viral docking and infection initiation via endocytosis, “cell eating/drinking”, which brings the viral particle into the cell. While this is only an example of one mutation that could cause inter-human transmissibility, with only a 13.5 kilobase size genome, the probability of any given site having variants (benign genetic changes) and mutations (pathogenic changes) is high.
Historically, this virus has caused up to 30% fatality in humans, so Lin et al. performed a genetic and structural analysis of the mutations necessary to fully switch host receptor recognition. A single glutamic acid to leucine mutation at residue 226 of the virus hemagglutinin was sufficient to enact the change from avian to human specificity. In nature, the occurrence of this single mutation could be an indicator of human pandemic risk.
—Editor’s note, Caroline Ash
Two other human cases of H5N1 have shown mutational propensity towards increased transmissibility
Time Magazine Reports:
What can we do?
Protect yourself. While the incoming US administration plans to gamble with our lives by selecting probably one of the most dangerous cabinets in modern history for public health, the current CDC has put out excellent guidance regarding the transmissibility of H5N1 from poultry and cattle to humans.
And the Canadian Centre for Occupational Health and Safety (CCOHS) has updated their guidance:
Conclusions
While H5N1 is not currently reported to transmit between humans, the potential for viral evolution in that direction is high. This makes the current outbreaks in dairy cattle and poultry of extreme concern to the public health of not just the US, but globally. Both epidemic and pandemic potential of the virus remain high.
Bibliography
Ting-Hui Lin et al., A single mutation in bovine influenza H5N1 hemagglutinin switches specificity to human receptors. Science 386, 1128-1134(2024). DOI:10.1126/science.adt0180
Cline, T.D., Karlsson, E.A., Seufzer, B.J. and Schultz-Cherry, S. (2013) 'The Hemagglutinin Protein of Highly Pathogenic H5N1 Influenza Viruses Overcomes an Early Block in the Replication Cycle To Promote Productive Replication in Macrophages', Journal of Virology, 87(3), pp. 1411–1419. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3554171/ (Accessed: 5 December 2024).
U.S. Department of Agriculture, Animal and Plant Health Inspection Service (APHIS) (2024) 'Confirmations of Highly Pathogenic Avian Influenza in Commercial and Backyard Flocks'. Available at: https://www.aphis.usda.gov/livestock-poultry-disease/avian/avian-influenza/hpai-detections/commercial-backyard-flocks (Accessed: 5 December 2024).
U.S. Department of Agriculture, Animal and Plant Health Inspection Service (APHIS) (2024) 'HPAI Confirmed Cases in Livestock'. Available at: https://www.aphis.usda.gov/livestock-poultry-disease/avian/avian-influenza/hpai-detections/hpai-confirmed-cases-livestock (Accessed: 5 December 2024).
Ducharme, J. (2024) 'The Bird Flu Virus Is One Mutation Away from Getting More Dangerous', TIME, 5 December. Available at: https://time.com/7200088/can-bird-flu-infect-humans/ (Accessed: 5 December 2024).
Centers for Disease Control and Prevention (CDC) (2024) 'How Infected Backyard Poultry Could Spread Bird Flu to People'. Available at: https://www.cdc.gov/bird-flu/media/pdfs/2024/07/avian-flu-transmission.pdf (Accessed: 5 December 2024)
Centers for Disease Control and Prevention (CDC) (2024) 'How H5 Bird Flu Could Spread on a Dairy Farm'. Available at: https://www.cdc.gov/bird-flu/media/pdfs/2024/11/Bird-Flu-Could-Spread-Poster.pdf (Accessed: 5 December 2024).
Canadian Centre for Occupational Health and Safety (CCOHS) (2024) 'Protecting Workers from Avian Influenza A(H5N1) in Dairy Cattle'. Available at: https://www.ccohs.ca/topics/hazards/health/zoonotic-diseases/dairy-cattle-infographic (Accessed: 5 December 2024).