#18,463
Over the past several years we've seen a steady stream of scientific papers come out of China describing the shift of many LPAI (Low Path Avian Influenza) viruses towards increased mammalian adaptation. A few (of many) include:
NPJ Vaccines: Impact of Inactivated Vaccine on Transmission and Evolution of H9N2 Avian Influenza Virus in Chickens
Cell: Early-warning Signals and the Role of H9N2 in the Spillover of Avian Influenza Viruses
Vet. Research: Emergence of a Novel Reassortant H3N3 Avian Influenza Virus with Enhanced Pathogenicity and Transmissibility in Chickens in China
Transboundary & Emerg. Dis.: H3 Avian Influenza Virus Isolated from China in 2021–2022 Showed the Emerging H3N8 Posed a Threat to Human Health
Viruses: Genetic and Biological Characteristics of Duck-Origin H4N6 Avian Influenza Virus Isolated in China in 2022
Viruses: Wild Bird-Origin H6N2 Influenza Virus Acquires Enhanced Pathogenicity after Single Passage in Mice
Just yesterday, in Preprint: Progressive Adaptation of H6N1 Avian Influenza Virus in Taiwan Enhances Mammalian Infectivity, Pathogenicity and Transmissibility we looked at a study describing a `. . . stepwise adaptation of avian influenza viruses to mammalian hosts', which the authors suggest may stem from prolonged circulation in in poultry.
While none of these LPAI (H3, H4, H5, H9, H10) viruses currently appear to pose the same level of threat as HPAI H5 and H7, they all are purportedly showing signs of gradual adaptation to mammalian hosts.
Although there may be (unidentified) species barriers that prevent most (or all) of these subtypes from ever posing a serious public health threat, the sharp increase in the number of known HPAI and LPAI viruses with zoonotic potential over the past 12 years has been remarkable.
In 2012, the CDC's IRAT (Influenza Risk Assessment Tool) list of zoonotic flu viruses with pandemic potential was pretty short (n=4):
By the end of 2016, the number of viruses on the IRAT list had tripled (n=12). And today there are two dozen on the list, and many others awaiting analysis.
All of which brings us to a new report, once again from China, on the zoonotic potential of LPAI H4N1 viruses, which are commonly found in wild birds and in poultry.
In 2012's in Seroprevalence Study: Avian Flu In Chinese Pigs, we looked at research that found low levels of H3, H4, and H6 subtypes of avian influenza in Chinese pigs while in 2015 we looked at reports of Avian H4N6 In Midwestern Swine.
While human infection with H4 viruses are believed to be both mild and rare, a 2011 PLoS One study (Evidence of infection with H4 and H11 avian influenza viruses among Lebanese chicken growers) presented serological evidence suggesting that `. . . H4 and H11 influenza viruses may possess the ability to cross the species barrier to infect humans.'
Today's study finds that - at least in the laboratory - the LPAI H4N1 virus is already surprisingly well adapted to infecting, and replicating within, mammalian hosts. This is a detailed report, and I've only posted the link and a few excerpts.
Follow the link to read it in its entirety. I'll have a bit more after you return.
Emerging zoonotic potential of H4N1 avian influenza virus: enhanced human receptor binding and replication via novel mutations
Virology Journal volume 22, Article number: 106 (2025) Cite this article
Abstract
Background
Avian influenza virus (AIV), a zoonotic pathogen found worldwide, includes multiple subtypes, one of which is the H4 subtype frequently detected in wild birds and poultry. Despite its prevalence, research on H4 subtype AIV has been scarce, with a focus predominantly on the H4N2 and H4N6 subtypes. The zoonotic potential of H4N1 has not been investigated to date.
Methods
In this study, we used gene sequencing in conjunction with bioinformatics methodologies to analyze wild-type H4N1 AIV strain and mutant strains emerging from serial passaging in cell culture. Furthermore, we assessed the zoonotic potential of H4N1 and the alterations caused by mutations via a series of phenotype assays, including evaluation of receptor binding affinity, immunofluorescence assays, analyses of growth kinetics across different animal cell cultures, and in vivo pathogenicity studies.
Results
Our research reveals that H4N1 AIV can bind to human receptors and exhibits an affinity for human lung and tracheal tissues. In vitro experiments demonstrate that H4N1 replicates efficiently in human cell lines. Furthermore, animal studies demonstrate that H4N1 can induce pneumonia in mice without the need for prior adaptation to the host. Notably, during passage in cell culture, H4N1 rapidly acquired two previously unreported mutations. These mutations significantly enhanced the virus’s ability to attach to human receptors and its capacity for replication.
Conclusions
In summary, our study provides preliminary experimental evidence for the emerging zoonotic potential of H4N1 AIV. These findings expand our knowledge of the H4 subtype AIV and reinforce the critical need for continued surveillance of AIV to prevent and prepare for potential outbreaks affecting human health.
(SNIP)
Discussion
In this study, we demonstrated that H4N1 AIV has the capacity to bind to human receptors, effectively adsorb onto human respiratory tract tissues, and replicate efficiently in human lung cells. These findings suggest its potential for cross-species transmission to humans, which is consistent with the hypotheses proposed by previous bioinformatics predictions [30].
Furthermore, H4N1 AIV induced symptoms of pneumonia and tracheitis in mice without the need for prior adaptation, highlighting their mammalian pathogenicity. Additionally, during a limited number of in vitro cell passages(4 and 7 passages), novel mutations arose in the HA and NP of H4N1 AIV. These mutations enhanced the virus’s binding affinity to human-type α2,6-linked sialic acid receptors, accelerated replication kinetics in mammalian cells, and increased murine pathogenicity.
These traits—receptor adaptation, enhanced replication efficiency, and heightened mammalian virulence—demonstrate H4N1 AIV’s capacity to rapidly evolve phenotypes conducive to cross-species infection under controlled laboratory conditions. While this does not equate to proven zoonotic transmission, it highlights the risks and emphasizes the necessity of actively monitoring H4N1 AIV.
PNAS Research: Intensity and Frequency of Extreme Novel Epidemics
BMJ Global: Historical Trends Demonstrate a Pattern of Increasingly Frequent & Severe Zoonotic Spillover Events
But forewarned is only forearmed, if we take steps now to prepare for the next crisis.
