#18,461
In the wide world of avian flu viruses, H5 and H7 viruses get the bulk of our attention, since they are the most destructive to the poultry industry and are known to occasionally spontaneously mutate into HPAI viruses when allowed to circulate in poultry.
But as we've discussed often, there are other zoonotic or potentially zoonotic avian flu viruses (H3, H4, H6, H9, H10, etc.) which are not reportable, and are often tolerated or ignored because they produce relatively minor financial losses to the poultry industry.
LPAI H9N2, which is now rife in poultry in Asia, the Middle East, and increasingly in Africa, is perhaps the most obvious threat. We've seen > 140 documented human infections (see ECDC chart below), mostly from China.
But increasingly China has warned on the spread (in wild birds and poultry) and zoonotic potential of other LPAI viruses, including H3Nx, H4Nx, H6Nx, and H10Nx.
If there is a common denominator among these varied LPAI viruses, it is that the longer they circulate in poultry, the greater the zoonotic threat they appear to pose. Their evolution may be far slower than H5 and H7 viruses, but that doesn't make them benign.
Only easier to ignore.
Twelve years ago - during the opening salvo of H7N9's first wave in China - Taiwan reported the world's first known human infection with LPAI H6N1 (see Taiwan CDC: Epidemiological Analysis Of Human H6N1 Infection). A year later, Taiwan reported an outbreak in dogs (see EID Journal: Influenza A(H6N1) In Dogs, Taiwan).
- In 2015, in the EID Journal: Seropositivity For H6 Influenza Viruses In China, researchers reported a low - but significant - level of antibodies, particularly among live bird handlers, to the avian H6 virus in China.
- Also in 2015, in Study: Adaptation Of H6N1 From Avian To Human Receptor-Binding, we saw a report citing changes the authors suggest are slowly moving the H6N1 virus towards preferential binding to human receptor cells instead of avian receptor cells
- In 2020's Nature: Evolution & Pathogenicity of H6 Avian Influenza Viruses, Southern China 2011-2017, we looked at H6's increasing adaptation to mammalian physiology, and again in 2022 in Study: Influenza A (H6N6) Viruses Isolated from Chickens Replicate in Mice and Human lungs Without Prior Adaptation.
Today we've a preprint (not yet peer-reviewed) - published on Preprints.org - which characterizes the evolution and biological characteristics of H6N1 viruses in Taiwan, and finds slow but continuous movement towards a more mammalian adapted virus.
The Taiwan H6N1 lineage provides a compelling example of the stepwise adaptation of avian influenza viruses to mammalian hosts, demonstrating how prolonged poultry adaptation can bridge avian-mammalian barriers.How far any of these LPAI viruses can go towards becoming a serious public health threat is unknowable, but their collective trajectory is concerning.
And if long-term carriage in poultry is truly a significant driver of mammalian adaptation, then far greater emphasis on biosecurity and virus control is essential.
Zuoyi Zheng,Xifeng Chen,Rutian Zheng,Zhigang Yan,Long Li,Rirong Chen,Lifeng Li,Yongmei Liu,Yi Guan *,Huachen Zhu *
Abstract
Interspecies transmission of avian influenza viruses remains a significant public health concern. H6 viruses have gained attention following the first human infection by a chicken-origin H6N1 virus (A/Taiwan/02/2013, Hu/13), which highlighted their zoonotic potential. To understand the evolutionary trajectory and mammalian adaptation of this Taiwan lineage, we compared two avian isolates (A/Chicken/Taiwan/CF19/2009, Ck/09; A/Chicken/Taiwan/2267/2012, Ck/12) and Hu/13 in vitro and in vivo. Hu/13 exhibited enhanced replication in MDCK cells, with larger plaques and higher viral titers than Ck/09 and Ck/12.
In BALB/c mice, Hu/13 caused high pathogenicity and mortality, while Ck/09 induced minimal morbidity. Hu/13 replicated efficiently in respiratory tissues, eliciting robust cytokine responses and severe pulmonary lesions, with Ck/12 showing intermediate virulence. In ferrets, Hu/13 demonstrated efficient transmission, infecting all direct-contact and one airborne-contact ferret, whereas Ck/09 failed to transmit. Histopathology confirmed escalating lung pathology from Ck/09 to Ck/12 and Hu/13. Whole-genome sequencing identified adaptive mutations in Hu/13 during ferret replication, though no canonical mammalian-adaptive changes (e.g., PB2-E627K or HA-Q226L) were detected.
These findings demonstrate progressive mammalian adaptation, replication efficiency, and transmissibility within the Taiwan H6N1 lineage, underscoring its zoonotic risk and emphasizing the need for enhanced surveillance to monitor mammalian-adaptive mutations, informing pandemic preparedness and public health strategies.
(SNIP)
4. Discussion
The interspecies transmission of AIVs continues to pose significant threats, particularly in densely populated regions with extensive poultry production, such as China. Among the most frequently detected influenza subtypes in aquatic birds, H6 viruses have sporadically spilled over into terrestrial poultry, leading to the establishment of enzootic clades [5,6,7,8]. The Taiwan H6N1 lineage exemplifies this evolutionary trajectory, having evolved into a distinct clade in chickens since 1997 and demonstrating increasing potential for mammalian adaptation over time [5,8,9,10,11,14]. This is evidenced by the first documented human and canine infections [9,10], as well as the progressive adaptation of temporally distinct strains in mammalian models, as highlighted in this study and previous research [8].
In this study, we compared the infectivity, pathogenicity, and transmissibility of three Taiwan H6N1 isolates (Ck/09, Ck/12, and Hu/13) in mammalian models. The results demonstrated a progressive enhancement in replication efficiency in MDCK cells, mice, and ferrets, with Hu/13 showing the highest capability, followed by Ck/12 and then Ck/09. Pathogenicity in mice and transmissibility in ferrets showed a similar trend, with Hu/13 exhibiting efficient contact transmission among ferrets. Notably, earlier H6N1 isolates from this Taiwan lineage, such as those from 1998 and 1999, were unable to replicate efficiently in mice, while isolates from 2002 and 2005 showed moderate replication [8]. In contrast, recent isolates like Ck/12 and Hu/13 displayed significantly enhanced replication in mammalian models, with Hu/13 causing severe morbidity and 100% mortality at 10⁶ PFU in mice.
These findings suggest that prolonged circulation and adaptation of H6N1 viruses in chicken populations enhance their replication efficiency and transmissibility in mammals. This pattern mirrors observations in chicken-adapted H9N2 in China and its related derivative viruses (e.g., H3N8, H7N9 and H10N8), which have also demonstrated increasing zoonotic potential over time [3,4,22,34,35]. Furthermore, the replication capabilities of Ck/12 and Hu/13 in mice exceed those of H6 viruses isolated from other regions of China, as shown in comparative studies [7,36,37,38]. Collectively, these results underscore the growing zoonotic risk posed by Taiwan H6N1 viruses and highlight the need for continuous surveillance and research to mitigate public health threats.
(SNIP)
While this study provides insights into the progressive evolution of H6N1 virus with increasing zoonotic potentials, several limitations must be acknowledged. First, the absence of post-2013 H6N1 isolates precludes assessment of ongoing evolution in Taiwanese poultry, and the use of limited number of strains may not fully recapitulate the genetic diversity and evolutionary dynamics of H6N1 in natural poultry populations. Second, while mouse and ferret models are widely used for influenza studies, they may not perfectly predict human infection, pathogenicity and transmission patterns. Future research should incorporate more field isolates and explore alternative models to validate these findings. Third, while we identified amino acid variations between the avian and human isolates (Table S6) and adaptive mutations in ferret-passaged Hu/13 (Table S5), functional validation of these changes is needed. Additionally, investigating the role of host immune responses in shaping viral adaptation could provide a more comprehensive understanding of the factors driving cross-species transmission.
The progressive adaptation of H6N1 viruses in mammalian systems highlights the need for enhanced surveillance and control measures. The detection of subclinical infections in ferrets and the increasing replication efficiency of recent isolates, coupled with the silent persistence of H6N1 in Taiwan, underscore the potential for H6 viruses to cause outbreaks in humans. Given the high prevalence of H6 viruses in poultry and their ability to acquire mammalian-adaptive traits, continuous monitoring of viral evolution in poultry populations is essential. This includes tracking key mutations in HA, NA, and internal genes that may enhance viral replication, transmission, and pathogenicity in mammals. A One-Health approach and proactive risk assessment, including reverse genetics studies to pinpoint transmission determinants, are critical to mitigate pandemic threats.
5. Conclusions
The Taiwan H6N1 lineage provides a compelling example of the stepwise adaptation of avian influenza viruses to mammalian hosts, demonstrating how prolonged poultry adaptation can bridge avian-mammalian barriers. The enhanced replication efficiency, pathogenicity, and transmissibility of recent isolates, such as Hu/13, highlight the zoonotic potential of H6 viruses. These findings underscore the importance of ongoing surveillance and research to identify and mitigate the risks posed by H6 viruses, particularly in regions with extensive poultry production and human-animal contact. By understanding the molecular mechanisms of viral adaptation and the role of intermediate hosts like chickens, we can better prepare for and prevent future influenza pandemics.
One of the reasons why I don't focus solely on avian H5 influenza in this blog is that there are many other ways the next pandemic could emerge. Last fall the WHO Released their 2024 Pathogens Prioritization Report, which identified 34 priority pathogens from 16 families (see graphic below).
