The convoluted road to a new reassortant avian H3N3 virus
#18,370
While HPAI H5 understandably has much of the world's attention, over the past few years we've seen a flurry of (avian/canine/swine) H3Nx-related studies coming out of China, warning of its continued evolution and potential public health threat.
Since all of the known human influenza pandemics (going back 130 years) have sprung from H1, H2, or H3 influenza viruses (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?), we can't afford to ignore new H1, H2, or H3 viruses when they emerge around the world.
Some recent blogs include:
Transboundary & Emerg. Dis.: H3 Avian Influenza Virus Isolated from China in 2021–2022 Showed the Emerging H3N8 Posed a Threat to Human Health
Emerg. Microb & Inf.: Emergence of Novel Reassortant H3N3 Avian Influenza viruses, China 2023
As you'll notice, while H3N8, H3N2, and even H3N6 have been featured, H3N3 has gotten a lot of attention over the past 18 months, with several new reassortants emerging in poultry. The H3N3 viruses we've looked at previously, however, are a different genotype than the one discussed today.
But in the past couple of years we've seen reports of increased pathogenicity and transmissibility of H3 viruses in Chinese poultry, and even some human infections (see EID Journal: Evolution of Avian Influenza Virus (H3) with Spillover into Humans, China).While China vaccinates heavily against H9N2 and H5+H7, they don't vaccinate against other LPAI viruses (like H3) that can also infect poultry, since they historically tend to cause little morbidity or mortality in birds.
Due to its length and technical nature, I've only posted the abstract and some excerpts from the discussion of today's study. Follow the link to read it in its entirety. I'll have a bit more after the break.
Emergence of a novel reassortant H3N3 avian influenza virus with enhanced pathogenicity and transmissibility in chickens in ChinaChunping Zhang,Conghui Zhao,Jiacheng Huang,Yang Wang,Bo Jiang,Hangyu Zheng,Mingzhi Zhuang, Yanni Peng,Xiaoxuan Zhang, Sha Liu, Haoxi Qiang, Huanhuan Wang, Xiancheng Zeng,
Guijie Guo, Ji-Long Chen & Shujie Ma
Veterinary Research volume 56, Article number: 56 (2025) Cite this articleAbstract
H3N3 avian influenza viruses (AIVs) are less prevalent in poultry than H3N8 viruses. However, although relatively rare, reassortant H3N3 viruses have been known to appear in both domestic poultry and wild birds. In this study, we isolated the H3N3 virus in chickens sourced from a live poultry market in China. A comprehensive genomic analysis revealed that the virus possessed a single basic amino acid in the cleavage site of the hemagglutinin (HA) gene.Phylogenetic analysis indicated that eight genes in the H3N3 virus belong to the Eurasian lineage. Specifically, the HA and NA genes were clustered with H3N2 and H11N3, respectively, while the internal genes were closely related to the H3N8 and H9N2 viruses. Furthermore, the H3N3 virus exhibited high and moderate stability in thermal and acidic conditions and efficient replication capabilities in mammalian cells.The H3N3 virus demonstrated that it could infect and replicate in the upper and lower respiratory tract of BALB/c mice without prior adaptation, triggering hemagglutination inhibition (HI) antibody titres ranging from 80 to 160; notably, the H3N3 virus replicated vigorously within the chicken respiratory and digestive tracts.The virus also transmitted efficiently and swiftly among chickens through direct contact, leading to higher levels of HI antibodies in both the inoculated and contact birds.These findings suggest that the H3N3 virus may be a novel reassortant originating from viruses circulating in domestic poultry, thus demonstrating an increased pathogenicity and transmissibility in chickens. Our study determines that H3N3 AIV potentially threatens the poultry industry and public health, highlighting the importance of active surveillance of AIVs.
(SNIP)
Discussion
In recent decades, H3N3 influenza viruses have been detected in multiple animal species, including domestic poultry, pigs, seals, wild birds, and humans. Compared to the H3N2 and H3N8 AIVs, the H3N3 viruses exhibit a more restricted distribution, are less frequently detected, and have historically attracted less focus from researchers. Moreover, recently, a novel H3N3 strain emerged in China; however, its characteristics are mostly unknown.
This study identified a novel reassortant H3N3 virus in a live poultry market. We observed that this virus was pathogenic to chickens and could be transmitted through direct contact among chickens. Phylogenetic analysis has revealed that the H3N3 is a novel reassortant virus, characterised by the HA and NA genes derived from the H3N2 and H11N3 viruses, respectively. The internal genes of the H3N3 virus exhibit a complex evolutionary lineage originating from two distinct subtypes of AIVs. Specifically, the PB1, PA, and M genes exhibit a closer genetic affinity to those of H9N2 viruses, whereas the virus’ PB2, NP, and NS genes show genetic clustering with H3N8 viruses.
(SNIP)
This study also indicates that the newly emerged AIVs capable of replicating in mammals may acquire key amino acid mutations to adapt to new hosts, enhancing viral pathogenicity and facilitating human droplet transmission. The substitution of E627K or D701N in PB2 dramatically increased the virulence and transmissibility of the H7N9 viruses. Such an outcome may be attributed to PA protein as the intrinsic driving force behind the emergence of PB2 627 K during H7N9 replication in mammals [1, 14, 51]. Moreover, molecular analysis disclosed that the H3N3 virus in this study was absent of PB2 627 K or 701N mutations but harbouring 292 V and 588 V, which are known to enhance viral pathogenicity in mammals.
Here, multiple mammalian-adapted mutations in PB1, PA, NP, M1, M2, and NS1 were also detected, suggesting that the H3N3 virus may threaten mammals, potentially enhancing its capacity for transmission and pathogenicity within this host group. Interestingly, the H3N3 virus exhibited mild pathogenicity in mice, confining viral replication to the respiratory tract. Additionally, no PB2 627 K or 701N mutations were detected in viruses recovered from the lungs and nasal turbinates of mice at 3 dpi.
Despite the H3N3 virus being categorised as a low pathogenic AIV subtype, the emerging novel H3N3 virus has shown a trend of increasing pathogenicity and efficient transmissibility in chickens. Although the H3N3 subtype virus in this study was replicated in mice without adaptation, our findings suggest that the virus is more adapted to chickens, indicating that effective control measures must be considered for poultry populations.
In summary, we identified a novel reassortant H3N3 virus, with internal genes derived from H3N8 and H9N2 viruses, in a live poultry market in China. Our research highlights the ongoing circulation and genetic reassortment of AIVs across various subtypes within domestic poultry populations. Thus, continued monitoring of AIVs in domestic and wild birds is imperative. It is particularly crucial to recognise their roles as reservoirs for the emergence of novel strains that may pose significant zoonotic risks to public health.
While the threat from H3N3 may not be huge - twice in my lifetime (1957 & 1968) a reassortment between seasonal flu and an avian flu virus - produced a pandemic influenza virus.
- The first (1957) was H2N2, which According to the CDC `. . . was comprised of three different genes from an H2N2 virus that originated from an avian influenza A virus, including the H2 hemagglutinin and the N2 neuraminidase genes.'
- In 1968 a novel H3N2 virus emerged (a reassortment of 2 genes from a low path avian influenza H3 virus, and 6 genes from H2N2) which supplanted H2N2 - killed more than a million people during its first year - and continues to spark yearly epidemics more than 56 years later.
