For a low pathogenic avian influenza (LPAI) virus that has only been identified in a few dozen patients over the past two decades (see FluTrackers List) - and generally causes only mild to moderate illness in humans - we spend a a lot of our time scrutinizing the (direct and indirect) pandemic potential of H9N2.
Despite its relatively innocuous facade, we do so for some very good reasons (which I've cited before):
- H9N2 has become widespread - even ubiquitous - among poultry across Asia and much of the Middle East
- Serological studies suggest human infection is far more common than case counts based on standard surveillance would have us believe
- H9N2 reassorts easily with other viruses, and its internal genes are often found inside many HPAI viruses (including H5N1, H5N6, and H7N9) - (see The Lancet's Poultry carrying H9N2 act as incubators for novel human avian influenza viruses)
- H9N2 viruses continue to accrue evolutionary changes, which include mammalian adaptations, and increased binding to human receptor cells (see PLoS Path: Genetics, Receptor Binding, and Transmissibility Of Avian H9N2).
A finding which has become increasingly common over the past decade.In 2010 (see Study: The Continuing Evolution Of Avian H9N2) we looked at computer modeling (in silica) that warned that H9N2 viruses have been slowly evolving towards becoming a `more humanized’ virus.
In 2016's Genomic Characteristics Of 2 A(H9N2) Virus Isolates From Humans In Anhui Province - 2015, researchers found:
The amino acid sequence alignment results showed that several mutations for human infection tropism presented in the two virus strains, including Q226L, H183N and E190T in HA; S31N in M2; 63-65 deletion in NA. In addition, the H9N2 influenza virus strains possessed the PSRSSR\GL motif in HA.This Q226L amino acid substitution - where the amino acid glutamine is replaced by leucine - is a commonly found in human flu viruses, not avian.
In 2015, in EID Journal: Replication Of Avian H9N2 In Pet Birds, Chickens, and Mammals, Bangladesh, scientists reported:
The H9N2 virus strain Env/9306 contains mammalian-like mutations in genes, including HAQ226L (H3 numbering) (5), which increase H9N2 virus transmissibility to and among mammals.And if that weren't enough, H9N2 continues to be extraordinarily promiscuous, reassorting with a wide variety of LPAI and HPAI viruses, and generating hybrids. A few examples include:
Reassortment After Co-infection Of Chickens With H4N6 and H9N2 influenza Viruses
A Canine H3N2 Virus With PA Gene From Avian H9N2 - Korea
PNAS: Reassortment Of H1N1 And H9N2 Avian viruses.
Molecular characterization and receptor binding specificity of H9N2 avian influenza viruses based on poultry-related environmental surveillance in China between 2013 and 2016
H9N2 avian influenza viruses (AIVs) have become panzootic and caused sporadic human cases since 1998. Based on the poultry-related environmental surveillance data in mainland China from 2013 to 2016, a total of 68 representative environment isolates were selected and further investigated systematically.
Phylogenetic analysis indicated that Y280-like H9N2 viruses have been predominant during 2013–2016 and acquired multiple specific amino acid substitutions that might favor viral transmission from avian to mammalians. Additionally, the viruses have undergone dramatic evolution and reassortment, resulting in an increased genetic diversity or acting as the gene contributors to new avian viruses. Receptor-binding tests indicated that most of the H9N2 isolates bound to human-type receptor, making them easily cross the species barrier and infect human efficiently.
Our results suggested that the H9N2 AIVs prevalent in poultry may pose severe public health threat.(SNIP)
In summary, the present study systemically analyzed the H9N2 environment isolates in mainland China from 2013–2016. Phylogenetic analysis indicated that Y280-like H9N2 viruses have been predominant during 2013–2016 and revealed that the viruses had acquired multiple specific amino acid substitutions that might favor viral transmission from avian to mammalians.
Additionally, the viruses have undergone dramatic evolution and reassortment, resulting in an increased genetic diversity. Receptor-binding tests indicated that most of the H9N2 isolates bound to human-type receptor, making them easily cross the species barrier and infect human efficiently. Therefore, urgent attention to H9N2 AIVs and diligent surveillance is becoming more and more essential.(Continue . . . )
Avian adapted flu viruses bind preferentially to the alpha 2,3 receptor cells found in the gastrointestinal tract of birds. While there are some alpha 2,3 cells deep in the lungs of humans, for a novel influenza to be successful in a human host, most researchers believe it needs to a able to bind easily to the α2-6 receptor cells abundantly found in the upper airway (trachea).
Although a preferential binding to human receptor cells is considered perhaps the biggest obstacle for an avian virus to successfully jump species – it isn’t the only one.Still, when you consider H9N2s growing diversity and geographic spread, its increased affinity for human (α2-6) receptor cells, and its ability to easily reassort with other - often more pathogenic viruses - and you have a low profile viral threat we can't afford to ignore.