|Range Of Endemic H9N2 Viruses|
Despite being an LPAI (low path avian influenza) virus, and producing relatively mild symptoms in humans , H9N2 is viewed by many as an important and influential player in the avian flu world.
- H9N2 has become widespread - even ubiquitous - among poultry across Europe and the Middle East (see map above).
- Serological studies suggest human infection may be far more common than standard surveillance (see FluTrackers List) would have us believe (see J. Infect & Public Health: High Seroprevalence Of Avian Influenza H9 Among Poultry Professionals In Pakistan)
- 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).
We've looked at scores of studies on H9N2 over the past 14 years, and over that time this subtype's reputation has increased. A few recent blogs include:
J. Virology:Genetic Compatibility of Reassortants Between Avian H5N1 & H9N2 Influenza Viruses
Virology: Receptor Binding Specificity Of H9N2 Avian Influenza Viruses
EID Journal: Two H9N2 Studies Of Note
Today we've a comprehensive review of the global spread and evolution of LPAI H9N2, published last week the Journal Viruses, that consolidates much of the information gleaned over the past decade on this under-appreciated pandemic threat.
As you would expect, this is a lengthy, and detailed open-access paper, and so I've only selected the abstract and a few excerpts (bolding mine) for this blog.I highly recommend following the link to read this review in its entirety. As for myself, I'll be keeping a copy on my desktop for future reference.
Viruses 2019, 11(7), 620; https://doi.org/10.3390/v11070620
Received: 6 June 2019 / Accepted: 1 July 2019 / Published: 5 July 2019
AbstractH9N2 avian influenza viruses have become globally widespread in poultry over the last two decades and represent a genuine threat both to the global poultry industry but also humans through their high rates of zoonotic infection and pandemic potential.
H9N2 viruses are generally hyperendemic in affected countries and have been found in poultry in many new regions in recent years. In this review, we examine the current global spread of H9N2 avian influenza viruses as well as their host range, tropism, transmission routes and the risk posed by these viruses to human health.
Keywords:H9N2; avian influenza viruses; zoonotic; pandemic potential; poultry
Conclusions and Perspectives
In recent years, outbreaks of H9N2 viruses have been found in an increasing number of countries, including for the first time, sub-Saharan Africa, far South-East Asia and Russia. Because of its expansive geographical range, it is speculated that H9N2 viruses may currently be causing greater economic damage to poultry production worldwide compared to highly pathogenic H5 or H7 subtypes which are generally more localised.
Moreover, the last four years have seen as many human H9N2 infections as the two decades before. These facts indicate a growing threat from H9N2 viruses to both animal and human health.
Although the virus mostly causes mild disease and low mortality, as compared to highly pathogenic viruses, there is clear potential for the virus to continue to adapt and become more pathogenic in chickens and better adapted to humans. Additionally, there remains a clear threat, as highlighted by the repeated novel zoonotic AIV viruses that have emerged in recent years such as H7N9, H10N8 and H5N6, posed by reassortant H9N2-origin viruses.
H9N2 viruses have been repeatedly isolated from non-human mammalian hosts such as swine and minks—these hosts pose a particular threat for emergence of novel pandemic viruses as they are highly susceptible to both human and avian influenza viruses and could drive the generation of novel reassortants.
Endemic countries across Asia and the Middle East, as well as, more recently, Africa, are most under threat from zoonotic H9N2 infections.
We have discussed how reassortants between H9N2 viruses and human seasonal influenza viruses are able to efficiently transmit between ferrets and there is, therefore, a real danger eventually such a reassortant could emerge in the field. Several H9N2 viruses have human receptor binding, pH stable HA proteins that could potentially allow efficient transmission between humans whilst other H9N2 viruses contain internal gene cassettes that allow extremely efficient replication in humans (i.e., genotype 57).
Overall there is a clear risk of both intersubtypic H9N2/human influenza virus reassortant emergence as well as an intrasubtyptic human binding HA/efficient mammalian polymerase reassortant emergence, either of which could pose a high zoonotic and pandemic threat.
These trends highlight a clear need for further surveillance efforts, particularly in countries where H9N2 has not been officially declared. Surveillance should also be continued in countries with endemic H9N2—in vaccinated poultry and poultry workers. Additionally, contemporary viruses circulating in poultry rearing systems need constant phenotypic characterisation to assess properties such as antigenic drift, viral pathogenicity and zoonotic potential.(Continue . . . )