Range Of Endemic H9N2 Viruses
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When we talk about avian influenza viruses with pandemic potential, H5 and H7 subtypes are often the first that come to mind, as several (H5N1, H5N6, H7N9, etc.) have a track record of causing severe and often fatal illness in humans (see HK Monitoring 4 More H5N6 Cases On the Mainland).
Luckily, none have acquired the ability to transmit easily from human-to-human, and therefore have only caused sporadic, albeit sometimes sizable, outbreaks in humans.
These aren't the only avian flu threats, however, and one of the other subtypes we spend a good deal of our time watching is LPAI (Low Pathogenic Avian Influenza) H9N2 which is common in poultry in Asia, the Middle East, and parts of Africa (see map above).
Unlike the more virulent H5 and H7 avian viruses, H9N2 is not considered a `reportable' disease by the OIE, and so our understanding of its range and continued spread is limited.
Control has been difficult, as the virus continues to mutate, and many countries continue to use outdated and ineffectual vaccines (see J. Virus Erad.: Ineffective Control Of LPAI H9N2 By Inactivated Poultry Vaccines - China).
Human infection with H9N2 has only rarely been reported over the past 2 decades (< 100 cases), but it almost certainly happens more often than we know (see J. Infect & Public Health: High Seroprevalence Of Avian Influenza H9 Among Poultry Professionals In Pakistan).
Whether it is due to better surveillance or higher incidence of infection, the number of human cases being reported - particularly by China - has increased markedly since 2015 (see FluTrackers Global Cumulative H9N2 Partial Case List 1998 - Current).
Today Hong Kong's CHP reports 6 more cases (3 from 2021 and 3 from 2022) in their Weekly Avian Influenza Report VOLUME 18, NUMBER 09. While data on these cases is limited (province, age, gender, onset date), these infections tend to be mild and self limiting.
Rare deaths have been reported (see ECDC: A Fatal H9N2 Case In China & Risk Assessment), but we rarely know about a patient's comorbidities. Most of these infections occur in parts of the world where influenza testing is limited, and medical care - particularly for mild viral illnesses - is seldom sought.
In May of 2020, the CDC added a new lineage (H9N2 Y280 lineage [A/Anhui-Lujiang/13/2018]) to their short list of novel flu viruses with at least some pandemic potential (IRAT), although it is not expected to have as severe of an impact as an H5 or H7 avian flu.
In their 2019 analysis, they do note the following traits :
- Human Infections: Sporadic human cases were reported. Limited information exists, but no confirmed reports of spread between humans. Information meets the definition of moderate risk for this element.
- Transmission in Laboratory Animals: In ferret studies, this virus transmitted well by direct contact between animals, and consistently by respiratory droplets, suggesting a high risk for this element.
- Receptor Binding: The predicted amino acid sequence of HA protein indicates that this virus possesses residues that may promote binding to human receptors. Only weak binding to 2,3 and 2,6 glycans has been observed in glycan array studies with recombinant HA.
- Population Immunity: Limited data are available, but the expectation of cross-reactive antibodies in humans to this virus is low. However, some level of cross-reactive antibodies may be possible in individuals with past exposure to seasonal viruses.
While concerning, H9N2's greatest claim to fame it its ability to reassort with other - sometimes far more worrisome - viruses (see PNAS: Evolution Of H9N2 And It’s Effect On The Genesis Of H7N9).
Three years ago, in EID Journal: Two H9N2 Studies Of Note, we looked at two reports which suggest that H9N2 continues to evolve away from current (pre-pandemic and poultry) vaccines and is potentially on a path towards better adaptation to human hosts.
While not exactly at the top of our pandemic worry list - at least as a standalone virus - H9N2's ability to reassort with potentially more dangerous avian, human, and swine flu viruses makes it one worth watching.
