HK CHP Reports 3 More Mainland H9N2 Cases

 #17,954

Although its incidence is almost certainly much higher than reported, over the past 26 years we've seen nearly 120 H9N2 human infections officially reported from China, along with a small number from other countries (see FluTracker's list).  

In January 6th's CDTR, the ECDC reported:

As of 3 January 2024, and since 1998, a total of 130 laboratory-confirmed cases of human infection with avian influenza A(H9N2) viruses, including two deaths, have been reported in eight countries: China (117), Egypt (4), Bangladesh (3), Cambodia (2), Oman (1), Pakistan (1), India (1) and Senegal (1). Most of the cases were children with mild disease.

As an LPAI (low path avian influenza), H9N2 is not considered a `reportable' disease by WOAH (formerly the OIE), even though it is zoonotic. We've seen seroprevalence studies which suggest people with exposure to infected poultry often develop H9 antibodies, suggesting mild or asymptomatic infection.

Cases are generally mild, are usually reported in children (likely due to testing bias), and are often only belatedly reported by China.

Last week, in WHO: Influenza At the Human-Animal Interface Summary and Risk Assessment (Feb 26th), we learned of the Mainland's first reported case of 2024 (a six-year-old boy from Anhui with onset on January 3, 2024). 

Of note, in mid-February Hong Kong reported their first H9N2 case since 2020. 

Today Hong Kong's CHP Weekly Influenza report lists 3 more cases from the mainland.  While details are scant, all occurred in Feb, all were in children under the age of 11, and they were spread across 3 provinces (Guangxi, Jiangxi, and Guangdong). 

Over the past 15 years we've watched as H9N2 has expanded its geographic range - spreading out of Asia into Europe, the Middle East and Africa. While it occasionally spills over into humans, H9N2's biggest threat may come from its unique ability to reassort with other, potentially more dangerous, avian viruses.  

 

Its internal genes have often been found inside many HPAI viruses (including H5N1, H5N6, H7N9, and most recently zoonotic H3N8) - (see The Lancet's Poultry carrying H9N2 act as incubators for novel human avian influenza viruses).  

Reassortment can also occur in humans, and with seasonal flu viruses. The possibility exists that someone could be co-infected with seasonal flu and H9N2, allowing a hybrid (reassortment) to emerge (see The `Other Mixing Vessel' For Pandemic Influenza).

While LPAI H9N2 ranks pretty far down our pandemic threats list - at least as a standalone virus - it's ability to reassort with potentially more dangerous avian, human, and swine flu viruses make any uptick in cases worthy of our attention.