WHO WPRO Reports 6th H10N3 Case & 3 Additional H9N2 Cases In China

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Over the past 6 months China has reported 16 H9N2 cases and 2 H10N3 cases, spread across 8 Provinces (see last May's HKCHP Reports 1 New H10N3 Case from the Mainland & 8 H9N2 Cases).  

While China has reported roughly 130 (mostly mild) H9N2 cases in the past decade, only 5 H10N3 cases had been reported to that date, the first being in July of 2021.

Given that many influenza cases are never hospitalized (and subsequently tested for novel flu), both of these numbers are presumed to significantly underrepresent the true burden from these zoonotic flu infections. 

Today's WHO WPRO Avian Influenza Weekly Update Number 1002 carries brief reports (excerpts below) on 3 additional H9N2 cases and a 6th H10N3 case, all from mainland China. After which, we'll take a deeper look at the concerns over the recent rise of both viruses. 

Human infection with avian influenza A(H9N2) virus

From 13 to 19 June 2025, three new cases of human infection with avian influenza A(H9N2) virus were reported to WHO in the Western Pacific Region. 

• The first case is a 52-year-old female from Zhoukou City, Henan Province, China, with onset of symptoms on 10 May 2025. The case was admitted to the hospital due to severe pneumonia on 12 May. Antiviral treatment was initiated, and she made a full recovery and was discharged on 23 May. 

• The second case is a six-year-old male from Deyang City, Sichuan Province, China, with onset of symptoms on 14 May. His clinical status was mild. 

• The third case is a 72-year-old female from Hengyang City, Hunan Province, China. She had onset of symptoms on 14 May 2025 and was hospitalized on 17 May 2025, due to her age, high fever and preexisting medical conditions. With antiviral treatment, the patient recovered and was discharged on 24 May. 

All three cases had poultry exposure, and all identified close contacts of the three cases completed 10 days of health monitoring, with no illness reported and no epidemiological link has been identified between the cases.

Since December 2015, a total of 133 cases of human infection with avian influenza A(H9N2), including two deaths (both with underlying conditions), have been reported to WHO in the Western Pacific Region. Of these, 130 were reported from China, two were reported from Cambodia, and one was reported from Viet Nam.

Human infection with avian influenza A(H10N3) virus

From 13 to 19 June 2025, one new case of human infection with avian influenza A(H10N3) virus was reported to WHO in the Western Pacific Region. The case, reported from Shaanxi Province, China, is a 70-year-old female farmer. While in Ordos City, Inner Mongolia, she developed symptoms including fever and chest tightness on 21 April 2025.

Her condition worsened and she was admitted to a hospital in Inner Mongolia with severe pneumonia on 25 April, then she was transferred to another hospital in Shaanxi Province. The case is still under treatment, and her condition is currently stable and improving. Close contacts were identified and all tested negative for influenza A and remained asymptomatic during the monitoring period.

To date, six cases of human infection with avian influenza A(H10N3) have been reported globally. Most previously reported human infections with avian influenza viruses were due to exposure to infected poultry or contaminated environments.

 Since avian influenza viruses, including avian influenza A(H10N3) viruses, continue to be detected in poultry populations, further sporadic human cases could be detected in the future. Currently, available epidemiologic information suggests that the avian influenza A(H10N3) virus has not acquired the ability for sustained human-to-human transmission, thus the likelihood of spread among humans is low.

While most of the (now 6) known H10N3 cases have produced severe illness, we have no idea how many mild, or subclinical, infections may have gone unreported.  

We continue to see cautionary reports, however, from Chinese researchers on the human health threat from this emerging subtype.

• Last July, in Frontiers: Phylogenetic and Mutational Analysis of H10N3 Avian Influenza A virus in China: Potential Threats to Human Health, we looked at a report that described 4 mutations of concern in the 2023 case (HA Q226L, PB2 D701N, PA S409N, and M2 S31N), along with the patient's treatment and course of illness.

• Last December Vet. Microbiology: The novel H10N3 Avian Influenza Virus Acquired Airborne Transmission Among Chickens: An Increasing Threat to Public Health reported the virus has become better adapted to poultry, is highly pathogenic in mice, can be transmitted via respiratory droplets between guinea pigs, and can also be transmitted via the airborne route by chickens.

• And two weeks ago Vet. Research: E627V Mutation in PB2 Protein Promotes the Mammalian Adaptation of Novel H10N3 Avian Influenza Virus reported that that PB2-626V is becoming increasingly common in AIV poultry isolates - including H10N3 - and that it significantly enhances mammalian adaptation while maintaining fitness in avian hosts.

H9N2, meanwhile, has been on the ascendent in recent years (see FluTracker's list), although stepped up screening for respiratory diseases in China may account for much of this increase.

 

H9N2  remains poorly controlled in Chinese poultry, despite the use of vaccines (see J. Virus Erad.: Ineffective Control Of LPAI H9N2 By Inactivated Poultry Vaccines - China), which has led to the creation and spread of numerous genotypes.

H9N2 also readily reassorts with, and often enhances, other novel influenza viruses (including H7N9, H5N1, and H5N6), making it an important viral co-conspirator (see Vet. Sci.: The Multifaceted Zoonotic Risk of H9N2 Avian Influenza).

While LPAI H9N2 is admittedly not at the very top of our list of pandemic concerns, the CDC has 2 different lineages (A(H9N2) G1 and A(H9N2) Y280) on their short list of influenza viruses with zoonotic potential (see CDC IRAT SCORE), and several candidate vaccines have been developed.

A recent review of H9N2's potential can be found at Nature: Genetic diversity of H9N2 avian influenza viruses in poultry across China and implications for zoonotic transmission.

Which is why continued reports of cases in China (and elsewhere) are worthy of our attention.