Guidance

Risk assessment of avian influenza A(H9N2)

Updated 9 August 2021

Epidemiology of A(H9N2) in humans

Avian influenza A(H9N2) is a common and geographically widespread disease of birds, which has the ability to directly infect humans. A total of 53 cases of human infections with avian influenza A(H9N2) have been reported from China since 2015 via the World Health Organization (WHO) reporting mechanisms. The latest case was a 30 year old from Guangdong province with an onset date in April 2021. Where exposure details are known, most cases in China have been exposed to infected poultry or their environments.

There have been 10 other WHO confirmed cases in other countries since 2015.

WHO has commented there has been no evidence of international disease spread among humans and the likelihood of community-level transmission is low.

There is no evidence of human-to-human transmission, no case clusters have been observed, and the current epidemiological and virological evidence suggests that this virus has not acquired the ability of sustained transmission amongst humans. Most human infections observed so far have resulted in a mild clinical illness. However, there have been some hospitalised cases and one fatal case in a person with underlying medical conditions.

Avian influenza A(H9N2) in avian species

H9N2 is considered to be a low pathogenic avian influenza (LPAI) in poultry but can cause symptoms including egg drop and morbidity. As a result of the low pathogenicity, circulation in poultry may go unnoticed or ignored.

LPAI H9N2 infection of birds is non-notifiable (according to the World Organisation for Animal Health (OIE) Terrestrial Animal Health Code, 2005). Therefore, information on the prevalence and distribution of the virus subtype is limited. However, it is considered to be enzootic in poultry in China where it causes significant losses to the poultry industry. Avian influenza A(H9N2) is known to affect both wild birds and domestic poultry. It is the most common subtype of avian influenza in Asia, the Middle East and North Africa, and sporadic detections have occurred in Europe.

Virology

Although avian influenza A(H9N2) can directly infect humans, it also causes considerable concern due to its ability to donate partial, or entire cassettes of genes through reassortment with other influenza viruses. Co-circulation of H9N2 has been directly linked to the evolution of the H7N9 avian influenza virus which has caused severe mortality and morbidity in humans in China.

WHO risk assessment

The WHO risk assessment for avian influenza A(H9N2) remains the same, an update with the Oman case was published on 10 May 2019. It states:

‘What is the likelihood that additional human cases of infection with avian influenza A(H9N2) viruses will occur?’

Most human cases follow exposure to the A(H9N2) virus through contact with infected poultry or contaminated environments. Human infection tends to result in mild clinical illness in most cases. Since the virus continues to be detected in poultry populations, further human cases can be expected.

‘What is the likelihood of human-to-human transmission of avian influenza A(H9N2) viruses?’

No case clusters have been reported. Current epidemiologic and virologic evidence suggests that influenza A(H9N2) viruses assessed by GISRS have not acquired the ability of sustained transmission among humans, thus the likelihood is low.

‘What is the likelihood of international spread of avian influenza A(H9N2) virus by travellers?’

Should infected individuals from affected areas travel internationally, their infection may be detected in another country during travel or after arrival. If this were to occur, further community level spread is considered unlikely as the A(H9N2) virus subtype has not been confirmed to have acquired the ability to transmit easily among humans.

UKHSA risk assessment

Whilst avian influenza A(H9N2) continues to circulate in poultry, there is a risk of sporadic human infections. Countries affected by human cases most recently include mainland China and Oman.

The risk to UK residents within the UK from Avian Influenza A(H9N2) is very low (as it is not currently present in the animal reservoir within the UK).

The risk of Avian Influenza A(H9N2) to UK residents who are travelling to affected countries (listed above) is very low but would be higher in those individuals with a history of poultry exposure and associated activities.

The probability that a cluster of severe acute respiratory illness in the UK is due to Avian Influenza A(H9N2) is very low but testing would be indicated. A history of travel to an affected country would increase this probability.

If compliance with infection control guidance is good, the risk to healthcare workers caring for any cases of A(H9N2) is very low. However, any respiratory illness among healthcare workers undertaking this work would be rapidly investigated for this infection.

The risk of infection to contacts of laboratory-confirmed cases of avian influenza A(H9N2) is currently considered to be low, but they would be subject to public health follow-up following their exposure. These individuals would be tested for infection if they developed any new febrile or respiratory illness.

References

WHO Influenza at the Human-Animal interface, 22 June 2021

EFSA/ECDC Avian influenza Overview, November 2018

Evolution of the H9N2 influenza genotype that facilitated the genesis of the novel H7N9 virus, PNAS 112 (2) 548-553, 13 January 2015