Avian influenza A (H5N1) virus infection

Centers for Disease Control and Prevention (CDC): case definitions for investigations of human infection with avian influenza A viruses in the United States[119]Centers for Disease Control and Prevention. Case definitions for investigations of human infection with avian influenza A viruses in the United States. May 2024 [internet publication]. https://www.cdc.gov/bird-flu/hcp/case-definition/index.html

Perform testing on people who meet epidemiological criteria and either clinical or public health response criteria.

Confirmed case:

• Avian influenza A virus infection in a person that is confirmed by the CDC’s Influenza Division Laboratory or a CDC-designated laboratory using methods mutually agreed upon by the CDC and the Council of State and Territorial Epidemiologists (CSTE).

Probable case:

• A person meeting criteria for avian influenza A virus infection (below) and for whom confirmatory laboratory test results do not provide a sufficient level of detail to confirm highly pathogenic avian influenza (HPAI) A(H5) virus infection.

Suspected case (case under investigation):

• A person meeting criteria for avian influenza A virus infection (below) and for whom confirmatory laboratory test results are not known or pending.

Epidemiological criteria:

• People with recent exposure (within 10 days) to avian influenza A viruses through one of the following:

  • Exposure to A(H5), A(H7), or A(H9) virus-infected birds

  • Exposure to an infected person

  • Laboratory exposure.

Clinical criteria:

• People with signs and symptoms consistent with acute or lower respiratory tract infection or conjunctivitis, or complications of acute respiratory illness without an identified cause.

Public health response criteria:

• Asymptomatic people for whom public health authorities, in consultation with the CDC, determine that testing is needed in order to assess the clinical spectrum of infection with avian influenza A virus as part of public health investigations.

CDC: case definitions for investigations of human infection with avian influenza A viruses in the United States Opens in new window

World Health Organization (WHO): case definitions for human infections with influenza A (H5N1) virus[120]World Health Organization. Interim guidelines for avian influenza case management. Sep 2007 [internet publication]. https://apps.who.int/iris/bitstream/handle/10665/205388/B0634.pdf

Person under investigation:

• A person whom public health authorities have decided to investigate for possible influenza A(H5N1) virus infection.

Suspected H5N1 case:

• A person presenting with unexplained acute lower respiratory illness with fever >38ºC (>100.4ºF) and cough, shortness of breath, or difficulty breathing AND one or more of the following exposures in the 7 days prior to symptom onset:

  • Close contact (within 1 metre [3 feet]) with a person (e.g., caring for, speaking with, or touching) who is a suspected, probable, or confirmed H5N1 case

  • Exposure (e.g., handling, slaughtering, defeathering, butchering, preparation for consumption) to poultry or wild birds or their remains or to environments contaminated by their faeces in an area where influenza A(H5N1) virus infections in animals or humans have been suspected or confirmed in the last month

  • Consumption of raw or undercooked poultry products in an area where influenza A(H5N1) virus infections in animals or humans have been suspected or confirmed in the last month

  • Close contact with a confirmed influenza A(H5N1) virus-infected animal other than poultry or wild birds (e.g., cat or pig)

  • Handling samples (animal or human) suspected of containing H5N1 virus in a laboratory or other setting.

Probable H5N1 case (notify WHO):

• Probable definition 1: a person meeting the criteria for a suspected case AND one of the following additional criteria:

  • Infiltrates or evidence of an acute pneumonia on chest radiograph plus evidence of respiratory failure (hypoxaemia, severe tachypnoea); or

  • Positive laboratory confirmation of an influenza A infection but insufficient laboratory evidence for influenza A(H5N1) virus infection.

• Probable definition 2: a person dying of an unexplained acute respiratory illness who is considered to be epidemiologically linked by time, place, and exposure to a probable or confirmed H5N1 case.

Confirmed H5N1 case (notify WHO):

• A person meeting the criteria for a suspected or probable case AND one of the following positive results conducted in a national, regional, or international influenza laboratory whose H5N1 test results are accepted by WHO as confirmatory:

  • Isolation of an influenza A(H5N1) virus

  • Positive influenza A(H5) PCR results from tests using two different PCR targets (e.g., primers specific for influenza A and H5 haemagglutinin)

  • A fourfold or greater rise in neutralisation antibody titre for influenza A(H5N1) virus based on testing of an acute serum specimen (collected 7 days or less after symptom onset) and a convalescent serum specimen. The convalescent neutralising antibody titre must also be 1:80 or higher

  • A microneutralisation antibody titre for influenza A(H5N1) virus of 1:80 or greater in a single serum specimen collected at day 14 or later after symptom onset and a positive result using a different serological assay: for example, a horse red blood cell haemagglutination inhibition titre of 1:160 or greater or an H5-specific western blot positive result.

UK Health Security Agency (UKHSA): case definition for possible cases of avian influenza with potential to cause severe human disease[128]UK Health Security Agency. Avian influenza: guidance for managing human cases. Feb 2024 [internet publication]. https://www.gov.uk/government/publications/avian-influenza-guidance-and-algorithms-for-managing-human-cases

Possible cases must fulfil the clinical AND exposure criteria below.

Clinical criteria:

• Fever >38ºC (>100.4ºF); or

• Acute respiratory symptoms (e.g., cough, hoarseness, nasal discharge or congestion, shortness of breath, sore throat, wheezing, or sneezing); or

• Other severe or life-threatening illness that is suggestive of an infectious process.

Exposure criteria:

• Close contact (within 1 metre [3 feet]) with live, dying, or dead domestic poultry or wild birds (including live bird markets), in an area of the world affected by avian influenza, or with any confirmed infected animal, in the 10 days before the onset of symptoms; or

• Close contact with a confirmed human case of avian influenza, or human case(s) of severe unexplained respiratory illness from avian influenza-affected areas, or human case(s) of unexplained illness resulting in death from avian influenza-infected areas, in the 10 days before the onset of symptoms.

  • Close contact includes handling laboratory specimens from cases without appropriate precautions, within 1 metre distance, directly providing care, touching a case, or within close vicinity of an aerosol-generating procedure, from 1 day prior to symptom onset and for duration of symptoms or positive virological detection.