National case definitions: Human infections with avian influenza A(H5N1) virus


The epizootic of A(H5N1) that began in late 2021 on the east coast of Canada and quickly spread across North America has led to increased attention to the potential spillover of the virus into humansFootnote 1.

The main public health response goal of emerging respiratory virus surveillance is early detection of any human case in Canada. Subsequently, such virus surveillance informs efforts at containment and/or mitigation of this novel respiratory pathogen. A secondary goal, in the context of the emergence or re-emergence of A(H5N1) outbreaks such as in the 2021-2023 North American epizootic, is to inform risk assessment by growing the evidence base on human infection risk and spectrum of illness, including asymptomatic or atypical presentations. This document outlines surveillance case definitions for Avian Influenza A(H5N1) Virus and provides instructions on reporting to the national level.

Surveillance case definitions are provided here for the purpose of case classification and reporting to the Public Health Agency of Canada. They are based on the current level of epidemiological evidence and uncertainty, as well as the above-noted public health response goals. Note that case definitions are subject to change as new information becomes available. These surveillance case definitions are not intended to replace clinician or public health practitioner judgment in individual patient management, or intended to be used for the purpose of infection control triage.

It should be noted that unusual severe acute respiratory illness (SARI) clusters in community or facility settings (and notably involving health care workers) should be appropriately investigated under the direction of local and provincial health authorities.

Initial diagnostic tests specific for Avian Influenza A(H5N1) can be performed in select laboratories (i.e. provincial public health and hospital-based laboratories). Such cases are considered probable pending confirmation. Jurisdictions without this capability should seek confirmation testing from Canada's National Microbiology Laboratory (NML) before being considered conclusive whereas jurisdictions that have alternative tests (e.g., sequencing or validated polymerase chain reaction [PCR]) may rely on NML or perform their own confirmatory testing. The latter jurisdictions may rely on NML to confirm the first A(H5N1) human cases and then later conduct confirmatory testing on their own. All positive samples must be shared with NML to fulfill their obligations as a National Influenza Centre and Canada's obligations under the International Health Regulations and other agreements. For more information on appropriate specimens or targets for laboratory testing, refer to the Protocol for Microbiological Investigations of Severe Acute Respiratory Infections (SARI).

Provincial/Territorial public health authorities should report confirmed and probable cases of H5N1 nationally within 24 hours of their own notification irrespective of illness severity. For more information on notification mechanisms and case report forms, refer to Emerging respiratory pathogens and Severe Acute Respiratory Infection (SARI) case report form. Persons under investigation (PUI) are not required to be reported nationally.

National surveillance case definitions are provided below - these are subject to change with ongoing monitoring and as understanding of A(H5N1) characteristics and risk assessments evolve.

National Surveillance Case Definitions for Avian Influenza A(H5N1)

Person under investigation (PUI):

Note: The surveillance mechanisms and systems for identifying a PUI may vary by jurisdiction according to perceived risk, resources, supporting structures, approach to asymptomatic individuals, and other context.

Note: Limited data suggest that A(H5N1) can present as a co-infection with other viral as well as bacterial pathogensFootnote 2. The identification of one causal agent should not exclude A(H5N1) where the index of suspicion may be high. In the context of high community circulation of other respiratory pathogens, an individual positive for another viral pathogen (e.g., SARS-CoV-2, seasonal influenza) in the absence of unusual disease does not comprise a situation where suspicion of A(H5N1) infection is high.

Probable Case:

Note: A positive non-seasonal influenza A test is appropriate when there is no alternative etiologic hypothesis. For example, an individual who meets the exposure and/or illness criteria and is positive for influenza A and is negative for A(H1) and A(H3) should be included in this definition of a probable case. However, an individual who tests positive for influenza A and an H3 infection is not a probable case.

Note: Efforts to obtain additional specimens to clarify case status may be warranted.

Confirmed Case:

Note: The NML can confirm detection of the virus using H5N1 specific reverse transcription polymerase chain reaction (RT-PCR) and/or further genetic analysis.

Exposure and Illness Criteria

Note: Where procedures or presentations are more likely to be associated with virus-laden aerosolization (e.g., CPR, intubation, ventilation, suction, sputum induction, nebulization, bronchoscopy, BiPAP) the distance considered in defining the sharing of a confined air space may be extended.

Note: Current evidence related to seasonal influenza indicates that viral loads in the 24 hours prior to symptom onset are substantially lower than once symptoms begin, peaking with symptom intensityFootnote 8. Effective transmission cannot be directly inferred from viral shedding, but transmission is also anticipated to be greater during the peak symptomatic period, particularly in association with projectile or aerosolizing symptoms such as cough or sneeze. Extension of the relevant exposure period for contacts to include one day prior to symptom onset in the case is thus intended to be a cautious approach for the purpose of emerging pathogen response. Asymptomatic or very mild H5N1 virus infections have occurred and have been reported in the literatureFootnote 9. There has been no evidence of sustained person-to-person spread of A(H5N1). In light of the lack of direct human evidence of an infectious period, it is reasonable to consider a typical exposure period for contacts spanning one day prior and through the symptomatic period of the caseFootnote 10 while recognizing the need for judgment and adjustment to these guidelines under some scenarios or based on additional local/practical considerations.

Many infectious diseases present with a spectrum of illness, including mild or asymptomatic infection. Clinician and public health judgment should be used in assessing patients with milder or atypical presentations, where, based on exposure, comorbidity or cluster history, the index of suspicion may be raised.

Clinician discretion, epidemiologic context, and local feasibility should be taken into account in discussion with local/provincial health authorities.


Footnote 1

Wille, M., & Barr, I. G. (2022). Resurgence of avian influenza virus. Science, 376(6592), 459-460.

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Footnote 2

To, K. K., Ng, K. H., Que, T. L., Chan, J. M., Tsang, K. Y., Tsang, A. K.,... & Yuen, K. Y. (2012). Avian influenza A H5N1 virus: a continuous threat to humans. Emerging microbes & infections1(1), 1-12.

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Footnote 3

Uyeki, T. M. (2009). Human infection with highly pathogenic avian influenza A (H5N1) virus: review of clinical issues. Clinical infectious diseases49(2), 279-290.

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Footnote 4

Public Health Agency of Canada. (2020). Flu (influenza): Symptoms and treatment. Retrieved from the Government of Canada website on July 18, 2022:

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Footnote 5

United States Centers for Disease Control and Prevention. (2022). Case Definitions for Investigations of Human Infection with Avian Influenza A Viruses in the United States. Retrieved from the Centers for Disease Control and Prevention website on July 18, 2022:

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Footnote 6

Public Health Agency of Canada. (2013). Severe Acute Respiratory Infection (SARI) Case Definition. Retrieved from the Government of Canada website on July 18, 2022:

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Footnote 7

Kurmi, B., Murugkar, H. V., Nagarajan, S., Tosh, C., Dubey, S. C., & Kumar, M. (2013). Survivability of highly pathogenic avian influenza H5N1 virus in poultry faeces at different temperatures. Indian Journal of Virology24(2), 272-277.

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Footnote 8

Suess, T., Remschmidt, C., Schink, S. B., Schweiger, B., Heider, A., Milde, J.,... & Buchholz, U. (2012). Comparison of shedding characteristics of seasonal influenza virus (sub) types and influenza A (H1N1) pdm09; Germany, 2007–2011. PloS one7(12), e51653.

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Footnote 9

Writing Committee of the World Health Organization (WHO) Consultation on Human Influenza A/H5. (2005). Avian influenza A (H5N1) infection in humans. New England Journal of Medicine353(13), 1374-1385.

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Footnote 10

National Center for Immunization and Respiratory Diseases. (2018). How Flu Spreads. Retrieved from the United States Centers for Disease Control and Prevention (CDC) website on August 16, 2022:

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Footnote 11

Public Health Agency of Canada. (2021). Flu (influenza): FluWatch surveillance. Retrieved from the Government of Canada website on July 18, 2022:

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