National Case Definition: Avian Influenza A(H7N9) Virus

Preamble

The main goal of emerging respiratory virus surveillance is early detection of any case in Canada. Subsequently, such virus surveillance informs efforts at containment and/or mitigation of this novel respiratory pathogen.  This document outlines surveillance case definitions for Avian Influenza A(H7N9) Virus and provides instructions on reporting to the national level. More detailed information on surveillance guidelines, including recommendations for surveillance objectives, activities, laboratory testing, and reporting of results, are described in the National Surveillance Guidelines for Human Infection with Avian Influenza A(H7N9).

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, and public health response goals. 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 screening tests specific for Avian Influenza A(H7N9) can be performed in select laboratories (i.e. provincial public health and hospital-based laboratories); however, confirmation of diagnosis should be sought from Canada's National Microbiology Laboratory (NML) before being considered conclusive. Such cases are considered probable pending NML confirmation. 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 H7N9 nationally within 24 hours of their own notification. National surveillance case definitions are provided below - these are subject to change with ongoing monitoring and as understanding of H7N9 characteristics and risk assessments evolve.

National Surveillance Case Definitions for Avian Influenza A(H7N9)

Person under investigation (PUI):

  • A person meeting exposureFootnote 1 and illnessFootnote 2 criteria

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

    Note: Limited data suggest that H7N9 can present as a co-infection with other viral as well as bacterial pathogens. The identification of one causal agent should not exclude H7N9 where the index of suspicion may be high.

Probable Case:

  • A person epidemiologically-linked through close contactFootnote c to a laboratory-confirmed case and meeting illness criteria2 but in whom laboratory diagnosis of H7N9 is not available or is indeterminate/unreliable (such as due to specimen quality, viral load or timing).

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


OR

  • A person meeting exposure1 and illness2 criteria and in whom a laboratory screening test for H7N9 was positive but not confirmed by the NML

Confirmed Case:

  • A person with laboratory confirmation of influenza A(H7N9) infection at Canada's National Microbiology Laboratory (NML).

    Note: The NML can confirm detection of the virus using H7N9 specific reverse transcription polymerase chain reaction (RT-PCR) and further genetic analysis.

Exposure and Illness Criteria

  1. Exposure criteria: Links within ten (10) daysFootnote a prior to illness onset2 to affected areasFootnote b (i.e. residence, travel history) or close contactFootnote c with a confirmed case or a probable case.
    1. Incubation period for H7N9 has been reported as 5 to 6 days (median), with a range from 1 to 15 days. This is considered prolonged compared to typical human influenza viruses (average 1 to 3 days). The available evidence supports exposure criteria based on 10 days prior to illness onset2 for the purpose of case identification and public health follow up of contacts within Canada. This is considered a reasonable approximation with some loss of surveillance sensitivity balanced against the consideration of local public health capacity to conduct public health investigation and follow-up of cases and contacts.
    2. Affected areas are defined as locations where animal or human infections due to H7N9 have recently been detected. As affected areas are subject to change, consult the website of the World Health Organization for up to date information.
    3. Close contact is defined as a person who provided care for the patient, including health care workers, family members or other caregivers, or who had other close physical contact OR who stayed at the same place (e.g. lived with or otherwise had close prolonged contact within two metres) as a probable or confirmed case while the case was ill (beginning 1 day prior to illness onset and continuing until resolution of illness).

      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 time and distance considered in defining the sharing of a confined air space may be extended. For more information refer to the National Interim Infection Prevention and Control Guidance for Acute Care Settings - Avian Influenza A(H7N9).

      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 intensity. 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 H7N9 virus infections have occurred, mainly in children but also adults, and have been reported in the literature; however studies have not conclusively established transmission from asymptomatic individuals. The full duration of the infectious period for influenza A(H7N9)  is unknown and may vary with factors such as age, immuno-suppression or other comorbidity or with the intensity/closeness of contact. In that context, it is reasonable to consider a typical exposure period for contacts spanning one day prior and through the symptomatic period of the case while recognizing the need for judgment and adjustment to these guidelines under some scenarios or based on additional local/practical considerations. 
  2. Illness criteria: Illness onset is defined by the earliest start of respiratory symptoms associated with the current episode. Focus is on the detection of severe acute respiratory illness (SARI) defined primarily by respiratory symptoms, i.e. fever (over 38 degrees Celsius) AND new onset of (or exacerbation of chronic) cough or breathing difficulty as well as clinical, radiological or histo-pathological evidence of pulmonary parenchymal disease (e.g. pneumonia, pneumonitis, or Acute Respiratory Distress Syndrome [ARDS]), typically associated with the need for hospitalization, intensive care unit monitoring and/or other severity marker (such as death).

Many infectious diseases present with a spectrum of illness, including mild or asymptomatic infection. Atypical H7N9 presentation with absent respiratory symptoms has been documented in the presence of comorbidity, notably immuno-suppression. Therefore, clinician and public health judgment should be used in assessing patients with milder or atypical presentations, where, based on contact, comorbidity or cluster history, the index of suspicion may be raised. Additional information can be found in the Interim Guidance For Containment When Imported Cases With Limited Human-To-Human Transmission Are Suspected/Confirmed In Canada.

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

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