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Human Health Issues related to Avian Influenza in Canada

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7 Public Health Risk Assessment

A risk assessment should be conducted from a population perspective in order to inform the public health response and specifically the need for the implementation of any community based measures (e.g., quarantine, cancellation of animal-oriented events). The management of individual contacts of a source of avian influenza virus should be based on the virus-specific risk, an individual exposure assessment and consideration of other factors specific to the situation or individual. This risk-focused strategy is also suggested for decisions regarding antiviral prophylaxis (see section 10).

7.1 Virus-specific risk

Influenza viruses are characterized into subtypes based on the surface glycoproteins. There are 16 hemagglutinin subtypes (H1-16) and nine neuraminidase sub-types (N1-9) for influenza A viruses. Not all potential combinations are known to exist and of the sixteen hemagglutinin subtypes to date only H1, H2, H3, H5, H7, H9, H10 and H11 have been shown to infect humans(5,16).

When an AI virus is detected in Canada, a risk assessment should be immediately carried out by analyzing the existing data (if any) on the occurrence and severity of human disease from the identified hemagglutinin (H) subtype and the specific (H and N) subtype/strain detected. The H type will likely be the first available laboratory result and many decisions may need to be made based on this preliminary finding, together with initial animal laboratory and clinical data. It is important to review these decisions and update the risk assessment as more information becomes available.

To facilitate a consistent approach to the management of these occurrences from a human health perspective the AI virus should first be classified based on the following four designations:

  • No data is available on the human illness risk of the subtype
  • Subtype has previously been identified and is not known to have caused human illness (e.g., H3N8, H6N1, H13N6)
  • Subtype is known to cause predominantly mild human illness (e.g., H7N3, H7N7, H9N2)
  • Subtype is known to cause predominantly severe human illness (e.g., H5N1 Asian strain)

This designation together with the exposure assessment should be used to guide the management of contacts of a source of avian influenza virus. The designation of the virus could change if confirmed (human) cases occur during the course of the outbreak; in which case the risk assessment and recommended management of contacts may also change.

7.2 Exposure risk

The World Health Organization (WHO) has stratified exposure risk into three categories; low, medium and high in the context of the H5N1 Asian strain epizootic(12). In developing the recommendations for this document these categories were reviewed and modified to reflect not only the H5N1 experience in Asia but experience with other AI viruses and the Canadian context. In addition, exposures to wild or non-commercial birds have been incorporated into these categories to facilitate use of this document beyond the commercial poultry outbreak setting. To be consistent with the WHO document the word "animal" is intended to include all avian species in the following context.

As the Canadian situation unfolds, the exposure risk groups might be modified based on experience and illness outcomes. Individuals who have exposures falling into more than one risk group should be managed based on their highest risk exposure.

High exposure risk groups:

  • Individuals with unprotected and very close exposure to a flock or group of sick or dead animals infected with AI or to particular birds that have been directly implicated in human cases (e.g., farm family member or worker who handled sick animals)
  • Personnel involved in handling sick animals or decontaminating affected environments (including animal disposal) as part of outbreak control efforts (e.g., cullers)

Moderate exposure risk groups:

  • Individuals who handle single or small groups of sick or dead animals infected with AI in an open air environment which is not densely populated by animals of the same species as the infected animal (e.g., single wild bird in a park)
  • Household/family contacts of a suspected or confirmed human AI patient ( defined as living under the same roof as the index case for >/= 24 hrs within the period when the case is presumed to be contagious)
  • HCWs (i.e., those working in a setting where health care is being provided) who had no, or insufficient, PPE in place when 1) in close contact (i.e., within 1 meter) of a strongly suspected or confirmed human AI case, or 2) in direct contact with respiratory secretions or other potentially infectious specimens from the case.
  • HCWs or laboratory personnel who might have unprotected contact (i.e., did not have or was wearing insufficient PPE) with specimens/secretions which may contain virus or with laboratory isolates.

Low exposure risk groups:

  • Personnel involved in culling non-infected or likely non-infected animal populations as a control measure (e.g., those exclusively culling asymptomatic animals in a control area outside of the infected and restricted zones)
  • Individuals who handle (i.e., have direct contact with) asymptomatic animals that may be infected with AI based on species and possibly proximity to a geographic area where AI has recently been identified (e.g., bird banders).
  • HCWs who used appropriate PPE during contact with human AI cases (i.e., in the absence of significant human to human transmission)
  • HCWs not in close contact (i.e., distance greater than 1metre) with suspected or confirmed human AI cases and having no direct or indirect contact with infectious material from that case(s)
  • Laboratory personnel working with the influenza virus using appropriate laboratory procedures and infection control precautions.

Initially, it is expected that those most likely to be exposed would include external employees (e.g. CFIA or Environment Canada workers) who are involved in outbreak control, culling of infected flocks or euthanasia of birds, disposal of carcasses, or cleaning of involved sites, as well as persons living and working on affected farms who have such contact.

If human illness is observed, the exposure history of these individuals should be documented and used to evaluate implemented infection control precautions. Close contacts of these cases should be managed as described in section 8.2. If human-to-human transmission is suspected, then a complete contact investigation should be conducted. This investigation will help inform the risk assessment of other settings where human cases are residing.

7.3 Other considerations

In order to target recommendations for contacts of an avian source or human source of avian influenza virus, it is important to consider other factors in addition to the epidemiology of the outbreak. Other factors that would likely influence recommendations for contacts include:

  • Degree of certainty that the bird population/domestic flock has been infected with the avian virus
  • Observation of human illness linked to the current outbreak and severity of illness
  • Timing of implementation of control measures
  • Individual risk factors in the exposed person (e.g. immunocompromising conditions)
  • Confidence that public health recommendations (e.g., for personal protective equipment, immunization, antiviral prophylaxis) are being or will be followed[14]
  • Number of cases/contacts (e.g., as numbers increase may advise self monitoring/quarantine rather than public health or health care system involvement)

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