Rapid risk assessment update: Avian influenza A(H5N1) clade 2.3.4.4b, public health implications for Canada

Assessment completed: November 29, 2024 (with data as of November 22, 2024)

Date of previous risk assessment: August 27, 2024

Reason for assessment

The Rapid Risk Assessment (RRA) is being updated as the following trigger was met: "Significant increase in human case reports, especially over short interval of time", due to the human cases of avian influenza A(H5N1) clade 2.3.4.4b in the United States (US) and Canada. In addition, the trigger "Cases in the general population with no/limited exposure to infected animals" is being closely monitored.

Risk question(s)

What is the likelihood and impact of human infection with avian influenza A (H5N1) clade 2.3.4.4b due to exposure to animals in Canada in the next 3 months?
If a person is exposed to animals known to be infected (e.g., during occupational activities on affected farms), what is the likelihood and impact of human infection following this exposure?
Since the last RRA, how may sporadic human cases and ongoing transmission in other animals in the US and Canada be influencing evolution of the virus?

Risk statement

The overall risk of avian influenza A(H5N1) clade 2.3.4.4b to the Canadian population remains low (low uncertainty); however, those with higher-level exposure^{Footnote a} to infected animals are at increased risk and should take appropriate precautions. Continued transmission of the virus in animals and sporadic cases in people may increase the chance of changes in the virus that could lead to human-to-human transmission.

The likelihood of infection for the general population in the next 3 months is very low to low, depending on region, and the impact on the general population in Canada is estimated to be minor since there is no evidence of transmission among humans or that circulating virus has acquired the capacity for sustained transmission among humans. The likelihood of infection for those with higher-level exposure^{Footnote a} to animals depends greatly on the likelihood of infection in the animals to which they are exposed. A random animal across Canada has a very low to low likelihood of being infected; however, the proportion of animals affected will vary by region and species. Those working with certain species, such as poultry or wild ducks, in certain regions, such as those experiencing outbreaks, will have a higher likelihood of infection. The impact on immuno-competent adults (those 18 and older) is estimated to be minor due to the mild clinical symptoms being seen in the United States in 2024; however, the possible impact on children (those under 18) and immuno-compromised adults is estimated to be major due to the severity of these historical infections and the recent case in Canada.
If a person is exposed to animals known to be infected, such as occupational exposure on affected farms, the likelihood of infection is estimated to be moderate (moderate uncertainty). This has increased from the June 2023 rapid risk assessment and April 2024 rapid risk assessment update. Poultry and livestock (cattle and swine) have been shown to shed sufficient amounts of virus for transmission and a rapidly increasing number of cases have occurred in cattle and poultry farm workers in the US, particularly in those not following personal protective equipment (PPE) recommendations. It is clear at this point that humans are susceptible to infection when exposed to a sufficient dose. In most cases, people with higher-level exposure^{Footnote a} to infected animals will be immuno-competent adults, for whom the estimated impact is minor. However, the impacts are potentially higher for children and immuno-compromised adults.
In addition to the human infection risk in the short term, it is important to consider evolution of avian influenza viruses. It is unknown how the current situation will affect evolution of the virus. However, since the last risk assessment update, continued transmission of avian influenza A(H5N1) clade 2.3.4.4b in wild and domestic bird populations, repeated spillover into diverse wild and domestic mammalian species, ongoing transmission between US dairy cattle and sporadic cases in humans exposed to poultry or cattle, increase the likelihood of viral reassortment and/or adaptation that could enable human-to-human transmission. Viral mutations previously recognized for potential mammalian adaptation have been observed in some human cases, including the Canadian case, but there remain uncertainties regarding interpretation of these mutations in conjunction with epidemiological findings. Surveillance and preparedness activities in both human and animal sectors remain important.

Risk assessment summary

Table 1. Likelihood and impact estimates of a human infection of avian influenza A(H5N1) clade 2.3.4.4b due to exposure to animals in Canada in the next 3 months
Question	Estimate [uncertainty]	Rationale
What is the likelihood that a random individual animal in Canada is infected with avian influenza A(H5N1) clade 2.3.4.4b during the assessment period?	Livestock: Very low [moderate] Poultry: Low [low to high]^{Footnote a} Wild animal^{Footnote b}: Low [low to high]^{Footnote c} Pet^{Footnote d}: Very low [moderate]	No indication of livestock infection in Canada to date Proportion of poultry and overall wild animal infection is low and has not increased compared to the June 2023 rapid risk assessment, though may have regional, species, and seasonal differences Likelihood that a random pet is infected has not increased from the June 2023 rapid risk assessment
What is the likelihood that an exposure involves a sufficient amount of virus to potentially cause an infection (to the average person)?	Higher level exposure^{Footnote e}: High [moderate] Lower level exposure^{Footnote f}: Very low to low [moderate]	Infected animals shed sufficient virus for transmission to humans with high intensity contact (see the June 2023 risk rapid assessment) No known cases due to low level exposures Uncertainty regarding type of exposure for some cases
What is the likelihood that a person who had exposure to sufficient amount of virus will develop an infection?	Moderate [moderate]	Assessed as low in the April 2024 rapid risk assessment US cases and serological studies demonstrate susceptibility of humans to infection
Likelihood Component for Risk Question 1: Overall likelihood of human infection with avian influenza A (H5N1) clade 2.3.4.4b due to exposure to animals in Canada	Higher level exposure^{Footnote e}: Very low to low [moderate] General population / lower level exposure^{Footnote f}: Very low to low [moderate]	Combination of animal infection, and human exposure and infection For those with higher exposure, the estimate is driven by the very low to low likelihood that a random animal across Canada is infected, but this will vary with regional, species, and seasonal differences
Likelihood Component for Risk Question 2: Overall likelihood of infection if a person has higher-level exposure^{Footnote e} to animals known to be infected	Moderate [moderate]	Combination of higher-level exposure and likelihood of infection This estimate does not include likelihood of a random animal being infected
In the event that human infection occurs, what would be the most likely spread scenario?	Zoonotic infection with no further transmission	No evidence of human-to-human transmission to date
What would be the impact on an individual infected?	Immuno-competent adult (≥18 years): Minor [moderate] Children (< 18 years) and immuno-compromised adults: Major [high]	The June 2023 rapid risk assessment estimated the impact as major for all groups Mild disease seen with occupational exposures in the US Severe disease seen globally in young people, including Canadian case Uncertainty regarding significance of viral mutations in Canadian case
What would be the impact on the Canadian population during the assessment period?	General population: Minor [moderate]	Anticipate only sporadic zoonotic cases Most cases likely to be in immuno-competent adults with mild disease
Footnotes: Footnote a Uncertainty ranges from low uncertainty for commercial poultry due to existing surveillance to high uncertainty for backyard poultry flocks due to limited surveillance in, and limited information on, the number, size, and geographic distribution of backyard flocks in Canada Return to footnote a referrer Footnote b Wild animals: limited to wild birds and wild mammals Return to footnote b referrer Footnote c Uncertainty ranges from low for wild birds due to existing surveillance to high for wild mammals due to limited surveillance and suspected variation in susceptibility between species Return to footnote c referrer Footnote d Pets: limited to dogs, cats, and ferrets (unmonitored, free-ranging animals such as feral cats and stray dogs are considered wild mammals for the purposes of this risk assessment) Return to footnote d referrer Footnote e Higher-level exposure: High intensity (within 2 metres and/or prolonged) contact with animals (i.e., wild birds, poultry, or mammals), or infected materials from these animals (e.g., feces, blood, secretions, or tissues), or an environment highly contaminated by infected animals. Return to footnote e referrer Footnote f Lower-level exposure: Low intensity (greater than 2 meters and/or transient) contact with animals (i.e., wild birds, poultry, or mammals), or infected materials from these animals (e.g., feces, blood, secretions, or tissues), or an environment not highly contaminated by infected animals (e.g., diluted recreational bodies of water, open-air environment). Return to footnote f referrer

Future risk in Canada

Future scenarios, drivers, and indicators have been explored in previous assessments, and these continue to be relevant.

The conclusions of this assessment are based on the amount of virus expected to be circulating in animals in Canada within the next three months, including that there is no indication of the virus being present in Canadian livestock to date. While the virus may enter Canadian cattle herds within this timeframe, it is likely to take longer to spread significantly. However, based on what has been seen in the US, it is likely that over the longer term, the virus enters and spreads within Canadian cattle herds, at least within a province (see the Canadian Food Inspection Agency Rapid Qualitative Risk Assessment). Cases are also likely to continue to be seen in wild birds, wild mammals, and poultry. This wider transmission in animal populations could then lead to more human exposures and infections, as could the infection of a new animal host species with more human contact or new exposure routes.

Moving forward, the likelihood of infection if a person has higher-level exposure may be lowered by increased and proper use of PPE, decreased shedding of the virus by animals, or decreased human or animal susceptibility (e.g., due to vaccination or natural immunity in humans or animals). The impact on affected individuals could increase if more vulnerable individuals participate in higher-level exposure activities.

Considerations regarding evolution of the virus and pandemic potential extend over a longer timescale and have higher uncertainty. As previously assessed, continued circulation of the virus in all animals, and especially mammals, increases the likelihood of changes to the virus. Further detections of the virus in pigs, that are known to be a mixing vessel for influenza A viruses or potentially other mammals, for which there is high uncertainty, could increase the likelihood of viral reassortment and/or adaptation that could enable human-to-human transmission. Circulation of human influenza viruses during the annual influenza season may increase the likelihood of simultaneous infection (of a human or animal) with both human and avian viruses, and the potential for reassortment.

This virus has also been assessed using the US CDC Influenza Risk Assessment Tool (IRAT) as of June 26, 2024, which assesses the potential pandemic risk posed by influenza A viruses.¹ This tool places avian influenza A(H5N1) clade 2.3.4.4b in the category of "moderate risk" for potential future emergence and public health impact, which is similar to previous assessments of earlier avian influenza A(H5N1) viruses.

Proposed actions for public health authorities and One Health partners

One Health coordination and governance on this issue remains vital. The proposed actions from the June 2023 rapid risk assessment and April 2024 rapid risk assessment continue to be important. The Public Health Agency of Canada will continue to engage One Health partners (federal, provincial, territorial, Indigenous and other non-government organizations) domestically and collaborate with international partners to assess public health risks associated with current and future avian influenza A strains.

The recommendations proposed below are based on the knowledge gaps identified during this update.

Surveillance, reporting, and risk assessment

Consider strengthening and integrating surveillance activities for influenza A(H5N1) across the One Health spectrum to:
- Detect asymptomatic or mild and severe human illness, especially in affected regions. See protocol for microbiological investigations for severe acute respiratory infections (SARI).
- Consider using the Public Health Agency of Canada's protocol for enhanced surveillance of avian influenza A(H5N1) on farms in Canada, when appropriate.
- Monitor genetic changes, species adaptation, and antiviral resistance in virus identified from any infected mammalian species, including humans.
- Conduct surveillance in pigs, particularly in mixed animal premises and/or premises experiencing outbreaks, for detection of viral reassortment.
Conduct public health risk assessments specific to region, activity, or species encountered, as needed.

Public health management

Refer to the guidance on public health management of human cases of avian influenza and associated human contacts in Canada.
Collaborate between One Health partners to thoroughly investigate all human cases of influenza A(H5N1) to identify the potential source of infection.
Work with federal, provincial, territorial and indigenous partners to enhance one health pandemic preparedness.
Continue to build awareness among health professionals on early detection and management of influenza A(H5N1) infection in people with acute respiratory illness or conjunctivitis in regions affected by avian influenza.

Risk communication

Continue regular communication on the current avian influenza A(H5N1) clade 2.3.4.4b virus outbreak with the public and those working with birds and animals. Share Wildlife and avian influenza: Handling guidelines to protect your health.

Research

Support data collection, science and research activities to address knowledge gaps and contribute to emerging evidence for public health decision making.

Technical annex: For further risk assessment details, including technical annex, please contact rap-per@phac-aspc.gc.ca.

Acknowledgements

Completed by the Public Health Agency of Canada in collaboration with partners from:

Canadian Food Inspection Agency
Environment and Climate Change Canada
Health Canada
Indigenous Services Canada
Alberta Health/Alberta Agriculture and Irrigation
British Columbia Centre for Disease Control
British Columbia Ministry of Agriculture
Cadham Provincial Laboratory
Government of Nova Scotia: Department of Health and Wellness
Government of Prince Edward Island
Institut national de santé publique du Québec
Manitoba Agriculture
Ministère de l'Agriculture, des Pêcheries et de l'Alimentation du Québec
Ministère de la santé et des services sociaux du Québec
Nova Scotia Health Authority
Ontario Ministry of Agriculture, Food and Rural Affairs
Ontario Ministry of Health
Public Health New Brunswick
Public Health Ontario

Footnotes

Footnote a

High intensity (within 2 metres and/or prolonged) contact with animals (i.e., wild birds, poultry, or mammals), or infected materials from these animals (e.g., feces, blood, secretions, or tissues), or an environment highly contaminated by infected animals. The June 2023 rapid risk assessment (Appendix B) and April 2024 rapid risk assessment update (Definitions) provide examples of occupational and recreational groups with potentially relevant exposures, such as farm workers, veterinarians, wildlife officers, and Indigenous harvesters, among others.

Return to footnote a referrer

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Date modified:: 2024-12-06

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