#Influenza at the #human – #animal #interface #summary and #assessment, 1 November 2024 – 28 September to 1 November 2024
Source: World Health Organization, https://www.who.int/publications/m/item/influenza-at-the-human-animal-interface-summary-and-assessment–1-november-2024
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Influenza at the human-animal interface
Summary and risk assessment, from 28 September to 1 November 2024{1}
• New human cases{2}: From 28 September to 1 November 2024, the detection of influenza A(H5) virus in 30 humans and influenza A(H9N2) virus in one human were reported officially.
• Circulation of influenza viruses with zoonotic potential in animals: high pathogenicity avian influenza (HPAI) events in poultry and non-poultry continue to be reported to the World Organisation for Animal Health (WOAH).{3} The Food and Agriculture Organization of the United Nations (FAO) also provides a global update on avian influenza viruses with pandemic potential.{4}
• Risk assessment{5}: Based on information available at the time of the risk assessment, the overall public health risk from currently known influenza viruses at the human-animal interface has not changed, and the occurrence of sustained human-to-human transmission of these viruses is currently considered unlikely. Although human infections with viruses of animal origin are infrequent, they are not unexpected at the human-animal interface.
• IHR compliance: All human infections caused by a new influenza subtype are required to be reported under the International Health Regulations (IHR, 2005).{6} This includes any influenza A virus that has demonstrated the capacity to infect a human and its haemagglutinin gene (or protein) is not a mutated form of those, i.e. A(H1) or A(H3), circulating widely in the human population. Information from these notifications is critical to inform risk assessments for influenza at the human-animal interface.
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{1} This summary and assessment covers information confirmed during this period and may include information received outside of this period.
{2} For epidemiological and virological features of human infections with animal influenza viruses not reported in this assessment, see the reports on human cases of influenza at the human-animal interface published in the Weekly Epidemiological Record here.
{3} World Organisation for Animal Health (WOAH). Avian influenza. Global situation. Available at: https://www.woah.org/en/disease/avian-influenza/#ui-id-2.
{4} Food and Agriculture Organization of the United Nations (FAO). Global Avian Influenza Viruses with Zoonotic Potential situation update. Available at: https://www.fao.org/animal-health/situation-updates/global-aiv-withzoonotic-potential.
{5} World Health Organization (2012). Rapid risk assessment of acute public health events. World Health Organization. Available at: https://iris.who.int/handle/10665/70810.
{7} World Health Organization. Case definitions for the 4 diseases requiring notification to WHO in all circumstances under the International Health Regulations (2005). Case definitions for the four diseases requiring notification in all circumstances under the International Health Regulations (2005).
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A(H5), United States of America (USA)
Since the last risk assessment of 27 September 2024, influenza A(H5) virus has been detected in 30 humans in the USA; of these, 20 cases were detected in California and 16 of those have been confirmed through partial or whole genome sequencing as clade 2.3.4.4b A(H5N1) viruses, closely related to those detected in dairy cattle. Whole genome sequences from nine of these indicate the viruses are of the B3.13 genotype which is the same genotype detected in dairy cattle.
The ten cases reported in Washington reportedly had exposure to poultry infected with avian influenza viruses. Genetic sequencing of three of these cases confirms that all are A(H5N1) viruses from clade 2.3.4.4b and that all are closely related genetically to the viruses causing infections in poultry on the farm where depopulation was conducted. CDC has successfully obtained partial gene sequences for viruses from three cases in Washington with other cases pending sequence analysis.{7} All cases were reported as adults > 18 years of age; all had reported mild illness and none were hospitalized, and all had recovered or were recovering at the time or reporting. All cases reported mild symptoms. Cases had onsets of illness between 28 September and 26 October 2024. Intensive epidemiological investigation of the case previously reported in Missouri with unknown exposure could not identify any link to an animal or animal product exposure; serology testing of the case and six contacts was negative for five contacts and showed weak immune signals in the sample from the case and one household contact who reported identical symptoms onset. These findings support a single common exposure, which remains unknown, rather than human-to-human transmission.{8,9} Low pathogenicity and high pathogenicity avian influenza (HPAI) viruses have been detected in birds in the United States. Since 2022, the HPAI A(H5) virus has been detected in commercial and backyard flocks in 48 states, impacting over 100 million birds. To date, 37 people have tested positive for A(H5) virus in the United States since 2022, with all but one of these cases occurring in 2024. All cases have been associated with exposure to either A(H5N1)-infected poultry or dairy cattle, except for one case where the exposure source could not be identified. A(H5N1) virus infections in dairy cattle and wild and domestic birds continue to be reported in the USA.{10} According to reports received by WOAH, various influenza A(H5) subtypes continue to be detected in wild and domestic birds in Asia, Europe and the Americas. Infections in non-human mammals are also reported, including in marine and land mammals.11 A list of bird and mammalian species affected by HPAI A(H5) viruses is maintained by FAO.{12}
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{7} CDC A(H5N1) Bird Flu Response Update November 4, 2024. Available at: https://www.cdc.gov/birdflu/spotlights/h5n1-response-11012024.html.
{8} World Health Organization. Influenza at the human-animal interface, Summary and risk assessment, from 20 July to 27 September 2024. Available at: https://cdn.who.int/media/docs/default-source/influenza/humananimal-interface-risk-assessments/influenza-at-the-human-animal-interface-summary-and-assessment–from20-july-to-27-september-2024.pdf?sfvrsn=355e503c_1&download=true.
{9} United States Centers for Disease Control and Prevention. CDC Report on Missouri H5N1 Serology Testing. Available at: https://www.cdc.gov/bird-flu/spotlights/missouri-h5n1-serology-testing.html.
{10} United States Department of Agriculture. Highly Pathogenic Avian Influenza (HPAI) Detections in Livestock, 19 July 2024. Available at: https://www.aphis.usda.gov/livestock-poultry-disease/avian/avian-influenza/hpaidetections/livestock.
{11} World Organisation for Animal Health (WOAH). Avian influenza. Global situation. Available at: https://www.woah.org/en/disease/avian-influenza/#ui-id-2.
{12} Food and Agriculture Organization of the United Nations. Global Avian Influenza Viruses with Zoonotic Potential situation update. Available at: https://www.fao.org/animal-health/situation-updates/global-aiv-withzoonotic-potential/bird-species-affected-by-h5nx-hpai/en.
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Risk Assessment for avian influenza A(H5) viruses:
{1.} What is the current global public health risk of additional human cases of infection with avian influenza A(H5) viruses? Most human cases so far have been infections in people exposed to A(H5) viruses, for example, through contact with infected poultry or contaminated environments, including live poultry markets, and occasionally infected mammals and contaminated environments. While the viruses continue to be detected in animals and related environments humans are exposed to, further human cases associated with such exposures are expected but unusual. The impact for public health if additional cases are detected is minimal. The current overall global public health risk of additional human cases is low.
{2}. What is the likelihood of sustained human-to-human transmission of avian influenza A(H5) viruses? No sustained human-to-human transmission has been identified associated with the recent reported human infections with avian influenza A(H5). There has been no reported human-to-human transmission of A(H5N1) viruses since 2007, although there may be gaps in investigations. In 2007 and the years prior, small clusters of A(H5) virus infections in humans were reported, including some involving health care workers, where limited human-to-human transmission could not be excluded; however, sustained human-to-human transmission was not reported. Available evidence suggests that influenza A(H5) viruses circulating have not acquired the ability to efficiently transmit between people, therefore sustained human-to-human transmission is thus currently considered unlikely at this time.
{3}. What is the likelihood of international spread of avian influenza A(H5) viruses by travellers? Should infected individuals from affected areas travel internationally, their infection may be detected in another country during travel or after arrival. If this were to occur, further communitylevel spread is considered unlikely as current evidence suggests these viruses have not acquired the ability to transmit easily among humans.
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