The influenza A(H5N1) virus currently circulating in the Americas belongs to an HPAI genotype resulting from recombination in wild birds in Europe and low pathogenic strains in wild and domestic birds during its global dissemination. In 2021, the virus spread through waterfowl migratory routes to North America and, in 2022, to Central and South America. By 2023, epidemic outbreaks in animals were reported by 14 countries and territories, mainly in the
Americas.
Since 2022, 19 countries on three continents have reported outbreaks in mammals to the World Organisation for Animal Health (WHO). In recent years, the detection of A(H5N1) viruses in non-avian species has increased worldwide, including terrestrial and marine mammals, both wild and domestic (companion and production).
Although reports of transmission of clade 2.3.4.4b viruses among mammals worldwide are rare, the current situation of contagion in dairy cattle in the United States of America, as well as reported mass deaths in marine mammals and infections in mink and fox farms in Europe, point to transmission among mammals, with further investigations needed for confirmation. Historically, from early 2003 to 12 December 2024, 954 human cases of avian influenza A(H5N1), including 464 deaths (case fatality rate 48.6%), were reported to the World Health Organization (WHO) in 24 countries globally (8). Between early 2021 and 12 December 2024, 92 detections of influenza A(H5N1) virus in people were reported to WHO, of which 64% (n=59) occurred in the United States.
From 2022 through epidemiological week (EW) 52 of 2024, a total of 19 countries and territories in the Region of the Americas reported 4,388 outbreaks of H5N1 avian influenza in animals to WHO, representing an additional 740 outbreaks since the last epidemiological update published by the Pan American Health Organization/World Health Organization (PAHO/WHO) on November 15, 2024.
A total of 71 human infections caused by avian influenza A(H5) have been reported in four countries in the Americas from 2022 to January 17, 2025, that is, 22 additional cases compared to the last epidemiological update published by PAHO/WHO. 68 cases were reported in the United States, one case in Canada confirmed on November 13, 2024, one case in Chile reported on March 29, 2023, and one case in Ecuador reported on January 9, 2023. During 2024 alone and through January 17, 2025, 68 human cases have been reported in Canada and the United States, of which 54 were reported between October 2024 and January 2025.
Situation by country and/or territory in outbreaks in birds
From 2022 to EW 52 of 2024, a total of 19 countries and territories in the Region of the Americas reported 3,137 outbreaks of avian influenza in domestic and/or wild birds to WHO, that is, 150 additional outbreaks compared to the last epidemiological update published by PAHO/WHO. Of these outbreaks, 2,389 occurred in domestic birds and 748 in wild birds. Between EW 1 and EW 52 of 2024, eight countries and territories in the Americas (Brazil, Canada, Colombia, Ecuador, the Falkland Islands, Mexico, Peru, and the United States) have reported outbreaks in both domestic and wild birds.
Situation by country and/or territory in outbreaks in mammals
From 2022 to EW 52 of 2024, eight countries and territories have reported 1,284 outbreaks of avian influenza A(H5N1) in mammals in Argentina, Brazil, Canada, Chile, the United States, the Falkland Islands, Peru and Uruguay. This represents 590 additional outbreaks since the last epidemiological update published by PAHO/WHO. Four countries and territories in the Americas (Argentina, Canada, the United States and the Falkland Islands) have reported 982 outbreaks of avian influenza in mammals during 2024.
Situation by country and/or territory with human cases
A summary of the situation in Canada and the United States regarding human infections with avian influenza A(H5N1) is presented below. During 2024 and through January 17, 2025, human cases of avian influenza A(H5N1) have been reported in Canada and the United States (10, 11). Since the last epidemiological update published by PAHO WHO, 21 new cases of influenza A (H5N1) have been confirmed, all in the United States.
On November 14, 2024, Canada confirmed its first human case of influenza A(H5N1) in a teenager in British Columbia, initially reported on November 9. Genomic sequencing linked the virus to the outbreak in poultry in the region (clade 2.3.4.4b, genotype D1.1) and detected the E627K mutation in the PB2 gene, associated with increased replication in mammals. The source of infection is still unknown, and no additional cases have been reported.
In the United States, during 2024 and through January 17, 2025, 67 human cases of influenza A(H5N1) have been confirmed in California (n=38), Colorado (n=10), Iowa (n=1), Louisiana (n=1), Michigan (n=2), Missouri (n=1), Oregon (n=1), Texas (n=1), Washington (n=11), and Wisconsin (n=1). Of these cases, 40 have been linked to exposure to sick or infected dairy cattle, while 23 have been linked to exposure to poultry. The source of exposure for two cases in California and one case in Missouri could not be determined, and investigations have been completed and human-to-human transmission has been ruled out. As of January 17, 2025, human-to-human transmission of avian influenza A(H5N1) virus has not been reported.
From March 24, 2024, to January 17, 2025, dedicated surveillance efforts for avian influenza A(H5) have monitored more than 13,400 people after exposure to infected animals and more than 600 of them have been tested.