Novel Highly Pathogenic Avian Influenza A Virus, Argentina, 2025
Vanstreels RE, Nelson MI, Artuso MC, Marchione VD, Piccini LE, Benedetti E, Crespo-Bellido A, Pierdomenico A, Wolff T, Uhart MM, Rimondi A. Novel Highly Pathogenic Avian Influenza A (H5N1) Virus, Argentina, 2025. Emerging infectious diseases. 2025;31(12).
Genomic sequencing of reemerging highly pathogenic avian influenza A(H5N1) virus detected in Argentina in February 2025 revealed novel triple-reassortant viruses containing gene segments from Eurasian H5N1 and low pathogenicity viruses from South and North American lineages. Our findings highlight continued evolution and diversification of clade 2.3.4.4b H5N1 in the Americas.
Highly pathogenic avian influenza (HPAI) viruses were introduced to South America in 2022 by migratory birds from North America. The viruses belonged to the 2.3.4.4b clade of HPAI A(H5N1) virus that became widespread in Europe in 2020 and spread to North America in 2021. The trajectory of H5N1 in South America has differed from H5N1 in North America in several critical ways. First, nearly all South America outbreaks stem from a single introduction of H5N1 viruses from North America, whereas the North America epizootic was reseeded by multiple independent introductions from Europe and Asia (A1–A6). Second, South America H5N1 outbreaks were driven by a single genotype (B3.2) that was introduced from North America and remained genetically stable during its spread across South America. In contrast, H5N1 viruses in North America underwent frequent reassortment with low pathogenicity avian influenza (LPAI) viruses, prompting new genotype nomenclature (using B, C, D). Third, South America’s H5N1 epizootic is unique in establishing mammal-to-mammal transmission in marine mammals, enabled by the H5N1 (B3.2) virus acquiring mammalian-adaptive polymerase basic (PB) 2 mutations (Q591K and D701N). That pattern has not occurred in North America, where H5N1 spillover into terrestrial and marine mammals was transient, except in United States dairy cattle.
Beyond the ecologic devastation among coastal wildlife, in 2023, H5N1 (B3.2) virus spread widely in birds across mainland South America, leading to poultry and wild bird outbreaks. Although in 2024 HPAI outbreaks occurred in Brazil and Peru (World Organisation for Animal Health), there were no detections in Argentina during March 2024–January 2025.