Molecular Changes Between H5N1 Viruses Between Europe, North America and South America and their Possible Relationship with their Pathogenicity and Infection in Mammals

Avian influenza viruses (AIVs) are classified as either low-pathogenicity (LP) or high-pathogenicity (HP). LPAIVs generally cause mild infections, whilst HPAIVs can cause high mortality in a wide range of avian species. AIV subtypes are defined based on their surface glycoproteins, haemagglutinin (HA; H1-H16) and neuraminidase (NA; N1-N9), and HPAIVs appear restricted to the H5 and H7 subtypes. HPAI viruses are of major concert for their pandemic potential and the impact of the socioeconomic agricultural outbreaks. Specifically, the goose/Guangdong H5 HPAI viruses, which emerged in 1996, are the only HPAI viruses known to be sustained in wild waterfowl populations. Driven by mutations and reassortment events those viruses evolved rapidly and spread globally. In 2005, hey were identified as Z-genotype and then defined by clade 2.2. Next, goose/Guangdong H5 HPAI viruses established two distinct lineages of virus, the 2.3.2 and 2.3.4 clade that co-circulated for some time in Asia, which further divergent into distinct groups via genetic drift and disseminated globally. These viruses made their way to the United States and Canada in 2014. Recently, to update and unify the nomenclature of clade 2.3.4.4 eight genetic groups (a–h) have been recognised (WHO, 2020).

Since 2020, H5N1 HPAI has been responsible for over 90 million poultry deaths across Africa, Asia, Europe, and North America. In November 2022, HPAI(H5) arrived in Peru, next spread towards Panama, Honduras, Costa Rica, and Guatemala. HPAI H5N1 was also reported in captive wild birds in zoo in Havana, Cuba (7 February 2023). The first occurrence of H5 in Argentina and Uruguay was also reported. Most of these viruses are reassortant between Eurasian and North American strains (WOAH, March 2023). Along with the outbreaks in avian species, the HPAIV H5N1 virus spillover to mammals continue to be reported. A mass mortality event of more than 3,000 sea lions was observed in early 2023 in Peru (Gamara-Toledo 2023). These viruses belong to the HPAI A/H5N1 lineage 2.3.4.4b and are 4:4 reassortant, where PA, HA, NA and MP belong to Eurasian lineage that initially entered North America from Eurasia and remaining PB2, PB1, NP and NS came from American lineage that was already circulating in North America (Leguia et al 2023, Biorxiv). As reported by Leguia et al (Biorxiv 2023) these viruses do not enquire mutations linked to mammalian host adaptation and enhanced transmission (such as PB2 E627K or D701N), but at least 8 novel polymorphic sites were found in their genome. Occurrence of the H5 virus in unusual host was also described in sea lion in Chile. Three human infections caused by avian influenza A(H5) have been reported: the first in the United States of America, reported in April 2022; the second in Ecuador, reported in January 2023; and third in Chile in April 2023. The epidemiological situation will be discussed in more detail during the presentation.

In general, virus evolution and host adaptation mechanisms are caused by the antigenic drift and antigenic shift of the virus. There are many host factors that limit AIV replication in humans, including host cell surface receptor, sialic acid (SA). Generally, AIV prefers to bind α2,3 SA, while human influenza viruses prefer to bind α2,6 SA. To achieve efficient and stable transmission between humans, AIV would need to switch binding preference from α2,3 SA to α2,6 SA via amino acid substitution at the RBS of HA1. According to published studies, H5N1 could potentially establish persistent human-to-human and full airborne transmission via Q226L, N224 or G228S mutations in the HA gene. However, current natural H5N1 and its variants have been reported to preserve dominant binding to α2,3 SA, showing that only partial adaptation to α2,6 SA has occurred. Another interesting and known mutation is E627K in the PB2 that interacts with a host protein, acidic nuclear phosphoprotein 32 family member A (ANP32A). This mutation support replication efficiency in mammalian cells. More examples of adaptational mutations and analysis of the current influenza strains obtained from mammals will be discussed during the talk.