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The main strains of swine influenza virus affecting pigs in France had remained unchanged for 30 years when, in 2020, a new strain named H1avN2#E (referred to as H1N2 clade 1C.2.4 in the international nomenclature) spread across farms in just a few months. Likely originating in Denmark, this strain initially emerged in Brittany before rapidly spreading throughout France, replacing the historical strains that had remained unchanged for three decades.
Anses scientists, notably at the National Reference Laboratory for Swine Influenza in Ploufragan-Plouzané-Niort, conducted an in-depth study on samples collected between January 2019 and December 2022, mainly through the Résavip surveillance network. They have established that this lineage is genetically and antigenically distinct from its predecessors, enabling it to evade existing immunity in pigs, whether from past infections or vaccination.
The emergence of H1avN2#E coincided with a sharp rise in swine influenza outbreaks, with 661 cases reported in 2020, compared with an annual average of 400 previously. This strain has also triggered more severe clinical forms, including severe respiratory disorders, hyperthermia and abortions.
In addition to its effects on pig health, this strain poses a significant zoonotic risk. It has been implicated in turkey infections and was responsible for a severe human case in 2021. The possibility of viral reassortment between swine, human, and avian strains makes it imperative to consolidate biosecurity measures, notably by reinforcing quarantines on the introduction of new animals, and maintaining virological surveillance even in the absence of regulatory obligation.
May 7, 2025/ ANSES/ France.
https://www.anses.fr
