Highly Pathogenic Clade 2.3.4.4b H5N1 Influenza Virus in Seabirds in France, 2022–2023

In 2022, a very high number of wild bird deaths associated with the detection of highly pathogenic (HP) H5 avian influenza virus (AIV) lineage Gs/GD/96, clade 2.3.4.4b viruses were unusually observed in Europe between May and September, whereas prior to 2022 most of these HP H5 AIVs detected in wild birds in Europe were almost all detected between October and March and few between April and September. In France, wild birds affected by this virus during this unusual period were mainly seabirds, including larids and sulids. Although the abnormal mortalities in larids and sulids were reported simultaneously, sequencing of the complete genomes of the viruses identified in these seabirds showed that sulids are mainly infected with genotype EA-2020-C, whereas larids are mainly infected with genotype EA-2022-BB. The identification of these two genotypes, therefore, confirmed that there was no direct link between the abnormal mortality observed in sulids and the abnormal mortality observed in larids. These two seabird mortality events can also be distinguished by the evolutionary pattern of the number of detections. Indeed, sulid mortality associated with the EA-2020-C genotype was observed in France only between July and September, corresponding to a single epidemic wave, whereas larid mortality associated with the EA-2022-BB genotype began in France and Europe in May 2022 and then this genotype continued to spread among larids in France in the form of several successive epidemic waves until at least September 2023.