A new scientific study published in Emerging Infectious Diseases has mapped the emergence and spread of a novel highly pathogenic avian influenza A(H5N1) virus genotype—EA-2023-DG—across Europe. Conducted by an international team of researchers led by the Avian Virology and Immunology Unit at Sciensano (Belgium) in collaboration with laboratories across 13 countries, the study reconstructs the reassortment origins of this virus and provides a detailed phylogeographic account of its transmission between 2023 and mid-2024.
Their findings not only underscore the dynamic nature of avian influenza virus evolution but also reinforce the critical importance of genomic surveillance in anticipating and mitigating zoonotic and agricultural threats.
Key Findings: A New Reassortant Emerges in the Baltic Region
The EA-2023-DG genotype was first identified in a wild swan in Finland in November 2023 and subsequently detected in poultry, captive birds, and a wild fox across 11 European countries. Genomic analysis revealed that this virus genotype emerged through a reassortment event in the southwestern Baltic Sea region—likely between July and August 2023.
Six of the virus’s eight gene segments were inherited from the dominant genotype EA-2021-AB. However, two polymerase segments (PB2 and PA) originated from low pathogenicity avian influenza (LPAI) viruses circulating in the region, highlighting ongoing genetic mixing among influenza viruses in wild bird populations.
Geographic and Species Spread: From Coastal Germany to Continental Reach
Following its initial detection, EA-2023-DG was reported in Germany, Sweden, Poland, Belgium, England, France, Norway, Slovakia, Switzerland, Austria, and Finland. Germany reported the highest number of cases, including 13 poultry farms and several wildlife detections. The virus was found in multiple bird families—Phasianidae (chickens, turkeys), Anatidae (ducks, geese), Accipitridae (hawks), and others—as well as in a single wild red fox.
By June 2024, the virus had spread from the Baltic Sea south to Lyon, France, west to England, and east to Slovakia. Despite wide dispersal, core circulation remained concentrated in Germany, Sweden, and Poland.
Tools and Methods: A Phylodynamic Reconstruction
Using full-genome sequences submitted to GISAID, the researchers applied both discrete and continuous phylogeographic modeling to retrace the virus’s evolutionary path. These approaches enabled them to not only pinpoint the likely time and place of emergence, but also track its subsequent migration across the continent.
The study highlights the power of phylodynamic tools—combining viral genome data with geographic and temporal metadata—to monitor real-time virus evolution and movement, even amid sampling bias and limited wildlife surveillance in some regions.
Broader Implications: Zoonotic Potential and Biosecurity Concerns
The emergence of EA-2023-DG is more than an academic concern. The virus has already served as a model for in vivo experiments in the United States assessing the replication of H5N1 viruses in cattle and domestic animals. Those studies showed that the virus can replicate efficiently in mammalian tissue and acquire adaptive mutations such as PB2 E627K—mutations that may increase its potential for cross-species transmission.
With ongoing cases of avian influenza in mammals, including cattle, cats, and even humans, the rapid evolution of clade 2.3.4.4b H5N1 viruses like EA-2023-DG raises alarm for public health and agricultural biosecurity alike.
Surveillance Remains a Cornerstone of Preparedness
While the study illustrates the significant role of genomics in understanding outbreak dynamics, it also exposes key gaps in cross-country surveillance—particularly in wildlife monitoring. Still, the integrated use of genetic, spatial, and temporal data offers a powerful model for future investigations.
Given the continuing evolution and spread of H5N1 viruses, the authors emphasize the need for coordinated, international surveillance systems that incorporate full-genome sequencing and real-time data sharing. Such systems are essential to inform timely public health and animal health responses, limit spillover risk, and prepare for emerging threats.
Van Borm S, Ahrens AK, Bachofen C, et al. Genesis and Spread of Novel Highly Pathogenic Avian Influenza A(H5N1) Clade 2.3.4.4b Virus Genotype EA-2023-DG Reassortant, Western Europe. Emerg Infect Dis. 2025;31(6). https://wwwnc.cdc.gov/eid/article/31/6/24-1870_article