Monthly summary of sampled birds
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April 2025
1. OVERVIEW
SENTINEL Wild Birds aims to enhance the understanding of highly pathogenic avian influenza (HPAI) virus dynamics in wild bird populations by conducting active surveillance at key locations in and near Europe. These locations are divided into the following surveillance nodes: Node 1 Gulf of Finland (Finland, Estonia), Node 2 Southern Baltic Sea (Sweden, Latvia, Lithuania, Poland), Node 4 Eastern Black Sea (Georgia), Node 6 Lake Constance (Germany, Austria, Switzerland), Node 7 Veneto Region (Italy), Node 8 Camargue (France), and Node 9 Gulf of Cadiz (Spain). This monthly summary provides an update on sampled wild birds as part of an early warning system to support wildlife management and disease prevention efforts. The data in this report are based on previously unpublished samples collected from January 2025 to April 2025, along with a few backlogged samples from Georgia in September 2024.
2. RESULT
2.1 DATA COLLECTION
Since the last monthly report (published on 2nd of April 2025; https://doi.org/10.5281/zenodo.15123571), and as of 15th April 2025, test results have been submitted for 1127 samples taken from 767 individual wild birds representing 21 taxa across four nodes in Europe (Table 1). Of the 1127 collected samples, 359 (32 %) were cloacal swabs, 353 (31 %) tracheal/oropharyngeal swabs, 296 (26 %) faecal samples, 37 (3 %) environmental samples, 21 (2 %) pooled organs, 21 (2 %) blood samples, 20 (2 %) feather samples, 19 (2 %) nest swabs, and 1 (<1 %) combined swabs (choana + cloacal). Of all samples, 10 (<1 %) from five different taxa were positive for avian influenza virus, of which none were positive for highly pathogenic avian influenza (HPAI) virus (Figure 1; Table 2).
The overall bird-level prevalence of avian influenza virus in these recently submitted samples was 1.3 %. The highest bird-level prevalence was found in Georgia (1.6 %; Table 1), and among the most frequently sampled birds, Mallard had a prevalence of 4 % (6 of 150 individuals), Yellow-legged Gull 1 % (1 of 104 individuals), Northern Pintail 0.5 % (1 of 190 individuals), and Black-headed Gull 0.5 % (1 of 214 individuals) (Table 1).
TABLE 1 Total number of individuals sampled in the wild (including recaptures of some birds), as well as number of individuals tested positive for avian influenza virus in the respective country. The table includes previously unpublished samples from September 2024 to April 2025.
FIGURE 1 Sample sites for the 767 birds sampled in five European countries, yielding 10 samples positive for avian influenza virus. The figure includes previously unpublished samples from September 2024 to April 2025.
TABLE 2 Total number of collected samples as well as number of samples positive for avian influenza virus, in the respective country. The table includes previously unpublished samples from September 2024 to April 2025.Total number of collected samples as well as number of samples positive for avian influenza virus, in the respective country. The table includes previously unpublished samples from September 2024 to April 2025.
FIGURE 2 Weekly summary of samples collected at each node, as well as all nodes combined, from week 34, 2024 to week 14, 2025. In total, 13060 samples (negative samples in blue; avian influenza virus-positive samples in orange) have been collected at seven nodes between August 2024 and April 2025, yielding 1040 samples positive for avian influenza virus, including 24 samples positive for HPAI virus. The figures also include samples published in previous reports.
2.2 GENOMICS SUMMARY
Since the last report was published on 2nd of April (https://doi.org/10.5281/zenodo.15123571), and as of 15th April 2025, a total of three new sequences have been generated from samples positive for H5 avian influenza virus. The samples were collected from Mallards in Georgia (Node 4 Eastern Black Sea) and was determined to be of the subtype H5N1. Genetic analysis of the haemagglutinin (HA) gene sequence from this sample found that one of these samples belonged to the Eurasian non-Goose Guangdong (EA-nonGsGd) low pathogenicity avian influenza (LPAI) clade, whilst the other two samples belonged to the H5 clade 2.3.4.4b highly pathogenic avian influenza (HPAI) clade (Figure 3A). Of the two H5 clade 2.3.4.4b sequences, one showed similarity to poultry and wild bird sequences from Israel and Croatia, whilst the other sequence showed similarity to European H5 sequences (Figure 3B). The H5 EA-nonGsGd sequence showed similarity to other H5 LPAI sequences from the Netherlands (Figure 3C). The Nextstrain builds are available on the SENTINEL Wild Birds Group (https://nextstrain.org/groups/SentinelWildBirds).
Apart from the H5 genome sequences, no further sequences were generated in this reporting period.
FIGURE 3 Phylogenetic analyses of the HA sequences generated by Node 4 Eastern Black Sea. A. Image of the whole HA tree with the sequences from Node 4 Eastern Black Sea highlighted. B and C. Zoom images of the HA tree showing the H5 clade 2.3.4.4b and EA-non-GsGd sequences, respectively. The sequences from Node 4 Eastern Black Sea are annotated.Phylogenetic analyses of the HA sequences generated by Node 4 Eastern Black Sea. A. Image of the whole HA tree with the sequences from Node 4 Eastern Black Sea highlighted. B and C. Zoom images of the HA tree showing the H5 clade 2.3.4.4b and EA-non-GsGd sequences, respectively. The sequences from Node 4 Eastern Black Sea are annotated.
3. CONCLUSION
Apart from 24 samples collected in Georgia in September 2024, all samples included in this report were collected in 2025. This indicates that the backlog of samples from autumn 2024 has now been processed, and future reports will only include recently collected material.
The previously observed trend in samples collected during 2025 persists, showing a low prevalence of AIV-positive results. To date, only 1% of all samples collected in 2025 have tested positive for avian influenza virus (45 out of 4,125).
However, genetic analyses of samples from the overwintering period on the Black Sea Coast, in Georgia, two hunted mallards were H5N1 HPAI-positive in January and February respectively. Sequence analyses of one female mallard in January identified a novel genotype, with the same genetic composition of genotype EA-2024-DI, except for the NP gene, which clusters with LPAI viruses. This combination has not been identified previously. Analyses of another female in February identified that it belongs to genotype EA-2024-DI (sublineage 2).
The detection of this new genotype in a Mallard during the overwintering period on the Black Sea coast suggests ongoing evolution and reassortment of avian influenza viruses in wild bird populations. This finding highlights the need for continued surveillance to rapidly detect emerging variants with potentially altered phenotype, host range or transmission dynamics.
Trapping of wintering birds has now concluded in most regions. Few or no samples have been collected so far in 2025 from the Gulf of Finland (Node 1) and the Southern Baltic Sea (Node 2). No samples have been collected since the previous report (March) in the Veneto region (Node 7) or the Gulf of Cádiz (Node 9). In Camargue (Node 8), sampling has shifted toward breeding gull colonies.
The Eastern Black Sea region (Node 4) remains active, with continued sampling of Mallards, Eurasian Teals, and Black-headed Gulls in relatively high numbers. Around Lake Constance (Node 6), various primarily resident species are still being sampled, albeit in low numbers.
During the upcoming breeding season, active surveillance will focus primarily on gull colonies in Camargue (Node 8) and the Eastern Black Sea region (Node 4), as well as tern colonies around Lake Constance (Node 6).
