Monthly summary of sampled birds
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The Nextstrain builds are available on the SENTINEL Wild Birds Group - https://nextstrain.org/groups/SentinelWildBirds
March 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 The data in this report are based on previously unpublished samples collected from January 2025 to March 2025, along with backlogged samples from Sweden and Georgia dating back to August 2024.
2. RESULT
2.1 DATA COLLECTION
Since the last monthly report (published on 26th of February 2025; https://doi.org/10.5281/zenodo.14929946), and as of 15th March 2025, test results have been submitted for 3899 samples taken from 2200 individual wild birds representing 40 taxa across six nodes in Europe (Table 1). Of the 3899 collected samples, 2012 (52 %) were tracheal/oropharyngeal swabs, 801 (21 %) cloacal swabs, 800 (21 %) faecal samples, 254 (7 %) feather samples, 24 (<1 %) combined swabs (choana + cloacal), 7 (<1 %) pooled organs, and one (<1 %) choana swab. Of all samples, 101 (2.6 %) from eight 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 4.5 %. The highest bird-level prevalence was found in Georgia (8.2 %; Table 1), and among the most frequently sampled birds, Eurasian Teal had a prevalence of 5.9 % (24 of 410 individuals), Mallard 5.3 % (67 of 1268 individuals), Pintail 1.4 % (1 of 72 individuals), and Marbled Duck and Black-headed Gull 0 % (0 of 79 and 0 of 117 individuals, respectively) (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 in the respective country. The table includes previously unpublished samples from August 2024 to March 2025.
FIGURE 1 Sample sites for the 2200 birds sampled in eight European countries, yielding 101 samples positive for avian influenza virus. The figure includes previously unpublished samples from August 2024 to March 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 August 2024 to March 2025.
FIGURE 2 Weekly summary of samples collected at each node, as well as all nodes combined, from week 34, 2024 to week 10, 2025. In total, 11933 samples (negative samples in blue; avian influenza virus-positive samples in orange) have been collected at seven nodes between August 2024 and March 2025, yielding 1030 samples positive for avian influenza virus, including 22 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 26th of February (https://doi.org/10.5281/zenodo.14929947), and as of 15th March 2025, a total of one new sequence has been generated from a sample positive for H5 avian influenza virus. The sample was collected from a Mallard in Sweden (Node 2 Southern Baltic Sea) and was determined to be of the subtype H5N3. Genetic analysis of the haemagglutinin (HA) gene sequence from this sample found that it belonged to the Eurasian non-Goose Guangdong (EA-nonGsGd) low pathogenicity avian influenza (LPAI) clade and showed similarity to H5N2 and H5N3 LPAI viruses previously detected at Node 2 Southern Baltic Sea (see February 2025 report: https://doi.org/10.5281/zenodo.14929947), as well as sequences from across Europe and Asia (Figure 3, Group 1).
The Nextstrain builds are available on the SENTINEL Wild Birds Group
(https://nextstrain.org/groups/SentinelWildBirds).
FIGURE 3 Phylogenetic analyses of the HA sequences generated by Node 2 Southern Baltic Sea. Image of a subset of the HA tree displaying the sequences from Node 2 Southern Baltic Sea. The two phylogenetic groups are indicated.
In addition to the H5 genome sequence, a further 2 non-H5 genome sequences were generated from Node 2 Southern Baltic Sea (N=1) and Node 6 Lake Constance Region (N=1). The sample from Node 2 Southern Baltic Sea was collected from a Mallard in Sweden and was of the H2N2 subtype. The sample from Node 6 Lake Constance Region was collected from a Mallard in Austria and was of the H12N2 subtype.
3. CONCLUSION
With close to 12,000 samples analysed since August 2024—including large numbers of Mallards (primarily from Sweden and Georgia), Eurasian Teals (mostly from France and Italy, but also from Georgia and Sweden), Marbled Teals (from Spain), and Black-headed Gulls (mainly from Georgia)—a pattern of two peaks of higher prevalence during autumn becomes evident. A first peak occurs in September, primarily driven by Mallards in Georgia and Eurasian Teals in Italy. A second peak follows in early November, mainly driven by Mallards in Sweden. These peaks are consistent with previous studies on avian influenza virus surveillance in Europe (EFSA, 2025; Munster et al., 2007; Wallensten et al., 2007) and Georgia (Lewis et al., 2013). Many of the individuals testing positive for avian influenza virus are juveniles, which are more susceptible to infection.
Positive detections of avian influenza virus in Mallards and Teals are commonly observed in the Southern Baltic (Munster et al., 2007; Wallensten et al., 2007) and Eastern Black Sea regions (Lewis et al., 2013; Waldenström et al. 2022) during autumn migration, and in France (Briand et al., 2017) and Italy (Gobbo et al., 2021) during the winter period.
Although the proportion of AIV-positive samples has been relatively high (4.7-45%), the number of confirmed HPAI cases has remained low. In 2024, 22 cases were detected—all in Eurasian Teals in Italy during November and December.
The winter months are generally associated with reduced bird movement at many nodes, leading to limited trapping activity at migratory stopover sites such as the Southern Baltic, the Gulf of Finland, and the Eastern Black Sea. However, at other locations—such as the Veneto region, Camargue, and the Gulf of Cádiz—trapping of wintering birds has continued. In 2025, only a small number of samples have tested positive for avian influenza virus, and no cases of HPAI have been confirmed to date.
With the onset of spring migration in early March, we anticipate an increase in AIV-positive samples at breeding sites further north in Europe. This trend is already evident in Sweden, with several confirmed cases of HPAI reported in late February (SVA, 2025).
Unfortunately, a backlog of unprocessed samples remains. Addressing this is essential to achieve near real-time analysis of collected material and improve our understanding of the current epidemiological situation.
