Capsule
An outbreak of high pathogenicity avian influenza (HPAI) in 2023 caused mass wild bird mortalities in Senegal and The Gambia, heavily impacting seabirds.

Aims
To present a comprehensive overview of an outbreak of HPAI (H5N1) in wild birds in Senegal and The Gambia during 2023.

Methods
Field surveys were conducted at 14 locations in Senegal and The Gambia before, during and after the HPAI outbreak to assess the impact on birds, particularly focusing on key non-breeding and colonial breeding sites where the virus led to the greatest mortality.

Results
In early 2023, a major outbreak of HPAI (H5N1) occurred in Senegal and The Gambia. At least 25,000 bird mortalities were recorded from 32 different species, with gulls and terns disproportionately affected. Increased bird mortality was first detected in April 2023, peaking in May, with the majority of dead birds were found at sites in The Gambia rather than Senegal. West African Crested Terns Thalasseus albididorsalis exhibited the largest mortalities (>10,000 dead birds recorded), followed by Caspian Terns Hydroprogne caspia (>4,000), Grey-headed Gulls Chroicocephalus cirrocephalus (>2,000) and Sandwich Terns Thalasseus sandvicensis (>2,000). By September 2023, no further mortalities were reported from the survey sites in Senegal and The Gambia.

Conclusion
A significant mortality of migratory and resident seabirds occurred in Senegal and The Gambia during 2023, mostly likely due to HPAI (H5N1), highlighting the vulnerability of these populations. A significant impact of HPAI on wintering populations of Sandwich Terns compounded the influence of the virus at breeding sites. Limitations in the response to the outbreak highlighted an urgent need to implement systematic surveillance, enhance biosafety measures and foster regional collaboration to mitigate future occurrences of HPAI and to protect wild bird populations in West Africa.