Abstract
Background:
Highly pathogenic avian influenza (HPAI) H5N1 has continued to result in severe mortalities among wild birds globally.

Aim:
The present study aimed to investigate the mass mortality in Slender-billed Gulls (Chroicocephalus genei) and Gull-billed Terns (Gelochelidon nilotica) at Dukan Lake, Iraq, during the 2024 breeding season.

Methods:
Field monitoring was conducted at breeding colonies, and dead carcasses were collected for necropsy, histopathology, and molecular analysis. Virus detection and genetic characterization were performed by reverse transcription polymerase chain reaction (RT-PCR) and sequencing. In addition, viral lineage was inferred by phylogenetic analysis of the HA and NA genes.

Results:
More than 3200 gulls and terns were found dead, exhibiting severe neurological signs and systemic illness. Necropsy and histo-pathological investigations indicated significant multi-organ damage, with gulls showing more pronounced visceral lesions than terns, while terns exhibited greater neurological damage. Molec-ular characterization confirmed the virus as HPAI H5N1, and analysis of the HA and NA genes revealed that the strain from Dukan Lake falls within clade 2.3.4.4b, displayed a high genetic re-semblance (>98%) with the recently identified H5N1 strain isolated from Mute swans in Kazakh-stan. The virus showed multibasic cleavage sites characteristic of high pathogenicity but main-tained receptor-binding specificity for avian hosts. Moreover, the results demonstrate the recent introduction of the virus via migratory flyways and the significance of gulls and terns in the disease ecology of HPAIV.

Conclusion:
This outbreak is the first recorded mass mortality of gulls and terns due to HPAI H5N1 in Iraq. The results emphasize the necessity for improved surveillance at mi-gratory stopover and breeding sites to observe viral evolution, host susceptibility, and potential risks for cross-species transmission.

Key words: Highly pathogenic avian influenza; H5N1; Gulls and terns; Dukan Lake; Iraq.