Highly Pathogenic Avian Influenza A(H5N1) Virus–Induced Mass Death of Wild Birds, Caspian Sea, Russia, 2022

T he coastal and estuarine wetlands of the north- ern Caspian Sea, which borders southeast Russia, provide support for millions of waterfowl and shorebirds during nesting, molting, migration, and wintering periods (1,2); >300 species of birds are found in this region.The area is crossed by several migration flyways (Figure 1), of which the Black Sea/Mediterranean Flyway is the main migratory route (3).The region plays a critical role in the reproduction of colonial nesting birds, such as pelicans, cormorants, herons, gulls, and terns.Maliy Zhemchuzhniy Island is located in the northern part of the Caspian Sea (Figure 1).Monitoring data on waterbirds has shown the high ecologic importance of this area, not only during the nesting period but also during bird migration.The island has had >150 species of birds registered since 2016.A breeding colony of Caspian gulls is located on the island, along with colonies of Great black-headed gulls and Caspian terns, which are all listed in the Red Data Book of Russia.We investigated mass deaths of wild migratory waterbirds on Maliy Zhemchuzhniy Island that occurred in May 2022.The study was approved by the Committee on Biomedical Ethics at the Federal Research Center of Fundamental and Translational Medicine in Novosibirsk, Russia (protocol nos.2013-23, 2019-3, and 2021-10).The Study On April 28, 2022, near the end of the egg incubation period, we had counted a total of 26,769 Great black-headed gull nests, 7,340 Caspian gull nests, and 5,267 Caspian tern nests on Maliy Zhemchuzhniy Island.In May, 1 week later, we detected mass deaths of waterbirds on the island comprising 25,157 Great black-headed gulls, 3,507 Caspian gulls, 5,641 Caspian terns, and 14 Dalmatian pelicans (Appendix 1 Figure 1, https://wwwnc.cdc.gov/EID/article/29/12/23-0330-App1.pdf).Nearly all gull and tern chicks died during the nesting period.The mass death event began during hatching of Great black-headed gulls.We only found the corpses of chicks (with down but without feathers) that were similar in age.We assume that not all of the chicks actually died from disease; death of adult birds likely led to the deaths of chicks in their nests.The Caspian terns were still incubating eggs at that time; consequently, the death of adult terns led to the death of egg clutches in their nests.We