H5N1 D1.1 Expanding

A fast-moving H5N1 bird-flu variant called D1.1 has spread across North American wild birds and pushed aside earlier local strains. (nature.com) Influenza viruses swap gene segments the way decks get reshuffled, and D1.1 is one of those new mixes. Researchers reported that it first appeared in surveillance in September 2024 and then spread during the 2024 fall migration across several bird flyways in the United States and Canada. (nature.com) The new paper, published April 15, 2026 in *Nature Medicine*, said D1.1 formed a single expanding lineage and displaced earlier H5 genotypes in multiple flyways. The authors used active and passive surveillance data from Canada and the United States to track that shift. (nature.com) Wild-bird spread matters because migratory birds can carry highly pathogenic avian influenza long distances without obvious illness and expose poultry along the way. The U.S. Department of Agriculture says its wild-bird surveillance system is meant to serve as an early-warning network for exactly that route of spread. (aphis.usda.gov) D1.1 has also shown up outside wild birds. A joint assessment from the Food and Agriculture Organization, World Health Organization and World Organisation for Animal Health said that by April 17, 2025, D1.1 was the most frequently detected H5N1 genotype across North America and had been found in wild birds, poultry and mammals. (woah.org) That same assessment said the first D1.1 detection in U.S. dairy cattle was confirmed in Nevada on January 31, 2025, followed by Arizona on February 13, 2025. The report described those findings as separate bird-to-cow spillovers, not evidence of sustained spread from people. (woah.org) Human cases tied to D1.1 have been occasional but real. The *Nature Medicine* paper linked the variant’s expansion to 17 human cases, including four that were severe or fatal, while CDC says there is still no known person-to-person spread of H5 bird flu in the United States and the public-health risk remains low. (nature.com) (cdc.gov) Several of the best-documented D1.1 infections came through poultry or wild-bird exposure rather than dairy work. CDC said the Louisiana patient who died in late 2024 was infected with D1.1 linked to backyard poultry and wild birds, and British Columbia said its hospitalized teenager was infected with the D1.1 strain circulating in local wild birds and poultry. (cdc.gov) (news.gov.bc.ca) Washington State poultry workers infected in October 2024 were also carrying D1.1 viruses closely related to poultry-outbreak strains, according to a CDC *Emerging Infectious Diseases* report. All of those workers recovered. (cdc.gov) Researchers said the wild-bird D1.1 viruses they analyzed did not carry the mammal-adaptation markers seen in some human cases. They also reported that existing candidate vaccine viruses still showed antigenic cross-reactivity against D1.1, meaning lab tests suggest those prepandemic vaccine seeds still recognize it. (nature.com) U.S. and international agencies are still treating the immediate risk to the general public as low, but they are tracking animal exposures, poultry outbreaks and new sequences closely. The next test for D1.1 is not whether it stays in wild birds, but how often it keeps jumping into farms and other mammals. (cdc.gov) (aphis.usda.gov)