The H5N1 bird flu virus responsible for the current U.S. outbreak in dairy cows is increasingly adapting to spread in mammals, new research in marine mammals suggests. Some experts worry this development could presage eventual human-to-human transmission.
In a preprint study that has not yet been peer-reviewed, researchers from the University of California, Davis and the National Institute of Agricultural Technology (INTA) in Argentina found evidence of the virus being spread among elephant seals and other marine mammals. They also found versions of the virus that could both spread between mammals and infect birds.
"The implication that H5N1 viruses are becoming more evolutionary flexible and adapting to mammals in new ways could have global consequences for wildlife, humans, and/or livestock," the researchers wrote in the new study, which was posted to the preprint database bioRxiv June 1.
This version of H5N1 began to spread widely among birds in 2020 — first in Europe and then in South Africa. The virus appeared in North America in 2022 and subsequently spread to South America. In August 2023, it was found at the very tip of South America, on the Tierra del Fuego archipelago — not in birds, but in sea lions.
Then, in October 2023, researchers from UC Davis and INTA found that the flu was ripping through a colony of elephant seals at Punta Delgada on the coast of Península Valdés in Argentina. The disease killed more than 17,000 elephant seals, including 96% of pups born that season.
Genetic sequencing of the deadly virus revealed it to be a particular lineage called clade 2.3.4.4b, genotype B3.2. This lineage spread from migratory birds into mammals in South America multiple times in late 2022 and 2023, with one of those spillovers evolving into a new lineage that can spread easily from mammal to mammal, the researchers found. The genetic data linked mammal outbreaks in Argentina, Chile, Peru and Brazil — the first known multinational mammal-to-mammal spread of the virus, the researchers reported.
"This is increased evidence that we should be alert, especially for marine mammals," co-lead author Dr. Marcela Uhart, a veterinarian with the UC Davis Karen C. Drayer Wildlife Health Center and its Latin America Program, said in a statement. "The more it adapts to mammals the more important it becomes for humans."
The virus can infect humans, but reported cases are rare. The Centers for Disease Control and Prevention has found four cases in humans in the U.S. — one from 2022 and three from 2024. The 2022 case had direct exposure to poultry, while the 2024 cases were transmitted from dairy cattle.
"This virus is capable of adapting to mammals, as we can see from the mutations that are consistently found in the viruses belonging to the mammalian clade," study co-leader Agustina Rimondi, a virologist at INTA, said in the statement.
Both Uhart and Rimondi said continued monitoring of the virus in wildlife is key to understanding the potential consequences for human health.
