Two recent studies describe deadly avian flu outbreaks among Argentina’s elephant seal population and terns found on islands off of Washington state, with both studies suggesting animal populations are significantly smaller in the wake of the infections.
In the first study on elephant seals, published today in Nature Communications, researchers from UC Davis and the National Institute of Agricultural Technology (INTA) in Argentina tracked the seal colony population one year after a deadly outbreak killed more than 17,000 elephant seals, including about 97% of pups.
The 2023 outbreak showed H5N1 spread efficiently among marine mammals and resulted in the introduction of the virus in marine mammals in five countries in southern South America.
Once introduced to elephant seals, the virus evolved into separate avian and marine mammal clades.
Our study also shows that H5 marine mammal viruses are able to jump back to birds, highlighting the need for increased surveillance and research cooperation in the region.
"This virus is capable of adapting to marine mammal species, as we can see from the mutations that are consistently found in the viruses belonging to this clade," said co-leading author Agustina Rimondi, PhD, of INTA in a press release. "Very importantly, our study also shows that H5 marine mammal viruses are able to jump back to birds, highlighting the need for increased surveillance and research cooperation in the region."
New potential reservoirs in seals
Mass mortality events among seals are rare, the authors said. The authors posit the pups were mostly infected via seal-to-seal transmission, likely enhanced by multiple introductions from sea lions.
The authors said with more than half of the reproductive population of elephant seals killed in the 2023 outbreak, it will take decades until the population rises to 2022 levels. Moreover, if seals become a reservoir for H5N1viruses, the mammals could infect wild birds, and coastal bird species could be repeatedly affected by spillover infections.
"Amidst growing evidence that mammal-to-mammal transmission played a role in H5N1 HPAI outbreaks in dairy cows in North America and in fur farms in Europe, the outbreak among elephant seals in Peninsula Valdes represents another case where mammal-to-mammal transmission was potentially involved in the spread of H5N1 HPAI infections," the authors concluded.
Outbreak killed over half of terns
In 2023, Rat Island, Washington, saw 56% of a large breeding colony of Caspian terns die from highly pathogenic avian influenza. In Frontiers in Veterinary Science, authors from the Washington Department of Fish and Wildlife (WDFW) as well as Washington State University described the outbreak and said no birds have successfully bred on the island since last year.
During the outbreak, a total of 1,101 adults and 520 chicks were killed by the virus. Alongside the birds, 15 dead harbor seals were found in the area, which usually only has 1 or 2 seal fatalities.
“This Caspian tern event was the first big marine environment avian flu outbreak for Washington. It caused a significant, punctuated mortality for the Caspian terns, which were already a species in decline throughout this flyway,” said lead author Katherine Haman, DVM, in a press release from Washington State University.
