More than four out of every five dead black vultures examined by University of Georgia researchers tested positive for highly pathogenic avian influenza, according to a new study published in Nature's Scientific Reports.
The actual toll of the virus on the integral species is likely exponentially higher, though, the researchers warned.
These birds may "represent tens or hundreds of thousands of black vultures," said Dr. Nicole Nemeth, lead author of the study and head of UGA's Southeastern Cooperative Wildlife Disease Study's research and diagnostic service. Nemeth is also an associate professor in UGA's College of Veterinary Medicine.
Black vultures' indiscriminate scavenging appears to sustain transmission of the virus beyond the typical bird flu season, the study found.
This year-round cycle of disease is concerning.
Sustained transmission means more opportunities for the virus to mutate. And more opportunities to mutate mean the potential to become more virulent and pose a bigger threat to other species, including humans. (There is currently no evidence this is happening in the vulture population, but the potential remains unknown.)
"Every day, more carcasses would appear, and we could see that the black vultures were eating their dead cohorts," Nemeth said. "From a survival perspective, that's smart. It's a ready-made, available food source.
"But it perpetuates this cycle of disease within the vulture population year-round."
Bird flu likely even more widespread than known
Researchers in the cooperative tested 134 dead black vultures from seven states in 2022 and 2023. More than 84% of the birds tested positive for highly pathogenic avian influenza, or H5N1.
"This is just a portion of the population that dies from the virus," said David Stallknecht, co-author of the study and a professor emeritus of wildlife diseases and population health at UGA. "And the portion that dies is just part of the bigger picture."
"This is a very important group of birds, ecologically speaking," Nemeth said. "In areas where vulture populations have been diminished, there are a lot more dead animals left in the landscape, which actually has been shown to contribute to increased diseases in people, wildlife and domestic animals."
Black vultures are abundant in the Southeast, but vulture species have been declining globally. Rampant bird flu won't help.
"We're not really thinking about just one virus," said Rebecca Poulson, co-author of the study and an assistant professor of wildlife diseases and virology. "Avian influenza is incredibly diverse. These highly pathogenic viruses continue to circulate and mix up, and it's creating a huge potential diversity of different viruses, many of which wildlife species in North America may not have seen before."
That could be devastating to not just bird populations but other animals vulnerable to the disease as well.
A silver lining: Some animals survive infection
Previous research has suggested about half the vultures that are infected with bird flu survive. Scientists found antibodies in the survivors, meaning they should have some protection against future outbreaks.
"That means that the virus is incredibly transmissible in these populations," Stallknecht said. "But from a conservation standpoint, it means there's survival as well."
If there's a silver lining to this study, that's it, the researchers said. The U.S. black vulture population is substantial enough that Stallknecht isn't too worried about the species' decline - not yet anyway.
This virus is scary. It's changing. And it's doing continual damage to our wildlife.
Nicole Nemeth, College of Veterinary Medicine
But for endangered species, even a few deaths could spell disaster.
A 2023 outbreak of bird flu in California condors, for instance, killed at least 20 birds. There are fewer than 600 condors. About a third of them are housed in captivity.
"When you have a population of only a few hundred animals, 20 dying is a big deal," Stallknecht said.
Avian influenza is changing
Also referred to as bird flu, this highly pathogenic H5 influenza was first detected in the 1990s and has wreaked havoc on wild and domesticated bird species on and off ever since.
It began infecting other animals from dairy cows and house cats to dolphins and bears.
"Everyone is always on the edge of their seat worried about whether it's going to mutate enough to go from person to person, mammal to mammal," Nemeth said. "This virus is scary. It's changing. And it's doing continual damage to our wildlife - on top of all the other challenges these wildlife already face."
The study was co-authored by UGA's Victoria Andreasen, Paul Oesterle, Dr. Chloe Goodwin, Dr. Xuan Hui Teo and Dr. Mark Ruder. Additional co-authors include Alisia Weyna, Robert Sargent, Mark Cunningham, Melanie Kunkel and Rebecca Hardman.
