Feb. 16, 2023 – A highly infectious strain of bird flu – known as H5N1– is sweeping across the globe, devastating backyard flocks and commercial farms alike. While this virus mostly infects birds, growing reports of mammals infected with the virus have some experts worried. The main concern, they say, is as H5N1 continues to spread and multiply in different species, there are more opportunities for the virus to potentially gain mutations that could help it more easily infect humans.
“Just by the sheer numbers game, the more chances it has to infect humans or [other] mammals, the more chance you're going to get that rare event happening,” says Richard Webby, PhD, the director of the World Health Organization Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds at St. Jude Children’s Hospital in Memphis.
Since May 2022, the U.S. Department of Agriculture has detected 121 mammals infected with the H5N1 virus, including red foxes, skunks, bears, and even seals. In October 2022, an outbreak at a mink farm in Spain resulted in the culling of over 51,000 animals, and earlier this month, Peru confirmed 585 sea lions had died from the virus in coastal nature preserves. While H5N1 infections have been detected in mammals over the past 25 years, “we’re certainly seeing an uptick in reported cases over the past 12 months,” Webby says.
The World Health Organization has stated that the risk to humans remains low, “but we cannot assume that will remain the case, and we must prepare for any change in the status quo,” WHO Director-General Tedros Adhanom Ghebreyesus, PhD, said at a news briefing this month.
Human Infections Remain Rare
H5N1 infections in humans are rare. They mostly happen when someone has unprotected, direct exposure to birds infected with H5N1 or surfaces contaminated with the virus. Since 1997, there have been about 870 human cases reported to the WHO, with roughly half dying from the infection. Even if someone is infected with the virus, it is even rarer that the infection would spread to another human.
“H5N1 viruses currently circulating in wild birds and causing poultry outbreaks are well-adapted to spread among birds,” Tim Uyeki, MD, chief medical officer of the Influenza Division's Office of the Director at the CDC, said in an online Q&A. "However, these H5N1 bird flu viruses do not have the ability to easily bind to receptors in the upper respiratory tract of humans, or to transmit among people.”
Even with the record-breaking numbers of infected wild birds and poultry, human infections have become rarer in the last few years. Since the beginning of this century, it was not uncommon to have at least 30 to 40 recorded human H5N1 per year. Since 2021, there have been fewer than 10 infections reported around the world, according to the CDC.
More Spread in Mammals
H5N1 infections in wild mammals are not unheard of, though they are also rare events. Most of these cases likely occur when an animal eats an infected bird and is exposed to the virus. Like in humans, these infections have been sporadic. But a report last month about an outbreak at a mink farm in Spain caused experts to worry, as the researchers suggested the virus may have spread by one mink infecting others. Minks are related to ferrets, which scientists use as an animal model for studying flu transmission.
“If we see something that transmits in mink, it’s not a stretch to think, ‘Maybe that is something that could also transmit in ferrets,’” Webby says. “We certainly would use transmission in ferrets as an alarm bell for human risk."
But so far, there is no evidence that the virus has picked up the ability to more easily infect humans, says William Schaffner, MD, a professor of infectious diseases at the Vanderbilt University Medical Center in Nashville.
“In fact, one of the reassuring things is in that mink farm in Spain, the people who work on the farm – and had very close contact with those mink – were all tested, and they were found to be negative,” he says. The minks were also very closely confined in cages – which may have aided spread of the virus, he notes.
The more recent report of infections in sea lions had some people speculating online about possible spread of the virus among the animals, but these marine mammals may have also been exposed to birds infected with H5N1.
What is clear is that the virus is widespread, and is popping up in more and more animals, says Jennifer Nuzzo, DrPH, who heads the Pandemic Center at the Brown University School of Public Health in Providence, RI. Why that is happening is less obvious.
“Does that mean that [H5N1] has gained some fitness to mammals? What does that mean for the potential to infect humans? We just don't know,” she says.
Will Bird Flu Become the Next Pandemic?
Unfortunately, it’s not possible to predict what type of virus will cause the next pandemic. But the WHO Global Influenza Surveillance and Response System is already hard at work monitoring influenza strains around the world, Schaffner says .
“It’s like a radar system, trying to detect the enemy” as early as possible, he says. The network not only tracks infections in humans, but also in birds and other animals.
If the bird flu did begin to pose a real threat to the public, the U.S. already has a stockpile of bird flu vaccines.
“If we need them, they can be updated to the latest strain and immediately start to be manufactured,” Schaffner says.
While experts agree that the public’s risk for H5N1 is low, people should avoid contact with sick or dead wild birds, poultry, and wild animals. Do not eat uncooked or undercooked poultry, eggs, and other poultry products. It is safe to consume cooked poultry, poultry products, and eggs. Additional protective measures can be found on the CDC’s website.