by Hundreds of wild sea lions in South America, a mink farm in Europe and more than 58 million backyard birds died.
All of these animals have fallen victim to the impact of bird flu, a virus that circulates rapidly around the world, killing wild and domestic animals, disrupting ecosystems and disrupting food supplies.
Human health is inextricably linked to animal health and these events are a disturbing reminder that a widespread epidemic in animals has potential consequences for humans.
In the United States, the latest wave of bird flu affected 17 mammals and more than 160 birds. It is the largest H5N1 outbreak since it emerged as a concern in China in 1996.
The virus has been under close scrutiny by scientists, even more now that it has spread far and wide.
“This is the number one potential pandemic virus that everyone has been interested in for a long time,” said Richard Webby, an infectious disease researcher at St. Jude Children’s Research Hospital in Memphis, Tenn., and director of the Collaborating Center for the World Health Organization. for studies on the ecology of influenza in animals and birds.
Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, said Wednesday that the spread of avian influenza on mammalian species needs to be monitored closely and that the risk to humans remains low for now.
“But we cannot assume it will stay that way and we need to prepare for any change in the status quo,” he said.
The sheer amount of H5N1 in circulation has increased the risk that the virus could spread to other species, develop the ability to transmit between people and become a pandemic.
But the virus has yet to unlock a complicated set of mutations or genetic changes that would allow it to spread more quickly in people.
“It’s a series of events, each of which is quite unlikely. This is why I say the risk to humans is currently low. Evolutionary barriers are high,” said Anice Lowen, a virologist and associate professor at Emory University School of Medicine. “It’s a numbers game. So that’s one reason why the scale of the current avian epidemic is concerning.”
Researchers are especially concerned about this version of bird flu, H5N1, because most humans have never dealt with it before.
“We don’t have an immune response against H5. That’s why the virus has pandemic potential,” Lowen said.
The scientists also observed high death rates and serious illness in chickens and mammals that contracted H5N1, leading them to fear that the virus could cause serious illness in people as well.
The United States has counted a single human case of H5N1 in the United States: an inmate in a Colorado prison culling infected birds on a farm. Previous spillovers in humans, primarily in people in Southeast Asia and North Africa who likely handled infected birds directly, had high mortality rates, although those numbers may be influenced by limited reports of mild cases.
H5N1 has long been a major pandemic concern. The version that circulated in ducks and other wild birds has evolved and adapted for efficient dissemination.
As those animals travel, so does the virus, through droppings, saliva and nasal secretions.
Wild birds are shedding viruses and infecting animals “over greater numbers and a larger geographic footprint than ever before,” said Bryan Richards, the coordinator of emerging diseases at the US Geological Survey’s National Wildlife Health Center. More than 6,100 wild birds have tested positive for the virus in the United States
Poultry farm birds are likely to become infected by contact with wild bird feces or other secretions.
Scavengers like bears, raccoons and foxes have also become infected, likely after eating a dead or sick bird, Richards said. Marine mammals, including dolphins, have also tested positive.
Recently, more than 500 sea lions were found dead with H5N1 in Peru. It’s unclear whether the virus was spreading among these species or if the animals were infected through food.
The virus is poorly adapted for spreading in people.
“The avian virus is not as good at attaching itself to human cells as it is in the respiratory tract of birds. They’re just not adapted to humans,” said Dr. Helen Chu, an infectious disease physician and influenza expert at UW Medicine in Seattle.
To spread effectively in humans, the virus would need to make several genetic changes. This process would probably have occurred in other mammals.
Webby and Lowen said there are two main processes the virus could take to develop better binding to receptors in human respiratory cells.
The first would be for the virus to change rapidly through reassortment, an “evolutionary shortcut” in which an animal becomes infected with both an avian flu virus and a human virus, Lowen said. During coinfection, the two segmented flu viruses could exchange bits of genetic code and combine to create a chimeric virus.
That virus would then likely need more replications to fix the mismatched genes and develop traits that would allow it to thrive in humans.
“The reassortments would be very concerning to see, but they probably still wouldn’t be able to transmit in humans,” Lowen said. “Probably what would be needed is more evolution to correct the discrepancies.”
Previous bird flu pandemics — starting in 1957 and 1968 — required both reassortment and further mutations before they could spread widely among humans, Webby said. Each of these pandemics killed about 1 million people worldwide and about 100,000 people in the U.S.
The second option is for the virus to mutate within a dense group of animals. Researchers have become concerned after an apparent H5N1 outbreak at a mink farm in Spain.
“They are in small cages very close together. There’s a level of transmission efficiency,” Chu said of the mink.
Studies more than a decade ago showed that ferrets could pick up mutations for airborne H5N1 transmission after serial infections.
In the recent mink farm outbreak, the virus likely spread from mink to mink, Webby said. Although he detected a troubling mutation, it remained largely bird-adapted. The minks have been culled.
“Luckily it was cancelled,” Webby said.
The virus faces steep genetic hurdles, but the more it spreads in animals, the better chance it has of overcoming those hurdles.
Lowen said governments should invest further in surveillance of potential hosts that could help the virus reach humans, consider measures such as vaccination to limit the spread in poultry, and invest in science that will help determine what genetic changes could be of concern to people.
This article was originally published on NBCNews.com
