A large black-backed gull that migrated from Europe to eastern Canada last winter may have been the first carrier in North America of the deadly strain of avian influenza that has killed tens of millions of domestic birds and devastated wild bird populations. .
Large-scale outbreaks have provided researchers with a new opportunity to refine their understanding of the disease by studying which species, behaviors, and ecologies of wild birds play a key role in transmission.
“Previous studies looking at avian flu made these broad categorizations of wild and domestic birds,” said Dr. Nichola Hill, assistant professor of biology at the University of Massachusetts Boston and lead author of a new paper on the topic.
But “wild birds are incredibly rich in species,” he said, adding that “each of them has a unique natural history and behavior.”
Knowing which migratory species carry the pathogen, for example, can help predict when and where it might arrive based on migration routes.
After the migratory gull made landfall, highly pathogenic avian influenza, also known as the H5N1 virus, exploded in North America. More than 77 million poultry, most raised in the crowded conditions that fueled the spread and evolution of the virus, have been culled in dozens of countries.
For some experts, the toll caused by this H5N1 strain in wild birds (it has affected more than 100 species so far) has been alarming and unprecedented in its depth and breadth. Among wild birds, the spread can be very difficult to contain, posing a greater threat of contagion to other wildlife. And some wild bird species, such as cranes and some seabirds, are particularly vulnerable, especially those with low reproductive rates and those already in danger of extinction.
The World Organization for Animal Health estimates that more than 383,000 wild bird deaths can be attributed to the virus since October 2021, although the count may be well underestimated due to how difficult it is to track sick and dead birds.
The pathogen has spread rapidly across various regions and species, at much higher rates than during the last outbreak in 2014-2015.
“It’s affecting a larger host range and it’s not ending up in a dead end in wild birds like it used to,” Dr Hill said. “It stays in wild birds, and that’s a scary prospect. For so many of us in this field, my gosh, what do we do when we spill into a wild animal for which there is no control?
It has long been assumed that the main hosts of bird flu are ducks, such as mallards, teals and shovelers, which feed on the surface and just below with their rumps in the air. They are critical for spread because they have mild or no symptoms and they spread it everywhere. The new study, however, found that other birds, such as geese, played an underestimated role due to their natural history.
“Geese are a bit more tolerant of areas disturbed by humans,” said Dr. Hill. “Imagine a commercial poultry operation or a backyard operation where they spread grain.” That attracts “geese and other scavenging birds, like gulls, ravens and magpies, so there’s an interface between them,” he said.
The unique natural history of the black-backed gull, the world’s largest gull, for example, plays a role in transmission. “Gulls were really rare hosts for highly pathogenic forms of the virus,” said Dr Hill. “When they took it, on those rare occasions, they spread it very quickly. There’s nothing like a seagull for really fast spread of the virus and really long distances. They will catch a tailwind and cross the Atlantic in 24 hours.”
The study may help other researchers track not only the continued spread of this year’s pathogen, but also the paths taken by other viruses that are harmful to wildlife.
“Knowing that seagulls, geese and ducks may be carrying this virus in different ways is a huge contribution to understanding or eventually more accurately modeling how to expect a virus like this to spread,” said Jonathan Runstadler, professor and chair of the Department of Infectious Diseases and Global Health at Tufts University Cummings School of Veterinary Medicine and co-author of the paper.
The data “allows us to predict if a virus is emerging, when that bird might enter North America, and what bird populations we might be able to monitor for it,” Dr. Runstadler said.
This year’s highly pathogenic avian flu lineage originated around 1996 and was first found in a domestic goose in China. It has been circulating around the world in wild and domestic birds ever since, evolving as it travels from host to host.
In 2005, after a decade of evolution, the strain caused a major outbreak in wild birds in China’s wetlands.
The strain first appeared in the United States in 2014, traveling in migratory birds from Eurasia across the Pacific to Alaska and further east, causing outbreaks on US poultry farms that resulted in the deaths of 40 million of turkeys and chickens.
However, after it reached the Midwest, mass culling stopped it, eliminating viral spread for both wild and domestic populations.
“We don’t have a vaccine,” said Dr. Hill. “All we have in our toolkit is the exchange of all our poultry, which is horrible, but to some extent it was successful.”
But killing infected poultry hasn’t worked this time, in part because the virus has been able to find a home in so many wild birds, sparking the largest outbreak of bird flu in history.
In some places, officials have been warning chicken farmers and even people who keep backyard flocks to keep their birds indoors, while in other places, the threat appears to have passed.
“This virus is so good because it ping-pongs back and forth between the wild and the domestic,” Dr. Hill said. “There is no better way to amplify a virus than to take a wild reservoir and tame a close relative. That is exactly what we have done with chickens and ducks. Highly pathogenic forms of the virus only occur when the virus reaches farm animals.”
On Quebec’s Magdalen Islands, wildlife officials recently discovered the carcasses of thousands of northern gannets that had been wiped out by the flu.
There is no way to predict whether flu outbreaks will decrease or worsen.
Some species, such as raptors, seabirds, and shorebirds, are also at high risk of contracting the virus due to their behaviors. Dozens of bald eagles are known to have died from the flu, largely because they feed on ducks and other birds that carry the pathogen.
Birds that gather in large numbers are also at risk. “There are a lot of birds in flocks, shorebirds, terns and seabirds, that form massive, massive groups and that could be a field day for the virus,” said Dr. Hill.
The extent of the devastation of various species is difficult to assess because surveillance is lacking. Better tracking along migration routes would help experts find ways to mitigate the spread of the virus.
Fatalities of large numbers of shearwaters and other seabird species have been reported along the Atlantic coast in Massachusetts, Maine, New Hampshire, and Connecticut. Avian flu is suspected, although tests have not confirmed it.
“The geographic extent of detection, the number of species we’re getting with detections, the number of diseases we’re seeing in wild birds, all of this is unprecedented,” said Andy Ramey, research wildlife geneticist with the Service. US Geological Survey in Alaska. that he studies avian influenza. “It’s uncharted territory and it’s hard to know what to expect.”
There is also concern that during this year’s breeding season for many species, parents may transmit the disease to the young in the nest, who have underdeveloped immune systems. Young wild birds are often exposed to low pathogenic viruses, which are common and can serve almost as inoculations, helping to strengthen their immune systems.
One endangered species being monitored is the roseate tern in Buzzards Bay, off the coast of Massachusetts. Testing is just beginning and no sick birds have been found yet.
“It looks to be a tough feeding year for terns,” said Carolyn Mostello, a shorebird biologist with the Massachusetts Division of Fish and Wildlife. “Nesting has been slow. Let’s hope we don’t have a combination of food shortages and bird flu; that could act together to really harm populations.”
Experts say bird flu poses a very low risk to people and has so far been detected in only two humans. However, as it persists and evolves, it could acquire the ability to pose a serious contagion threat to humans.
Dr. Hill said a big handicap in better understanding the outbreak has been a lack of funding for efforts to track the spread. “Surveillance is very, very, very bad,” he said. “We are spending very little money and time to get ahead of this.”