A great black-backed gull migrating from Europe to eastern Canada last winter may have been North America’s first carrier of the deadly strain of bird flu that has killed tens of millions of domestic poultry and devastated wild bird populations.
Large-scale outbreaks have provided researchers with a new opportunity to refine their understanding of the disease by studying the wild bird species, behaviors and ecologies that play a key role in transmission.
“Previous studies looking at avian flu have 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 subject.
But “wild birds are incredibly species-rich,” she said, adding that “each one 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 landed, the highly pathogenic bird flu, also known as the H5N1 virus, exploded across North America. More than 77 million poultry, most raised in crowded conditions that have fueled the spread and evolution of the virus, have been slaughtered in dozens of countries.
For some experts, the toll caused by this H5N1 strain on wild birds – it has struck more than 100 species so far – has been alarming and unprecedented in its depth and scale. Among wild birds, the spread can be very difficult to contain, posing a greater threat of spread 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 threatened.
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 number may be vastly underestimated due to the difficulty of tracking 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 impacting a wider host range and not ending up like it did in wild birds,” Dr Hill said. “It’s held in wild birds, and that’s a scary prospect. For many of us in this field, my God, what do we do when we encounter a wild animal that there’s no control over? »
It has long been assumed that the primary hosts of bird flu are dabbling ducks, such as mallards, teals and shovelers, which feed on and just below the surface with their rumps up. They are essential for the spread because they have mild or no symptoms and they spread it very far. The new study, however, found that other birds, such as geese, played an underappreciated role due to their natural history.
“Geese are a bit more tolerant of areas disturbed by humans,” Dr Hill said. “Imagine a commercial poultry farm or a backyard operation where they spread grain.” It attracts “geese and other scavenger birds, like gulls, crows and magpies, so there’s an interface between them,” she said.
The unique natural history of the black-backed gull, the largest gull in the world, for example, plays a role in transmission. “Gulls were really rare hosts for the highly pathogenic forms of the virus,” Dr Hill said. “When they wore it, those rare occasions, they spread it very quickly. There’s nothing like a seagull to disperse the virus really quickly and over very 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 pathways taken by other wildlife-damaging viruses.
“Knowing that gulls, geese and ducks can move this virus in different ways is a great contribution to understanding or possibly more accurately modeling how to expect a virus like this to spread,” said said Jonathan Runstadler, professor and chair of the Department of Infectious Diseases and Global Health at Tufts University’s Cummings School of Veterinary Medicine and co-author of the paper.
The data “allows us to predict if a virus emerges, when that bird might enter North America, and which bird populations we might target for surveillance to detect it,” Dr. Runstadler said.
This year’s highly pathogenic bird flu line originated around 1996, first being discovered in a domestic goose in China. Since then, it has been circulating worldwide in wild and domestic birds, evolving as it moves from host to host.
In 2005, after a decade of evolution, the strain caused a major outbreak among wild birds in wetlands in China.
The strain first appeared in the United States in 2014, traveling in migrating birds from Eurasia across the Pacific to Alaska and further east, causing outbreaks on US poultry farms that led to the death of 40 million turkeys and chickens.
After reaching the Midwest, however, mass culls halted it, eliminating viral spread for both wild and domestic populations.
“We don’t have a vaccine,” Dr Hill said. “All we have in our toolkit is swapping all of our poultry, which is awful, but to some extent it’s been successful.”
But killing infected poultry didn’t work this time around, in part because the virus was able to find a home in so many wild birds, spawning the biggest bird flu outbreak of all time.
In some places, authorities have warned chicken farmers and even people who raise backyard flocks to keep their birds indoors, while in other places the threat appears to have passed.
“This virus is so good because it jumps back and forth between wild and domestic,” Dr Hill said. “There is no better way to amplify a virus than to take a wild reservoir and domesticate a close relative. That’s exactly what we did with chickens and ducks. Highly pathogenic forms of the virus only occur when the virus enters livestock.
In the Magdalen Islands, Quebec, wildlife officials recently discovered the carcasses of thousands of white gannets decimated by the flu.
There is no way to predict whether flu epidemics will decrease or worsen.
Certain species, such as raptors, seabirds and shorebirds, are also at high risk of catching the virus due to their behaviors. Dozens of bald eagles are known to have died of the flu, largely because they feed on ducks and other birds carrying the pathogen.
Birds that congregate in large numbers are also at risk. “There are lots of birds flocking – shorebirds, terns and seabirds – forming massive, massive groups and this could be a field day for the virus,” Dr Hill said.
The extent of the devastation of various species is difficult to assess, as monitoring is lacking. Better tracking along migration routes would help experts find ways to mitigate the spread of the virus.
The deaths of large numbers of shearwaters and other seabird species have been reported along the Atlantic coast in Massachusetts, Maine, New Hampshire and Connecticut. Bird flu is a suspect, although tests haven’t confirmed it.
“The geographic extent of the detections, the number of species we are getting with the detections, the amount of disease we are seeing in wild birds, all of this is unprecedented,” said Andy Ramey, a wildlife geneticist. US Geological Survey research center in Alaska. who studies avian flu. “It’s uncharted territory and hard to know what to expect.”
There are also concerns that during this year’s breeding season for many species, parents could transmit the disease to offspring in the nest, which 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 boost their immune system.
A monitored endangered species is the roseate tern on Buzzards Bay off the coast of Massachusetts. Testing is just beginning and no sick birds have yet been found.
“It looks like this is going to be a tough feeding year for terns,” said Carolyn Mostello, coastal bird biologist for the Massachusetts Division of Fish and Wildlife. “Nesting was slow. Hopefully we don’t have a combination of poor food resources and bird flu; who could act together to really harm the populations.
Experts say bird flu poses a very low risk to humans and so far it has only been detected in two humans. However, as it persists and evolves, it could acquire the ability to pose a serious threat of spreading to humans.
Dr Hill said a major handicap to better understanding the outbreak was the lack of funding for efforts to track the spread. “The surveillance is really, really, really bad,” she said. “We spend very little money and time anticipating this.”