A great black-backed gull that migrated from Europe to eastern Canada last winter may have been the first vector to North America of deadly avian influenza, which has killed tens of millions of domestic fowl and devastated wild bird populations.
The large-scale outbreaks have provided researchers with a new opportunity to refine their understanding of the disease by examining which wild bird species, behaviors and ecologies play key roles in transmission.
“Previous studies looking at avian flu made these big categorizations of wild and domestic birds,” said Nichola Hill, an assistant professor of biology at the University of Massachusetts Boston and lead author of a new paper on the subject.
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But “wild birds are incredibly biodiverse,” she said, adding that “each of them has a unique natural history and behavior.”
For example, if you know which migratory species carry the pathogen, you can predict when and where it might arrive based on the migration routes.
After the migratory gull made landfall, the highly pathogenic bird flu, also known as the H5N1 virus, spread across North America. More than 77 million poultry, most raised in cramped conditions that have fueled the spread and development of the virus, have been culled in dozens of countries.
For some experts, the toll this H5N1 strain is taking on wild birds – it has afflicted more than 100 species to date – was alarming and unprecedented in its depth and breadth. In wild birds, spread can be difficult to contain, posing a greater risk of spreading to other wildlife. And some wild bird species, like cranes and some seabirds, are particularly at risk, particularly those with low reproductive rates and those that are already endangered.
The World Organization for Animal Health estimates that more than 383,000 wild bird deaths have been attributed to the virus since October, although the number may be a gross underestimate due to the difficulty of tracking sick and dead birds.
The pathogen has spread rapidly across regions and species, at rates much higher than during the last outbreak in 2014-15.
“It’s affecting a wider host range and isn’t ending up like it used to be in wild birds,” Hill said. “It’s maintained in wild birds and that’s a scary prospect. Many of us in this area, my God, what do we do when we encroach on a wild animal that has no control?
It has long been thought that the primary hosts for avian influenza are pond ducks such as mallards, teals and shovelers, which feed on the surface and just below with their hindquarters in the air. They are critical to spread because they have mild or no symptoms and carry them far and wide. However, the new study found that other birds, such as geese, played an underestimated role due to their natural history.
“Geese are a little more tolerant of human-disturbed areas,” Hill said. “Imagine a commercial poultry farm or backyard operation where grain is distributed.” That “attracts geese and other scavengers like gulls, crows and magpies, so there’s an interface between them,” she said.
For example, the unique natural history of the African Black-legged Gull, the largest seagull in the world, plays a role in the transmission. “Gulls have been really rare hosts for highly pathogenic forms of the virus,” Hill said. “If they carried it with them, on those rare occasions, they spread it very quickly. There’s nothing like a seagull for really quick spread of the virus and really long distances. They will get a tailwind and will cross the Atlantic in 24 hours.”
The study could help other researchers track not only the further spread of this year’s pathogen, but also the routes of other viruses that are harmful to wildlife.
“Knowing that seagulls, geese and ducks may transmit this virus in different ways goes a long way towards understanding or eventually modeling more accurately the spread of such a virus,” said Dr. 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 article.
The data “allows us to predict if a virus will emerge, when this bird might enter North America, and what bird populations we might target for surveillance to detect it,” Runstadler said.
The highly pathogenic lineage of this year’s avian influenza emerged around 1996 and was first found in a domestic goose in China. It has since circulated in wild and domestic birds around the world, evolving as it migrates from host to host.
In 2005, after a decade of evolution, the strain caused a major outbreak in wild birds in wetlands in China.
The strain first appeared in the United States in 2014, migrating 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 turkeys and chickens.
However, after it reached the Midwest, it was stopped by mass culls, eliminating the spread of the virus for wild and native populations.
“We don’t have a vaccine,” Hill said. “All we have in our toolbox is replacing all our poultry, which is terrible but has been successful to some extent.”
But killing infected poultry hasn’t worked this time, in part because the virus has made a home in so many wild birds and spawned the largest outbreak of avian flu.
In some places, officials have warned chicken producers 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 oscillates between wild and domestic,” Hill said. “There’s 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.”
On Quebec’s Magdalen Islands, animal rights activists recently discovered the carcasses of thousands of white gannets that had been wiped out by the flu.
It is not possible to predict whether the flu epidemics will decrease or worsen.
Some species such as raptors, seabirds and shorebirds are also at high risk of contracting the virus because of their behavior. Dozens of bald eagles are known to have died from the flu, mostly because they hunt ducks and other birds that carry the pathogen.
Birds that gather in large numbers are also at risk. “There’s a lot of flocks of birds — shorebirds, terns, and seabirds — forming huge, huge groups, and that might just be a field day for the virus,” Hill said.
The extent of the devastation of different species is difficult to assess due to the lack of monitoring. Better tracking along migration routes would help experts find ways to stem the spread of the virus.
Deaths 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 a suspect, although tests have not confirmed this.
“The geographic scale of the discoveries, the number of species we get with the discoveries, the amount of disease we’re seeing in wild birds, it’s all unprecedented,” said Andy Ramey, a wildlife geneticist with the US Geological Survey in Alaska of the studied bird flu. “It’s uncharted territory and hard to know what to expect.”
There is also concern that during this year’s breeding season for many species, parents could pass the disease on to their offspring, which have underdeveloped immune systems. Young wild birds are often exposed to low pathogenic viruses, which are widespread and can act almost as a vaccine to boost their immune systems.
One critically endangered species that is being monitored is the Roseate Tern in Buzzards Bay off the coast of Massachusetts. Testing has just started and no sick birds have been found yet.
“It seems to be a difficult feeding year for the terns,” said Carolyn Mostello, a shorebird biologist with the Massachusetts Division of Fisheries and Wildlife. “Nesting was slow. Hopefully we don’t have a combination of poor food resources and bird flu; which could act together to cause real harm to the population.”
Experts say bird flu poses a low risk to humans and has only been detected in two people so far. However, if it persists and evolves, it could acquire the ability to pose a serious threat to humans.
Hill said a major obstacle to a better understanding of the outbreak has been the lack of funding for efforts to track the spread. “Surveillance is really, really, really bad,” she said. “We’re spending very little money and time driving this forward.”
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