At bird breeding grounds this spring and summer, the skies have been clearer and quieter, the flocks drastically thinned. Last year, more than 60 percent of the Caspian terns at Lake Michigan vanished; the flock of great skuas at the Hermaness reserve, in Scotland, may have shrunk by 90 percent. Now more broken bodies are turning up: a massacre of 600 arctic-tern chicks in the United Kingdom; a rash of pelicans, cormorants, gulls, and terns washed up along West African coasts. In recent months, Peruvian officials have reported the loss of tens of thousands of pelicans—by some estimates, up to 40 percent of the country’s total population.
The deaths are the latest casualties of the outbreak of H5N1 avian flu that’s been tearing its way across the world. In the past couple of years, more than 100 million domestic poultry have died, many of them deliberately culled; out in the wild, the deaths may be in the millions too—the corpses have just been too inaccessible and too numerous for scientists to count. “It’s been carnage,” Michelle Wille, a virologist at the Peter Doherty Institute for Infection and Immunity, told me. “For many species, we are losing decades of conservation work.”
For months, experts worried most about the outbreak’s magnitude—it struck so swiftly and lethally that it’s become the most deadly H5N1 epidemic recorded in North America. Now the looming concern is length: when, and if, the virus will withdraw.
History would seem to be on our side, at least in North America: Earlier versions of avian flu that made it to this side of the world flamed out within a year or two, quashed by the ebb and flow of migrating hosts and by concerted poultry culls. But this new strain is stubborn, pounding the continent more or less continuously. “That has never happened before,” Vijaykrishna Dhanasekaran, a virologist at Hong Kong University, told me.
[Read: Eagles are falling, bears are going blind]
The situation has grown dire enough that last month, the United States began, for the first time in history, to offer avian-flu vaccines to birds—starting with critically endangered California condors, which have lost more than 20 members of their very small population to the virus and cannot afford more, Ashleigh Blackford, U.S. Fish and Wildlife Service’s California-condor coordinator, told me. This choice is one of many grim and tacit acknowledgments that this virus is now so entrenched that it’s almost certain to circulate here indefinitely.
The longer the virus lingers, the greater the chance that it will pose a different danger: permanent tenancy among mammals, a group that, historically, the virus has not easily infected and spread among. Some experts worry that the virus has now managed to establish new methods of transmission in select communities of mink and foxes on fur farms, and maybe in wild seals. The chances of an outbreak among people—certainly, of another pandemic—are still, in absolute terms, low, Nídia Trovão, a virologist at the Fogarty International Center, told me. But the more new places H5N1 establishes itself for good, the more its threat to us will grow.
Scientists can’t yet say why this particular flu virus has found such unprecedented success in North America, only that it has. In the two years since its arrival, it has infiltrated more birds and mammals—including species not previously known to be vulnerable to avian flus—than has any other pathogen of its ilk. Among the most affected creatures have been wild birds, a major departure from previous strains that primarily attacked poultry. And although past viruses in this family have been relatively slow to evolve, this one keeps amping up its genetic diversity by mixing its genome with bits of other bird-borne flus—tweaks that may be helping H5N1 find even more new hosts and execute further genomic changes still.
Not all infected with H5N1 develop terrible disease—and in recent months, researchers have even spotted signs that the outbreak is on the wane. In certain parts of the U.S. and Europe, case counts in the wild seem to be slumping. And scientists are stumbling upon gannets in Scotland carrying antibodies to the virus—and whose eyes have, in the aftermath of presumed infection, bizarrely changed color from blue to black. Wendy Puryear, a virologist at Tufts University School of Veterinary Medicine, is now searching for similar hints of immunity in gray seals. Should this pattern continue, there’s hope that the virus, at least in this part of the world, could “just fizzle out,” Dhanasekaran said.
Wille and other researchers, though, are looking toward autumn with a degree of dread. Another surge of disease could appear then, especially as seasonal migrations return birds to their overwintering homes. Even without a fall comeback, the math is against elimination in North America, where the virus has infiltrated well over 100 species. At this point, H5N1 lingering in at least some creatures seems far likelier than a full and permanent retreat.
For people, the news is still, tentatively, okay. “In its current form, the virus has very, very low transmissibility to humans,” with just a handful of cases detected, says Ian Barr, the deputy director of the World Health Organization Collaborating Centre for Reference and Research on Influenza. But the list of mammals catching the virus is still growing. In some of these outbreaks, the root cause is likely avian, experts told me: cats eating raw poultry; raccoons, skunks, and other scavengers presumably snarfing the carcasses of infected birds. Other cases, though—seals at sea, foxes and mink crowded into fur farms—seem better explained by mammals catching H5N1 directly from another of their own kind. Given the sheer number of animals involved, “it’s hard to fathom that each was individually exposed to an infected bird,” Louise Moncla, an avian-influenza expert at the University of Pennsylvania, told me.
[Read: We vaccinate animals more than ourselves]
Since the virus arrived in North America, it also appears to have become deadlier, with particularly devastating effects when it infiltrates certain creatures’ brains, Richard Webby, a virologist and the director of the WHO Collaborating Centre for Studies on the Ecology of Influenza in Animals and Birds, told me. Fox kits have been found racked with tremors; bears have gone partially blind.
To truly transform into an even worse scourge, the virus would need to shape-shift from a pathogen that primarily plagues bird guts into one that could easily infiltrate mammalian lungs. The virus has already taken at least one step toward that: Researchers sampling H5N1-infected mammals, including foxes, mink, cats, seals, and even a person, have detected genetic tweaks that are helping the microbe replicate inside mammalian cells. But a boost in infective capability alone isn’t enough for a large-scale outbreak, Webby said. The virus would probably need to circumvent some mammal-specific immune defenses. And it would certainly need to acquire the ability to more efficiently transmit between mammalian hosts via, say, an airborne route. There’s not yet evidence, Webby told me, that the virus has pulled off such changes, though just a few might suffice: better binding to receptors on mammalian lung cells, a more stable structure so it could travel via aerosol.
That said, the criteria for properly mammalizing a flu virus “are not really well understood,” Moncla said. All that’s certain is that the chasm between birds and mammals is not uncrossable.
No single intervention will quash H5N1’s potential pandemic threat. Avian-flu vaccines have been developed for humans, but for now, Trovão, of the Fogarty International Center, told me, the priority is keeping the virus out of cramped, often unsanitary communities of mammals where it could rack up further mutations. Vaccinating poultry, too, would help, says Mariette Ducatez, a virologist at the National Veterinary School of Toulouse. The practice is already relatively common in countries where avian influenza is endemic, including China, Egypt, and Vietnam. This fall, France is likely to introduce shots for a subset of its domesticated birds; regulators in the U.S. have mulled eventually doing the same.
But avian-flu vaccines can be expensive, difficult to administer, and better at tempering disease severity than blocking spread. Without vigilant monitoring, the virus could spread silently in vaccinated flocks, especially those with spotty coverage, where the microbe could be pressured to evolve in new ways. On the global market, many poultry buyers are also reluctant to import vaccinated birds.
The stakes for vaccinating wild animals are less economic but no less fraught. And although researchers believe that the recently inoculated California condors are tolerating the new shots well, other species aren’t necessarily next in line. In most cases, vaccinating wild animals just isn’t practical, Trovão told me. Even for the condors, which are already closely monitored by scientists and used to frequent human contact, shots are “not sustainable, really, not long term,” Samantha Gibbs, a wildlife veterinarian at U.S. Fish and Wildlife, told me. Scientists still can’t say for sure how well the vaccines will work in the birds or how long protection might last. “This was a last resort,” Gibbs said, “just an attempt to get them through this.”
But with H5N1 still raging and the birds still vulnerable, Gibbs and her team suspect that they’ll have little choice but to continue vaccinating the condors on an annual basis. If this current threat doesn’t force their hand, maybe the next one will. Already, another bird flu appears to have been imported from across the Atlantic—an H5N5 that’s been detected in North American raccoons and gulls.
