As an outbreak of highly pathogenic H5N1 bird flu spread across North America this spring, researchers hoped for a repeat of what happened after another strain of bird flu arrived in the United States in December 2014. Although more than 50 million birds died or were destroyed in a matter of months, costing farmers more than $1.6 billion, by June 2015, the virus was essentially gone. Poultry outbreaks ended, wild birds stopped dying, and migratory waterfowl didn’t bring the virus back when they returned from their summer breeding grounds in Canada.
But this time it’s different. H5N1 infections in both wild bird and poultry have continued over the past summer in parts of the United States and Canada, with hopes that warmer temperatures will halt the spread. And while the 2015 outbreak primarily affected poultry farms in the Midwest, H5N1 has spread to practically the entire continental United States and infected at least 99 wild bird species, a record. Whether migratory birds will trigger additional introductions in the fall is “a million-dollar question,” said Bryan Richards, coordinator for emerging diseases at the U.S. Geological Survey’s National Wildlife Health Center.
Even if they don’t, scientists worry that the virus could continue to circulate year-round, posing a permanent threat to poultry farming and wild birds, including several endangered species. “The effects on wild birds can last for a very, very long time,” Richards says. Europe can show us what to expect: H5N1 has already become a fixture in wild birds there and has caused increasing outbreaks over the past 3 years, causing record damage to the poultry industry.
H5N1 first appeared in poultry in China’s Guangdong province in 1996 and has since caused several major outbreaks around the world. It has evolved to infect waterfowl species without causing significant damage, allowing the birds to spread the virus far and wide. In the current outbreak, waterfowl are thought to have transmitted the virus from Europe to Canada and then along the east coast. Bald eagles, owls and other predators died after eating infected waterfowl. In February, H5N1 reached Mississippi’s flight path, where snow geese and other species are migrating into northern Canada. Meanwhile, it infected poultry farms, forcing farmers to kill 40 million chickens and turkeys. Later in the spring, the virus slowly moved west.
To date, H5N1 has been detected in more than 2,000 wild birds in the United States, compared to just 99 during the 2015 outbreak; biologists suspect that the virus is much more transmissible than its predecessors. “It just exploded across the breadth of the species it was observed in,” said Wendy Puryear, a wildlife virologist at Tufts University.
In May, infestations started to decline, although some species were still affected. Black vultures, which pick up H5N1 when they scavenge carcasses, still die by the hundreds, says Rebecca Poulson, a wildlife disease researcher with the Southeastern Cooperative Wildlife Disease Study at the University of Georgia. “It still hits those scavengers pretty hard,” she says. And in June, researchers in New England were surprised when a second wave of infections hit seabirds. “Suddenly it was like a switch had been turned,” says Puryear.
Seabirds are particularly vulnerable because many nest in dense colonies. Gannets populations crashed in parts of eastern Canada, as they did in Europe. In Lake Michigan, Caspian terns – locally endangered – were very badly affected. H5N1 rarely infects mammals, but this wave has killed hundreds of harbor seals in Maine; Puryear and colleagues are trying to figure out whether the virus can spread between seals or whether they are all infected by birds or their feces. The United States and the United Kingdom have each seen one human H5N1 case so far.
Now all eyes are on the migratory birds, which fan out over a wide area as they return to the United States from the north and can spread the virus widely. Canadian Wildlife Service (CNS) researchers have collected samples from 1,000 snow geese at their Arctic breeding grounds, but testing them for H5N1 could take another month or two, says CNS waterfowl biologist Jim Leafloor. U.S. federal and state biologists are testing live and hunter-killed migrating ducks and geese.
Regardless of whether a new wave of infections is coming from the north, many researchers say the virus has already established itself in some parts of the United States. If those areas overlap large poultry farming areas, the consequences could be serious. Farmers can be at constant risk of major losses, and Richards says they need to maintain or tighten biosecurity measures, such as careful cleaning of boots and equipment.
There is still much to learn. In wild birds, for example, how H5N1 moves from one individual to another and between species is still a mystery, says Tufts wildlife virologist Kaitlin Sawatzki. “It’s going to be a very complicated story,” she says. “It’s hard to predict and we’re nervous.”