Publisher Health

When a highly contagious strain of avian influenza began racing across the United States this spring, the Raptor Center at the University of Minnesota, prepared for an influx of ailing birds.

“But we never could have anticipated the flood of patients that arrived,” said Dr. Victoria Hall, executive director of the center, which provides medical care for birds of prey.

From late March to early June of this year, Dr. Hall and her colleagues saw more than 180 flu-afflicted birds, including scores of great horned owls, red-tailed hawks and bald eagles. Many were severely ill, suffering from seizures or unable to see or stand. Caring for these animals — just one of which survived — was emotional, draining work that required long hours in personal protective equipment, including Tyvek suits and respirators.

So it came as an immense relief when cases tailed off this summer, falling to just one in July and zero in August.

But in September, the sick raptors returned. Last month, the center had 11 confirmed cases of the avian flu, Dr. Hall said: “It’s definitely coming back through.”

This year’s avian influenza outbreak is the worst the United States has experienced since 2015, the last time the country was hit hard by bird flu. So far, the virus has affected 47 million farmed birds, nearly the same number as in the 2014-15 season.

But by all other measures, this year’s outbreak is “radically different than what we’ve seen before,” said Bryan Richards, the emerging disease coordinator at the National Wildlife Health Center, which is part of the U.S. Geological Survey.

This year’s is more wide-ranging, spreading to almost every state, and is having a much greater effect on wild birds, infecting more than 100 species in North America, according to Mr. Richards. (The virus currently poses little risk to humans, experts say.)

And, unlike in 2015, the virus did not fizzle out over the summer. Instead, it continued to circulate in wild birds, many of which spend their summers in the Arctic.

Now, as wild birds fly south for the winter, they are bringing the virus with them. Cases are climbing again in a number of northern states and popping up for the first time in some southern ones, such as Mississippi, Arkansas and New Mexico.

Poultry farms are familiar with the risks of avian influenza, but for many zoos, rehabilitation centers and facilities that house wild birds, it is a formidable new threat. They are bracing themselves for a resurgence — and wondering whether the virus is here to stay.

“We’ll just have to be on alert,” said Dr. Trevor Zachariah, the director of veterinary programs at Florida’s Brevard Zoo, where infected wild vultures keep finding their way onto the property. “We may have to live with this.”

Spring surge

Avian influenza wings its way around the world in the bodies of migrating birds, especially in water birds such as ducks, geese and gulls. Some of these birds, which can carry the virus without showing symptoms, share summer habitats in the Arctic, where they may swap pathogens, picking up new flu strains.

“It’s like bringing the kids to day care from different suburbs,” said Dr. Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory. “As they commingle, that’s where the virus moves around on a global scale.”

As infected birds migrate, they shed the virus in their feces, mucus and saliva, seeding infections in flocks of farmed fowl, in which some flu strains can be fatal.

The current outbreak began in late 2021, when a highly pathogenic strain of flu known as Eurasian H5N1 was detected in birds in Eastern Canada. The virus made its way down the Atlantic coast to Florida and then exploded this spring, when migrating birds carried the pathogen north and west.

Along the way, it not only infiltrated hundreds of commercial flocks but also began felling wild birds, taking out entire families of owls and triggering die-offs of geese and gulls.

“We’ve never seen anything like this before,” said Ashton Kluttz, the executive director of the Bird Rescue Center in Sonoma County, Calif., which created extra makeshift hospitals to handle the patient load.

(The virus also found its way into mammals, from foxes to seals.)

Rehabilitation centers, sanctuaries and zoos have scrambled to put new precautions in place, moving vulnerable birds inside and suspending public programs. After Tri-State Bird Rescue & Research, in Delaware, identified its first case in February — triggering a three-week quarantine — it adopted new disinfection protocols and stopped accepting waterfowl from other rehabilitation centers.

“We have — fortunately, knock wood — not had an outbreak at our center,” said Lisa Smith, the rescue’s executive director. But, she added, “We know the virus is still out there.”

Fall flights

Cases ebbed in many places this summer, but the virus never completely disappeared. Now, it’s on the move.

Blue-winged teal, a species related to ducks, are among the first species to fly south in the fall. In mid-September, three hunter-harvested teal tested positive for the virus in Mississippi, marking the first detections in that state. The finding affirms that, “Yes, the virus persisted in northern latitudes through the summer,” Mr. Richards said. “And it’s coming back on the wings of wild waterfowl.”

Wisconsin began to see a spike in wild bird infections around Labor Day, Dr. Poulsen said, and recently documented its first poultry outbreaks since May. “We were really hoping that we were out of the woods,” he said.

The virus appears to be making a comeback in other northern regions, including Minnesota, North Dakota and Alberta, Canada. So far, experts have said, the surge has been smaller than it was in the spring. “But there’s lots more geese and ducks to come down out of the Arctic,” said Margo Pybus, a provincial wildlife disease specialist at Alberta’s Fish and Wildlife Division.

Farther south, wildlife facilities are preparing for a flood of fall migrants. Southern California was spared in the spring, but Dr. Hendrik Nollens, the vice president of wildlife health at the San Diego Zoo Wildlife Alliance, does not expect to be so lucky this fall.

The San Diego Zoo and its sister facility, the San Diego Zoo Safari Park, covered their outdoor aviaries and removed food and water sources that might attract avian interlopers, even going so far as to drain a pond popular with wild ducks. They also moved 900 birds into more protected habitats — “a herculean effort” that involved relocating hundreds of flamingos, Dr. Nollens said.

To pull off the feat, employees guided many of the flamingos into enclosed trailers, which delivered the animals to covered or indoor habitats. Staff members modified these new spaces to suit the leggy wading birds, adding feeding pools, trimming low-hanging tree branches and raising the sprinklers designed to keep their feathers in fine fettle.

When the flamingos will return home is unclear. “When do we stop doing these things is actually a tough challenge,” Dr. Nollens said. “We don’t know what’s going to happen from here.”

The long haul

The signs from Europe, where the outbreaks began earlier than they did in North America, were not encouraging. There, the virus has persisted through multiple migration cycles, fueling “the largest avian flu epidemic” ever observed on the continent, according the European Center for Disease Prevention and Control.

Although the scope and scale of future outbreaks is hard to predict, scientists said that they expected the virus to persist through the winter, then to travel north again in the spring. “I don’t see any reason to suggest this is going to go away anytime soon,” Mr. Richards said.

And if the virus does stick around? “It is going to have huge implications for anybody with either permanently captive or transitory captive wildlife in their care,” Dr. Hall said.

The Raptor Center plans to continue testing all incoming patients for the virus indefinitely and is considering building a more permanent triage and quarantine area, she added. In California, the Bird Rescue Center recently decided to add an avian influenza unit to its new facility, which is not slated to open until at least late 2023.

The Oregon Zoo, which moved many of its birds indoors in August, does not want to keep the animals cooped up for long. “It’s just not good welfare,” said Dr. Carlos Sanchez, the zoo’s head veterinarian. “They need the space to move. They need the sun.” So employees are working to reopen aviaries with new precautions in place, such as foot baths where visitors can disinfect their shoes.

In the long run, managing the risks may require facilities to be watchful and nimble, tightening and loosening precautions as outbreaks flare and fade.

At the Pittsburgh Zoo & PPG Aquarium, for instance, employees will return to wearing full P.P.E., as they did this spring, if the virus is reported in the surrounding county, said Dr. Justin Rosenberg, an associate veterinarian at the zoo.

After the spring surge, combined with more than two years of coronavirus-related precautions, there is definite “preparedness fatigue,” Dr. Rosenberg acknowledged.

But the zoo is eager to protect its avian charges, which include flamingos, ostriches and a 3-year-old duck with “one of the best names that I have encountered,” Dr. Rosenberg said. “We are doing whatever we can to keep Ritz Quacker safe.”