This year’s epizootic of highly pathogenic avian influenza has killed tens of millions of poultry as well as unknown numbers of wild waterfowl and raptors.
As of May 23, U.S. Department of Agriculture officials had recorded about 350 outbreaks among domestic birds in 35 states, with about 40 million birds dead from disease or from depopulation to control spread of the virus to more flocks and reduce the risk to humans. The highly pathogenic H5N1 avian influenza strain circulating in the U.S.—and much of the world—is unusual for also causing disease and deaths among a range of wild birds such as ducks, crows, and eagles, with crows and eagles likely becoming infected from eating remains of infected birds.
Federal and state agencies have recorded infections in 67 species, about triple the number found during the previous outbreak of highly pathogenic avian influenza in the U.S., in 2014-15.
Dr. Louise Dufour-Zavala, president of the American Association of Avian Pathologists, said the losses of domestic birds would be even higher if the poultry industry hadn’t improved its biosecurity since 2015. She said the number of outbreaks in commercial poultry houses reflects the wide geographic spread of the influenza virus, the high virus load in the environments near the poultry houses, and the diversity of wild bird species infected.
The egg industry has been the hardest hit, with about 30 million egg-laying hens killed. Dr. Dufour-Zavala said those losses have created a shortage of breaker eggs, which are used for liquid or powdered eggs, whereas the price and availability of meat from broiler hens remained largely unchanged. The outbreaks also had reduced the availability of turkey meat, but the effects were less severe than in the egg industry, she said.
The crisis also has been devastating for veterinarians, bird owners and growers, emergency responders, and poultry caretakers. Ending the lives of these animals, especially after caring for and protecting them, is difficult, stressful, and depressing, she said. Dr. Dufour-Zavala recounted conversations with veterinarians who were grieving the losses along with the farmers and those who kept poultry as pets.
“They take care of these animals,” she said. “They get attached to these animals. And it’s been very difficult.”
One person in the U.S. has tested positive for the H5N1 virus, and the Centers for Disease Control and Prevention considers the virus to be low risk for public health.
Animal health authorities in the U.S. and Canada also have identified H5N1 infections in wild red foxes that developed neurologic disease and died. In a May 13 announcement, Dr. Lindsey Long, Wisconsin Department of Natural Resources wildlife health veterinarian, indicated there was no evidence so far that foxes are significant sources of transmission of the virus.
Arrival from Europe
The strains of highly pathogenic H5N1 avian influenza virus currently circulating in North America were identified in Europe in fall 2020, and they spread through the continent and into Africa, the Middle East, and Asia, according to the CDC. They have been the dominant subtype globally since fall 2021.
The H5N1 virus was first detected in wild birds of eastern Canada in late 2021 and the U.S. East Coast in January 2022. In February, a turkey flock in Indiana became the first commercial flock in the U.S. with a confirmed outbreak.
Dr. Dufour-Zavala said that, unlike past epizootics that waned as the weather warmed, Europe’s outbreaks continued throughout 2021. That raises concerns North America’s outbreaks, too, could continue this year.
Bryan Richards, emerging disease coordinator for the U.S. Geological Survey’s National Wildlife Health Center, said the H5N1 virus appears to have spread from Europe to the U.S. through contact among migratory birds, potentially wild birds in Iceland.
Richards said the virus shares its lineage with the H5N2 and H5N8 viruses that emerged in 2014-15, which also were descendant from an H5N1 avian influenza virus that emerged in 1996 among farmed geese in southern China.
Richards said that in 2014-15, wildlife authorities typically found no more than two or three dead wild birds at a time, with some rare exceptions such as the deaths of eight goslings in Michigan. This year’s wildlife deaths have included larger die-offs.
The USGS has received reports of die-offs among snow geese in the Great Plains, often with more than 1,000 deaths in a single location and some surviving birds bobbing their heads and seeming disoriented. More than 1,000 lesser scaup also died in a single count in east-central Florida, which led to the deaths of vultures and other scavenger birds that ate the contaminated carcasses.
The disease also has been killing bald eagles, great horned owls, and red-tailed hawks, Richards said.
“Now these sorts of events are occurring in Saskatchewan, Alberta, likely Manitoba as well,” said Richards, who described a trail of disease and deaths moving “on the wings of white geese as they migrate north.”
Richards said the data so far do not suggest the virus is endangering the survival of any wild species in North America. None of the confirmed infections involves endangered whooping cranes, for example.
Wildlife infections precede poultry deaths
Pennsylvania had its first confirmed H5N1 infection March 24, when test results came back for a bald eagle found dead in the eastern half of the state. Dr. Kevin Brightbill, state veterinarian for Pennsylvania, said that, soon afterward, ducks in a group in Venango County, to the north of Pittsburgh, were unable to take off from the water and had neurological signs of disease. They, too, were positive for infection.
The disease reached the first commercial egg-laying hens by mid-April in Lancaster County, west of Philadelphia, and that county since had eight outbreaks in layer and broiler houses. Across the state, about 4 million egg-laying hens, 190,000 commercial ducks, and 70,000 broilers have died from infections or depopulation. The state has the third-highest losses behind Iowa and Nebraska.
“I’m happy to tell you that, in most cases, our producers are being very vigilant, and we are catching disease before we’re seeing large portions of the flock affected,” Dr. Brightbill said. “However, when we are not catching the disease early enough, we are seeing an abrupt uptick in both mortality and morbidity on these farms.”
The Pennsylvania Game Commission also has received reports of die-offs among turkey vultures, and Dr. Brightbill’s counterpart in Maryland told him turkey vultures in that state, too, had tested positive for infection.
Dr. Brightbill said these are tough times for farmers, animal health emergency responders, and the communities that support them. About 200 state and federal responders are working with veterinarians, the poultry industry, and academics to respond and support those dealing with infected flocks. That support includes a hotline to provide mental health support to farmworkers and their families.
Dr. Brightbill said biosecurity is paramount for disease prevention, and producers should report any concerns to their veterinarians.
Dr. Dufour-Zavala said the veterinarians and others who are responding to infections for days to weeks at a time struggle with such difficult work, even though they know they are doing what is best for the overall bird populations by helping prevent further spread of the virus. Most veterinarians love nature, she said, and seeing the effects of this virus on wildlife adds to their pain.
But she said veterinarians also know they are supported, and they are enduring this challenge together. She expects the poultry, avian, and wildlife veterinary communities will share experiences from this response and examine ways to better prevent influenza infections, fight the disease, and stop its spread.
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