More than three years after the avian flu pandemic, the virus continues to spread in the United States, with growing impacts on food production and livestock, affecting more than 80 million chickens, thousands of wild birds, and dozens of mammal species, including polar bears.
The virus is now raging among dairy cows, having been identified in 94 herds in 12 states since March, with the most recent animal to test positive being an alpaca on an Idaho farm.
“The scope and scale of this disease presence is enormous,” said Julianna Lennock, national coordinator for the USDA’s Wildlife Disease Program.
The scale and related concerns are reflected in rising egg prices, new warnings to thoroughly cook ground beef and eggs, and calls for dairy and poultry farmers to take extra measures to prevent the spread of the disease.
As the outbreak continues, concerns are growing about the risks to humans and whether rising temperatures and extreme weather could exacerbate this and future pandemics.
How widespread is this bird flu outbreak?
The highly contagious H5N1 virus has spread to six continents since it was first detected in Europe and Asia in 2020. It has been reported in much of North, Central and South America and has been found on every continent except Australia. It was also found in Antarctica last fall, raising alarms about the potential impact on penguins, one of the world’s most beloved bird species.
Timeline:From chickens to foxes, bird flu spreads across the US
The United States has had bird flu outbreaks before, but this one is longer-lasting and more widespread, affecting commercial, home, and farm flocks in every state except Louisiana and Hawaii, with more than 5.9 million birds infected since May 1 alone.
Since 2022, 14 million turkeys and 80 million chickens (including 71 million laying hens) have been reported to be infected. Poultry farmers must cull chickens or turkeys from their flocks if they test positive. Experts say the culling to prevent human infections is driving up egg prices.
Affected mammals have been found in 31 states, with the most commonly infected animals being foxes, rats, skunks, mountain lions, cats and harbor seals.
Research studies have found that the longer the virus exists and spreads, the greater the risk of genetic mutations that could make it more easily transmitted from animals to humans and between humans.
“The longer the virus is around, the more likely it is that changes will occur,” said Lennock, who oversees a federal program charged with tracking the virus in wild birds.
Can humans be infected with bird flu?
Yes, but the risk in the United States remains very low, federal officials reiterated in a briefing Thursday, saying the public should “be vigilant, but not fearful.”
Four people have tested positive in the United States since the virus arrived in the winter of 2021-2022. All had contracted the virus on farms. In the first case of 2022, a worker had helped remove infected poultry on a farm. All three patients this year had contact with dairy cows. Two reported only conjunctivitis or pink eye, while the third also experienced upper respiratory symptoms. No one has died from the virus in the United States, according to federal officials, but deaths have been reported internationally.
For now, ground beef and eggs are all considered safe for the general public, except for raw milk, as long as they’re thoroughly cooked, according to federal officials, which will kill any virus that may remain in egg yolks or ground beef once the infected dairy cows are shipped to market and slaughtered.
Pasteurization of milk kills active viruses and prevents infection, but leaves behind traces of harmless viruses that are estimated to be present in about 20 percent of the nation’s milk supply, according to a federal study.
The Food and Drug Administration says it’s unclear whether the virus can be transmitted through raw milk, but has urged states that allow the sale of raw milk to restrict sales as a precaution.
“The fact that 20 percent of the milk supply has this substance in it should be concerning to everyone because it means it’s already out there,” U.S. Health and Human Services Secretary Xavier Becerra said in a speech to the Western Governors Association last week. “If we start to see this substance spike, then we should be really concerned.”
Why are authorities concerned about human infection with bird flu?
A major concern among federal agencies and researchers is that the virus will evolve and become more contagious between humans, though so far the virus has been difficult for humans to infect and there have been no instances of person-to-person transmission.
Unless strict measures are put in place to prevent repeated transmission of the virus between wild and domestic animals,, Experts say there may be an increased risk to people.
The USDA and CDC are urging farmers to step up efforts to prevent the spread of infection by cleaning and disinfecting equipment, especially if the same equipment is used to handle fertilizer and feed, to keep farm employees safe.
Preventing the spread of infection from farm to farm is “really important,” officials said Thursday.
How is this bird flu outbreak different from previous ones?
The outbreak is “unusual” for “its global extent” and the wide range of bird and mammal species it has infected, concluded a study published earlier this year by Tufts University researchers Jonathan Ranstadler and Wendy Puryear. “While the risk to humans remains low, this unexpected outbreak highlights the need for continued vigilance and further research,” the study said.
Avian influenza has spread worldwide among birds, especially migratory birds such as ducks. Migratory birds are the natural hosts and travel long distances, sometimes across hemispheres. During migration, infected birds shed the virus in their mucus, saliva and excrement.
Lennock said that in previous outbreaks, wild birds would often be asymptomatic and simply carry the virus, but the current strain of the virus has evolved and is causing large numbers of infected and deadly diseases.
DeAnn Prosser, a wildlife researcher at the U.S. Geological Survey’s Eastern Ecological Science Center, said the virus is causing a lot of concern because it’s affecting human health and agriculture, as well as killing wildlife including seabirds, raptors and marine mammals.
Infections in dairy cows are rare, and officials aren’t sure how the outbreak began. One study released by federal officials this week suggested it probably started when wild birds infected cows in the Texas Panhandle. Cattle from that herd, which showed no symptoms at the time, were transported to Michigan, where the virus quickly spread to other states, the report said. The investigation is ongoing.
Typically, infections eventually die down naturally as wild birds develop immunity and stop spreading the virus, and that is expected to happen this time too, but it’s taking longer than usual.
How does bird flu spread?
Scientists don’t fully understand all the ways it spreads, but most involve bird droppings.
Wild waterfowl are the main vectors, said Maurice Pithesky, an associate professor of cooperative extension, poultry health and food safety epidemiology at the University of California, Davis.
The first four birds spotted in the U.S. were wild ducks taken by hunters in the Carolinas. Lennock said U.S. duck hunters, who have their own safety guidelines,, They have been invaluable in working with authorities to test wild waterfowl.
Pitesky gave some examples of ways the avian flu virus could be transmitted:
- The virus spreads through wild bird droppings in farm ponds and buildings.
- It can be aerosolized and released into the air.
- A group of stray cats drank raw milk, contracted the virus, and developed neurological symptoms and died.
- Animals feed on infected birds.
- Farm workers may bring in scrapes and soil from wetlands and farm fields that could carry the virus.
- Farm tractors and other equipment can carry infected material between farms.
Visual Guide:Bird flu outbreak
Could climate change be influencing this bird flu outbreak?
A key difference between this outbreak and past outbreaks is that global average temperatures have been rising, particularly since this outbreak began. Last year was the hottest on record, and this year appears set to continue that trend.
The possibility that rising temperatures could alter the transmission of viruses and contribute to global pandemics has been a long-standing concern for scientists studying potential links between climate change and the spread of viral outbreaks and pandemics.
Prosser is a co-author of a study last fall that found that changing weather patterns and extreme weather events were closely linked to an “unprecedented global spread of highly pathogenic avian influenza.”
There is no single answer to the link between climate change and bird flu outbreaks in the U.S., she said, and more research is needed. On the one hand, influenza particles may break down faster in warmer weather, making them less infectious. But when heat waves cause changes in food resources or immune stress in humans and animals, the virus may spread more easily, she said.
Could shrinking habitat space for birds be a factor?
Pitesky says it’s not just global warming that’s changing the environment — agriculture and human encroachment on the natural landscape may be bigger potential factors.
As agricultural land expands and droughts dry up existing wetlands, farms are proliferating near wilderness areas, especially wetlands.
“We’re producing more poultry than ever before, and with more poultry production comes less habitat for wild waterfowl and less habitat for animals to roost and feed,” Pithesky says. When wild areas overlap with poultry and cattle operations, the potential for disease transmission increases.
“California has lost 95 percent of its natural wetlands, primarily to agriculture,” he said. “There are obvious benefits in terms of feeding 8 billion people.” But an unintended consequence is that “waterfowl are now using less-than-optimal habitat.”
Dinah Voyles Pulver covers climate and the environment for USA TODAY. She can be reached at dpulver@gannett.com or @dinahv.p
