Stop the next influenza pandemic

In the spring, U.S. consumers found themselves having to look for something that used to be very easy to find: eggs. Some supermarkets were limiting customers to one carton per visit, but they still ran out of stock. Shoppers who managed to find eggs in stock paid high prices — 12 eggs sometimes sold for more than $11. For many people, this was the first sign that the country was in the midst of a devastating bird flu outbreak.

The H5N1 influenza virus that caused this outbreak hit poultry farms particularly hard. Egg shortages and soaring prices have resulted in the loss of tens of millions of chickens. The United States Department of Agriculture (USDA) requires the culling of poultry flocks that test positive for H5N1. Currently, egg prices are falling. But U.S. consumers could experience a new shock if they buy a turkey for their Thanksgiving meal in late November. Due in part to the impact of bird flu, this year's flock size is the smallest in decades. Some economists predict that turkey prices this year will be 40% higher than in 2024.

Nature Spotlight: Influenza

Some poultry farms were infected with H5N1 from waterfowl. However, some people contracted the virus from new sources. In March 2024, the United States announced that several herds of dairy cows were infected. Since the H5N1 virus first infected cattle, the virus has infected at least 1,000 cattle, tens of millions of chickens and turkeys, and 70 people in 18 states, killing one of them.

Infectious disease experts are increasingly concerned about the spread of the H5N1 virus to dairy cows, which come into close contact with humans. “This was not on the pandemic bingo card,” said Thomas Friedrich, a virologist at the University of Wisconsin-Madison. He added that while the virus has not yet developed the ability to easily transmit between humans, that doesn't mean it can't be transmitted. And “not enough is being done to take the threat as seriously as I think we should.”

In April 2024, the federal government began requiring dairy farmers to test their cattle for H5N1 before moving them between states. And late last year, it launched a milk testing program. But that's not enough, says Carol Cardona, an avian influenza researcher at the University of Minnesota in Minneapolis. “We want a comprehensive national plan that addresses all sensitive species,” she says. Dairy cows are one of the latest species to be infected with H5N1, but they won't be the last.

The clade of H5N1 currently circulating on Earth has been detected in dozens of animal species, including cats, seals, and mice. Experts fear that the longer the virus circulates in mammals, the more likely it is to acquire the mutations needed to transmit between humans, sparking a global pandemic. “The risk remains unless measures are taken and maintained to proactively detect infection in cattle and, if infection is detected, to stop the spread of the virus from farm to farm,” Friedrich said.

first contact

When Kay Russo, a veterinarian at RSM Consulting in Fort Collins, Colorado, heard about a mysterious disease circulating in dairy cows in Texas, she worried it might be bird flu.

Russo started his career as a dairy veterinarian, but since returning to university to specialize in poultry medicine, he has been tracking the spread of H5N1 strains from wild birds to various species. When she asked veterinarians in Texas about the status of the wild birds on her farm, one veterinarian replied, “All the wild birds are dead.” “This sounds crazy, but it looks like it could be the flu,” she remembers saying.

Evidence that the virus circulating in cattle came from wild birds in December 2023¹. In many commercial poultry farms, the birds remain indoors where they are protected from wildlife. However, cattle and wild birds mix freely in pastures, watering holes, and at mealtimes.

Friedrich said many researchers initially thought the “spill” phenomenon was an anomaly. “There was something unique about this particular gene cluster, which we call B3.13, that allowed the virus to jump from birds to cows.”

But since then, a different genotype of the virus, called D1.1, has also jumped from wild birds to cattle at least twice, in two different locations. Scientists don't yet know how or why this is happening, but it suggests the interface between dairy cows and wild birds requires more attention.

Although cows cannot be kept in enclosures like chicken coops, there are other ways to minimize contact with wild birds. Maurice Pitesky, an epidemiologist who studies poultry health at the University of California, Davis School of Veterinary Medicine, says the biggest problem is the built environment. “In some cases, farms are located near prime waterfowl habitat,” he says. In other areas, wild birds must rely on suboptimal habitats, such as poultry and livestock lagoons (see Close Contact).

While it's not feasible to rebuild dairy farms, it is possible to make the habitat less attractive and keep waterfowl away from dairy farms, Pitesky said. That might mean using nets in ponds to prevent birds from landing, or using lasers or sonic cannons to scare birds away. “We want to reduce the possibility of spillover,” he says.

cow conundrum

When an outbreak occurs, the focus shifts to controlling the spread of the virus from animal to animal and farm to farm.

In poultry, H5N1 is deadly. Therefore, poultry farms tend to have appropriate biosecurity measures in place. If a bird tests positive for H5N1, all poultry on that farm will be culled.

H5N1 is more difficult to sniff out in dairy herds, in part because the disease is milder. Cows infected with H5N1 may have reduced milk production or stop eating, but rarely die. Cattle H5N1 control strategies focus on identifying and isolating sick animals. “The initial hope was that this would burn up inside the cow and we wouldn't have to worry about it,” Russo said. He calls it a “head-in-the-sand approach.”

USDA recommends that producers implement biosecurity measures on dairy farms to prevent the spread of H5N1 infections. For example, it suggests farms disinfect tires on vehicles upon arrival, treat infected milk before disposal, and ensure farm workers wear personal protective equipment. However, these measures are time-consuming and costly, and many dairy producers do not have a strong incentive to implement them. “The only way to improve biosecurity is if the cows start dying,” says Keith Paulsen, director of the Wisconsin Veterinary Diagnostic Laboratory at the University of Wisconsin-Madison.

Researchers still don't understand how the virus moves from farm to farm. Theories include that the virus is transmitted by birds, insects, or farm workers. brought in by vehicle. Or it may be carried by the wind. The cow's udder could be the entry point, or it could be the respiratory tract. “We don't really know what the most dangerous pathway is,” says Michelle Krom, a consultant turkey veterinarian and food system risk management expert in Minneapolis, Minnesota. This lack of clarity makes it difficult to develop an optimal plan of action.

In the August preprint²researchers showed that the virus appears to be widespread on dairy farms in California. The research team also found infectious viruses in the air in milk, wastewater compost lagoons, and even in both the milking parlor and cow breath.

This suggests there may be some simple risk mitigation strategies, says Seema Lakdawala, a virologist at Emory University in Atlanta, Georgia, who co-led the study. In milking parlors, “there's a lot of aerosol and infectious virus,” she says. Face masks can protect workers from infection, and electric fans may help reduce the amount of virus carried in the air. Lakdawala also points out the importance of disinfecting milking equipment before disposal to inactivate the virus in milk from infected cows to prevent milk from entering the waste stream. “There's more than one way the influenza virus spreads,” she says. “One intervention won’t solve everything.”

Dairy herd management also makes control difficult. Poultry producers operate what is called an “all in, all out” strategy. This means that entire populations of animals on a particular farm come and go together. But on dairy farms, about a third of the animals are replaced each year, Russo said. This means there is a constant supply of cattle that have no immunity to the virus. And stopping new cattle migration is nearly impossible. Dairy production is often geographically dispersed. One farm raises calves, another takes in animals ready for breeding, and a third takes in lactating cows. Dairy farmers can't just flip a switch and stop production, Poulsen said.

As a result of these obstacles, little is being done to curb the spread of the virus. “The way dairy farms eliminate the virus is that it survives infection as it travels through the herd,” California veterinarian Annette Jones said in July, testifying before the U.S. House Livestock, Dairy, and Poultry Subcommittee. “Some of our herds have been isolated for over 300 days, which means they could be spreading the virus in the environment for that long.”

Infected dairy farms can be devastating to poultry producers. Russo says a single infected cow can shed enough virus to “wipe out an entire poultry farm.” “Of the 39 million eggs we lost last year, 28 million were related to dairy cow spills.”

This kind of spillover is exactly what happened at Hickman's Family Farm in Buckeye, Arizona, in May. The virus spread from a nearby dairy farm to one of the egg producer's cage-free farms. The company had to cull 6 million birds, 95% of the flock. “This is just a slow-motion train wreck,” President Glenn Hickman said on the show. podcast In July.