Bird Flu Has Taken Out Millions Of Chickens And Turkeys. What That Means For Humans.

The risk to humans is still low, but cases in mammals are growing globally, which is a troubling step on the road toward human infection.

More than 58 million domestic fowl in the US have died or been culled over the past 12 months due to a large outbreak of the avian flu — the deadliest outbreak in the country’s history, according to the Department of Agriculture.

The specter of an avian flu pandemic is always on health authorities’ minds. Should we be worried this time? According to the CDC, the risk to humans right now is low

“They’re not considering this anything that needs to be on people’s minds continuously,” said Geoffrey Lossie, an avian diagnostician at Purdue University's College of Veterinary Medicine.

However, this avian virus — the H5N1 strain of influenza — has a mortality rate in humans of 50%, which is one reason scientists are keeping a close watch on it. 

So many chickens have been killed it’s contributing to an egg shortage in the US that’s driving up prices. Egg prices more than doubled in 2022 but have since declined, and other factors, like inflation and demand, have played a role in the cost of eggs.

The H5N1 virus has been detected in more than 120 wild mammals in the US in 2022 and 2023, including red foxes, skunks, and even grizzly bears — a troubling step in the road toward human infection.

An outbreak at a mink farm in Spain suggested the virus can now spread in mammals, and 52,000 infected animals had to be culled. This week, four dead seals tested positive for bird flu in Scotland, as did 585 sea lions in Peru, suggesting cases are growing globally in mammals.

Past avian flus have moved from birds to other animals, but the number of mammals that have been infected recently is unusual, said Gregory C. Gray, an infectious disease epidemiologist at the University of Texas Medical Branch in Galveston. “There’s concern that it could further change and jump to humans,” he said.

The H5N1 strain of influenza is a mix of DNA from a Eurasian lineage of the virus and a North American lineage, Lossie said. The strain has had a 53% death rate among humans, based on 858 reported cases globally since 2003, according to the World Health Organization. The “H” and “N” refer to proteins on the surface of the influenza virus that affect how the virus behaves. 

What is bird flu?

Different influenza viruses tend to target different species, said Dr. William Schaffner, a spokesperson for the Infectious Diseases Society of America. 

Avian flu is carried by wild migratory birds like wild ducks and geese and usually follows certain patterns. The virus typically doesn’t sicken or kill wild birds, though it does kill domestic fowl. In this case, though, influenza is making some wild birds sick, Lossie said.

Usually the virus passes from wild birds to domestic birds through feces. In the current outbreak, there is some evidence that the avian flu is also passing through respiratory droplets in wild birds. It’s definitely being shed through the respiratory route in domestic birds like chickens and turkeys, Lossie said.

And while the number of birds who have perished from the avian flu is about the same as in a 2015 avian flu outbreak, the outbreak has now visited double the number of states

“This does suggest that bird flu is more widely distributed geographically,” Schaffner said.

Also, Lossie added that scientists have detected far more infections in wild birds during this outbreak than in past outbreaks.

How does bird flu get to humans?

It’s rare for the exact same virus that infects birds to reach humans. “The appropriate receptors in the mucous membranes in our throats, noses, and upper part of our respiratory tract are not very receptive to bird flu,” Schaffner said.

When it does happen, it’s usually in someone who lives and works closely with birds — and the infection stops there. Since the outbreak began, there have only been four cases in humans and one death. 

“Quite a number of different strains of avian flu have spilled over to humans, but they’ve had limited, if any, human-to-human transmission,” Gray said. “Often it’s a dead end.” Only one person in the US, a poultry worker in Colorado, was infected by the current strain in April while culling poultry infected with H5N1. He reported only fatigue and was treated with antivirals. He has since recovered, Lossie said. (There was also a second human case in South West England detected at the end of 2021.)

Usually there’s another step involved: The virus has to mutate enough to infect a mammal and mutate again to infect people. Every time a virus replicates, DNA can mutate, giving it opportunities to infect different life forms. New strains of a virus also happen through genetic drift, which is when two virus strains — say one human and one avian — combine to create a new variant, Lossie said.

“Most pandemics generate when there is a species jump and usually with an intermediate host where the virus can evolve,” said Dr. Cesar Arias, chief of infectious diseases at Houston Methodist Hospital. “The concern here is the Spanish report of mink transmission. There could be a genetic adaptation in which the virus can become airborne from human to human.”

So far, the CDC said that the circulating virus lacks the genetic changes that would allow it to easily cause illness in humans.

How deadly is bird flu in humans?

It can be very deadly, with a mortality rate as high as 50% in the cases where bird flu has jumped directly from birds to farmers and other handlers, Schaffner said.

Every time the virus replicates, it can mutate into a version the human immune system has never seen before. “That makes it harder for our immune systems to fight the infection,” Arias said. “When they get to humans, the adaptations tend to be accompanied by high virulence, which means infections are severe with high mortality.”

Basically, our bodies have never encountered these new viruses and have no defenses. This is effectively what happened with SARS-CoV-2, the virus that causes COVID.

How worried should we be?

Right now, the avian flu doesn’t warrant five-alarm concern, experts said.

But things could change. While it takes time for a pandemic to develop, “I think we’re headed in that direction,” Arias said.

Right now, the biggest risk is to humans who have a lot of contact with birds, especially poultry, Lossie said. That includes not only people in commercial poultry operations but also people with backyard flocks, which are on the rise due to the rising price of eggs in stores.

Are there ways to protect ourselves?

The current outbreak remains a matter mainly for public health organizations for the foreseeable future. The USDA has a surveillance system in place to track avian flu in birds — one they say is the strongest in the world

“We need to be sure the US government is very vigilant through surveillance and be alert for novel viruses that might spill into humans,” Gray said.

There aren’t any avian flu vaccines widely available, but there are prototypes. “The vaccines have basically been constructed then the question would be how quickly to manufacture them,” Schaffner said.

The CDC cautions against unprotected contact with wild or domestic birds. They recommend wearing an N95 respiratory or surgical mask, eye protection, disposable gloves, and boots as well as washing your hands after any contact and don’t touch your mouth, nose, or eyes, all possible points of entry for the virus.

If you have chickens or other birds (including pet birds), keep them protected from wild birds. “Try to shelter your backyard herd or flocks of chickens from migratory birds,” Schaffner said.

Avian flu in wild and domestic birds isn’t in and of itself a cause for panic, Schaffner added. “There’s no immediate threat but public health authorities are on the alert.” ●

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