Bird flu is highly lethal to some animals, but not to others. Scientists want to know why (2024)

NEW YORK (AP) — In the last two years, bird flu has been blamed for the deaths of millions of wild and domestic birds worldwide. It’s killed legions of seals and sea lions, wiped out mink farms, and dispatched cats, dogs, skunks, foxes and even a polar bear.

But it seems to have hardly touched people.

That’s “a little bit of a head scratcher,” although there are some likely explanations, said Richard Webby, a flu researcher at St. Jude Children’s Research Hospital in Memphis, Tennessee. It could have to do with how infection occurs or because species have differences in the microscopic docking points that flu viruses need to take root and multiply in cells, experts say.

But what keeps scientists awake at night is whether that situation will change.

“There’s a lot we don’t understand,” said Dr. Tom Frieden, a former CDC director who currently heads Resolve to Save Lives, a not-for-profit that works to prevent epidemics. “I think we have to get over the ‘hope for the best and bury our head in the sand’ approach. Because it could be really bad.”

Some researchers theorize that flu viruses that originated in birds were the precursors to terrible scourges in humans, including pandemics in 1918 and 1957. Those viruses became deadly human contagions and spread in animals and people.

A number of experts think it’s unlikely this virus will become a deadly global contagion, based on current evidence. But that’s not a sure bet.

Just in case, U.S. health officials are readying vaccines and making other preparations. But they are holding off on bolder steps because the virus isn’t causing severe disease in people and they have no strong evidence it’s spreading from person to person.

The flu that’s currently spreading — known as H5N1 — was first identified in birds in 1959. It didn’t really begin to worry health officials until a Hong Kong outbreak in 1997 that involved severe human illnesses and deaths.

It has caused hundreds of deaths around the world, the vast majority of them involving direct contact between people and infected birds. When there was apparent spread between people, it involved very close and extended contact within households.

Like other viruses, however, the H5N1 virus has mutated over time. In the last few years, one particular strain has spread alarmingly quickly and widely.

In the United States, animal outbreaks have been reported at dozens of dairy cow farms and more than 1,000 poultry flocks, according to the U.S. Department of Agriculture. Four human infections have been reported among the hundreds of thousands of people who work at U.S. poultry and dairy farms, though that may be an undercount.

Worldwide, doctors have detected 15 human infections caused by the widely circulating bird flu strain. The count includes one death — a 38-year-old woman in southern China in 2022 — but most people had either no symptoms or only mild ones, according to the U.S. Centers for Disease Control and Prevention.

There’s no way to know how many animals have been infected, but certain creatures seem to be getting more severe illnesses.

Take cats, for example. Flu is commonly thought of as a disease of the lungs, but the virus can attack and multiply in other parts of the body too. In cats, scientists have found the virus attacking the brain, damaging and clotting blood vessels and causing seizures and death.

Similarly gruesome deaths have been reported in other animals, including foxes that ate dead, infected birds.

The flu strain’s ability to lodge in the brain and nervous system is one possible reason for “higher mortality rate in some species,” said Amy Baker, an Iowa-based U.S. Department of Agriculture scientist who studies bird flu in animals. But scientists “just don’t know what the properties of the virus or the properties of the host are that are leading to these differences,” Baker said.

Unlike cats, cows have been largely spared. Illnesses have been reported in less than 10% of the cows in affected dairy herds, according to the USDA. Those that did develop symptoms experienced fever, lethargy, decreased appetite and increased respiratory secretions.

Cow infections largely have been concentrated in the udders of lactating animals. Researchers investigating cat deaths at dairy farms with infected cows concluded the felines caught the virus from drinking raw milk.

Researchers are still sorting out how the virus has been spreading from cow to cow, but studies suggest the main route of exposure is not the kind of airborne droplets associated with coughing and sneezing. Instead it’s thought to be direct contact, perhaps through shared milking equipment or spread by the workers who milk them.

Then there’s the issue of susceptibility. Flu virus need to be able to latch onto cells before they can invade them.

“If it doesn’t get into a cell, nothing happens. … The virus just swims around,” explained Juergen Richt, a researcher at Kansas State University.

But those docking spots — sialic acid receptors — aren’t found uniformly throughout the body, and differ among species. One recent study documented the presence of bird flu-friendly receptors in dairy cattle mammary glands.

Eye redness has been a common symptom among people infected by the current bird flu strain. People who milk cows are eye level with the udders, and splashes are common. Some scientists also note that the human eye has receptors that the virus can bind to.

A study published this month found ferrets infected in the eyes ended up dying, as the researchers demonstrated that the virus could be as deadly entering through the eyes as through the respiratory tract.

Why didn’t the same happen in the U.S. farmworkers?

Some experts wonder whether people have some level of immunity, due to past exposure to other forms of flu or to vaccinations. However, a study in which human blood samples were exposed to the virus indicated there’s little to no existing immunity to this version of the virus, including among people who’d had seasonal flu shots.

A more menacing question: What happens if the virus mutates in a way that makes it more lethal to people or allows it to spread more easily?

Pigs are a concern because they are considered ideal mixing vessels for bird flu to potentially combine with other flu viruses to create something more dangerous. Baker has been studying the current strain in pigs and found it can replicate in the lungs, but the disease is very mild.

But that could all change, which is why there’s a push in the scientific community to ramp up animal testing.

Frieden, of Resolve to Save Lives, noted public health experts have been worried about a deadly new flu pandemic for a long time.

“The only thing predictable about influenza is it’s unpredictable,” he said.


The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group. The AP is solely responsible for all content.

Bird flu is highly lethal to some animals, but not to others. Scientists want to know why (2024)


Bird flu is highly lethal to some animals, but not to others. Scientists want to know why? ›

It could have to do with how infection occurs or because species have differences in the microscopic docking points that flu viruses need to take root and multiply in cells, experts say.

How lethal is bird flu? ›

Worldwide, doctors have detected 15 human infections caused by the widely circulating bird flu strain. The count includes one death — a 38-year-old woman in southern China in 2022 — but most people had either no symptoms or only mild ones, according to the U.S. Centers for Disease Control and Prevention.

Does bird flu affect other animals? ›

Bird flu viruses have in the past been known to sometimes infect mammals that eat (presumably infected) birds or poultry, including but not limited to wild animals, such as seals, bears, foxes, skunks; farmed mink; stray or domestic animals, such as cats and dogs; and zoo animals, such as tigers and leopards.

Can humans get bird flu from eating eggs? ›

The U.S. Food and Drug Administrations says that there is no evidence that anyone has been infected with the avian flu by eating properly cooked eggs. Cooking eggs to 160°F (71°C) will kill the avian flu virus. The recommendation for cooking eggs well is supported.

Can you get bird flu from bird poop? ›

The virus is found in an infected bird's feces (poop) and fluids from the bird's eyes, nose, or mouth. Bird flu viruses don't usually infect people. However, this can happen if you: Touch your eyes, nose, or mouth after handling infected live or dead birds.

Is bird flu fatal to all birds? ›

Avian influenza, or “bird flu,” is a respiratory disease of birds caused by influenza A viruses. Wild birds, such as ducks, gulls, and shorebirds, can carry and spread these viruses but may show no signs of illness. However, avian influenza can kill domestic poultry (such as chickens, turkeys, ducks, and geese).

Can you survive bird flu? ›

The mortality (death) rate for bird flu in humans is high — over 50% for all known cases worldwide. This means half of all people diagnosed with bird flu die from it.

Is bird flu in milk? ›

So far, officials have not detected infectious virus in any supermarket milk samples. The finding comes as authorities are still identifying new infected herds in this year's unprecedented outbreak of H5N1 bird flu in dairy cattle.

What temperature kills bird flu? ›

165ºF (with no “pink” parts) – this temperature will kill the H5N1 virus as well as other illnesses. Make sure eggs are fully cooked so that the yolks are not runny or liquid. Keep raw meat separate from cooked or ready-to-eat foods, such as lettuce or fruit.

Is it safe to drink milk? ›

Pasteurization kills pathogens

Based on currently available information, the FDA says commercial milk is safe. Commercially available milk is pasteurized, a process that kills harmful bacteria and viruses by heating milk to a specific temperature for a set period of time.

Can you get bird flu from air? ›

Humans can have contact with avian flu virus when a person touches a surface with virus on it, then touches their eyes, nose, or mouth. The virus can also be inhaled from droplets or dust in the air. The spread of avian flu virus from one infected person to another is very rare.

Can bird flu spread through water? ›

Open bodies of water, including drinking water reservoirs, can become contaminated by birds that are actively shedding virus or by waterfowl carcasses. Surface runoff also represents a potential source of contamination for groundwater.

Is bird flu curable in humans? ›

CDC currently recommends treatment as soon as possible with flu antiviral drugs for people with suspected or confirmed avian influenza A(H5N1) virus infection.

What is the death rate of the bird flu? ›

“In the long term, in the coming years or decades, however, I'm much more concerned.” He gives two reasons: One is that there has been a mortality (or death) rate of about 50% in the almost 900 people around the world who have been infected with bird flu between 2003 and 2024.

How bad is bird flu for health? ›

H5N1 bird flu is widespread in wild birds worldwide and is causing outbreaks in poultry and U.S. dairy cows with one recent human case in a U.S. dairy worker. While the current public health risk is low, CDC is watching the situation carefully and working with states to monitor people with animal exposures.

How long does bird flu virus live? ›

The length of time that avian influenza viruses can survive on surfaces varies by the surface type and environmental factors such as temperature and humidity. They can survive longer under cold and wet conditions (weeks to months) than under warm, dry conditions (hours to days).

How do you get bird flu? ›

The most likely way for a human to catch bird flu is through close contact with infected birds, their faeces (poo) or feathers. See a doctor immediately if you have recently returned from a country that has had an outbreak of bird flu and you have flu like symptoms.

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