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

NEW YORK — Over the past two years, bird flu has been blamed for the deaths of millions of wild and domestic birds around the world. It killed legions of seals and sea lions, destroyed mink farms, and dispatched cats, dogs, possums, foxes, and even a polar bear.

But it seems like it barely touched people.

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

But what keeps scientists up at night is whether this situation will change.

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

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

Several experts consider it unlikely that this virus will become a deadly global contagion based on current evidence. But that’s not a sure bet.

As a precaution, U.S. health officials are preparing vaccines and making other preparations. But they are holding off on bolder measures because the virus does not cause serious illness in people and there is no strong evidence that it is spreading from person to person.

The flu that is currently spreading – known as H5N1 – was first identified in birds in 1959. It only began to concern health authorities when an outbreak occurred in Hong Kong in 1997, which involved serious human illness and death.

It has caused hundreds of deaths worldwide, 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 prolonged contact within households.

Like other viruses, however, the H5N1 virus has mutated over time. In recent years, a specific strain has spread alarmingly, quickly and widely.

In the United States, animal outbreaks have been reported in dozens of dairy cow farms It is more than 1,000 flocks of birds, according to the U.S. Department of Agriculture. Four human infections have been reported among the hundreds of thousands of people who work on poultry and dairy farms in the US, although this number may be an underestimate.

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

There’s no way to know how many animals have been infected, but certain creatures appear to be contracting more serious illnesses.

Take cats, for example. Flu is commonly considered a lung disease, but the virus can also attack and multiply in other parts of the body. In cats, scientists found the virus attacks the brain, damaging and clotting blood vessels and causing seizures and death.

Equally horrific deaths have been reported in other animals, including foxes who ate dead and infected birds.

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

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

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

Researchers are still figuring out how the virus has spread 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 is thought to be direct contact, perhaps through shared milking equipment or spread among the workers who milk them.

Then there is the issue of susceptibility. The flu virus needs to be able to attach itself to cells before it can invade them.

“If you don’t enter the cell, nothing happens. … The virus simply floats,” explained Juergen Richt, a researcher at Kansas State University.

But these anchoring points – sialic acid receptors – are not found uniformly throughout the body and differ between species. A recent to study documented the presence of avian flu-friendly receptors in the mammary glands of dairy cattle.

Eye redness has been a common symptom among people infected with the current strain of bird flu. People milking cows are at 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 to study published this month found that ferrets infected in the eyes ended up dying, as researchers demonstrated that the virus can be just as deadly entering through the eyes as it can through the respiratory tract.

Why hasn’t the same happened for US farmworkers?

That’s a difficult question to answer, experts say. Perhaps people have some level of immunity, due to previous exposure to other forms of flu or vaccinations, Richt suggested.

A more ominous question: What happens if the virus mutates 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 studied the current strain in pigs and found that it can replicate in the lungs, but the disease is very mild.

But all of this could change, which is why there is a push in the scientific community to increase animal testing.

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

“The only predictable thing about the flu is that it is unpredictable,” he said.

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