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Why is bird flu highly lethal to some animals, but not to others?

A dead sea bird lays beside a dead sea lion on the beach, 2023.
A dead sea bird lays beside a dead sea lion on the beach, 2023. Copyright Juan Macri/AP Photo
Copyright Juan Macri/AP Photo
By AP & Euronews
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Bird flu is highly lethal to some animals, but not to others. Scientists want to know why.

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Why has bird flu killed millions of wild and domestic birds, touched seals and sea lions, mink farms, cats, dogs and others but 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 the US state of 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 Centers for Disease Control and Prevention (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 scientists think flu viruses originating in birds were the precursors to pandemics in 1918 and 1957, but several experts think it’s unlikely this virus will become a deadly global contagion, based on current evidence.

But that's not a sure bet.

Officials are preparing just in case but currently, there's no evidence it's spreading between people.

H5N1 outbreak

The flu that is 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 US Department of Agriculture.

Four human infections have been reported among the hundreds of thousands of people who work at US poultry and dairy farms, though that may be an undercount.

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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 CDC.

Animal infections

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

Flu viruses can attack and multiply in other parts of the body besides the lungs. In cats, scientists have found the virus attacking the brain, damaging and clotting blood vessels and causing seizures and death.

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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 US 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, with those that did develop symptoms experiencing fever, lethargy, decreased appetite and increased respiratory secretions.

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Eye redness in humans

People infected with the current bird flu strain have had eye redness as a common symptom.

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 to US farmworkers?

Some experts wonder whether people have some level of immunity, due to past exposure to other forms of flu or to vaccinations.

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But a study where human blood samples were exposed to the virus indicated there's little to no existing immunity to this version of it.

Some are also concerned that it could mutate to become more lethal or spread more quickly.

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.

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