NEW YORK (AP) — Over the past two years, avian flu has been blamed for the deaths of millions of wild and domestic birds around the world. It has killed legions of seals and sea lions, wiped out mink farms and wiped out cats, dogs, skunks, foxes and even a polar bear.
But it seems that this hardly affected people.
It’s “a bit of a puzzle,” although there are likely explanations, said Richard Webby, an influenza researcher at St. Jude Children’s Research Hospital in Memphis, Tennessee. This could have to do with how 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 now runs Resolve to Save Lives, a nonprofit that works to prevent outbreaks. “I think we need to forget about the ‘hope for the best and bury our heads in the sand’ approach. Because it could be really bad.”
Some researchers hypothesize that influenza viruses originating in birds were precursors to terrible plagues in humans, including the pandemics of 1918 and 1957. These viruses became deadly human contagions and spread among animals and humans.
A number of experts believe this virus is unlikely to become a deadly global contagion, based on current evidence. But it’s not a sure bet.
Just in case, US health authorities are preparing vaccines and making other preparations. But they are refraining from taking bolder steps because the virus does not cause serious illness in humans and they have no solid evidence that it spreads from person to person.
The flu now spreading – known as H5N1 – was first identified in birds in 1959. It only really began to worry health officials with the outbreak in Hong Kong. Kong in 1997, which resulted in serious human illness and death.
It has caused hundreds of deaths worldwide, the vast majority of them due to direct contact between people and infected birds. Where there was apparent spread between people, this involved very close and widespread contact within households.
However, like other viruses, the H5N1 virus has mutated over time. In recent years, a particular strain has spread at an alarming speed and scale.
In the United States, animal outbreaks have been reported on dozens of dairy 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 on poultry and dairy farms in the United States, although this figure may be an underestimate.
Around the world, doctors have detected 15 human infections caused by the widespread strain of bird flu. The tally includes one death — a 38-year-old woman in southern China in 2022 — but most people had no or only mild symptoms, according to the U.S. Centers for Disease Control and Prevention.
There is no way to know how many animals have been infected, but some creatures appear to be suffering from more severe illnesses.
Take cats, for example. The flu is generally considered a disease of the lungs, but the virus can also attack and multiply in other parts of the body. In cats, scientists found that the virus attacked the brain, damaging and clotting blood vessels and causing seizures and death.
Equally 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 the “higher mortality rate in some species,” said Amy Baker, a U.S. Department of Agriculture scientist based in Iowa who studies avian flu in animals. I don’t know what properties of the virus or what properties of the host are leading to these differences,” Baker said.
Unlike cats, cows were largely spared. Illnesses have been reported in fewer 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 that the felines contracted the virus by drinking raw milk.
Researchers are still determining how the virus spread from cow to cow, but studies suggest the primary route of exposure is not the type of airborne droplets associated with it. coughing and sneezing. Rather, it is thought to be direct contact, perhaps via shared milking equipment or transmitted by the workers who milk them.
Then there is the question of susceptibility. The flu virus must be able to attach itself to cells before it can invade them.
“If he doesn’t enter a cell, nothing happens. … The virus is swimming everywhere,” explains Juergen Richt, a researcher at Kansas State University.
But these docking points – sialic acid receptors – are not found uniformly throughout the body and differ between species. A recent study documented the presence of avian influenza-friendly receptors in the mammary glands of dairy cattle.
Redness of the eyes is a common symptom among people infected with the current strain of bird flu. The people who milk the cows have their eyes at udder level and splashing is common. Some scientists also note that the human eye has receptors that the virus can bind to.
A study published this month found that ferrets infected in the eyes eventually died, with researchers demonstrating that the virus could be as deadly entering through the eyes as through the respiratory tract.
Why didn’t the same thing happen to American farm workers?
This question is difficult to answer, experts say. Perhaps people have some level of immunity, due to previous exposure to other forms of flu or vaccines, Richt suggested.
A more ominous question: What happens if the virus mutates in a way that makes it more deadly to humans or allows it to spread more easily?
Pigs are a concern because they are considered ideal mixing vessels for avian flu, as they can potentially combine with other flu viruses to create something more dangerous. Baker studied the current strain in pigs and found that it can replicate in the lungs, but the disease is very mild.
But all that could change, which is why the scientific community is working to step up animal testing.
Frieden, of Resolve to Save Lives, noted that public health experts have long worried about a new deadly flu pandemic.
“The only predictable thing about the flu is that it’s unpredictable,” he said.
___
The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Education Media Group. The AP is solely responsible for all content.