by NEW YORK (AP) – Over the past two years, bird flu has been blamed for the deaths of millions of wild and domestic birds around the world. He killed legions of seals and sea lions, destroyed mink farms, and dispatched cats, dogs, taps, foxes and even a polar bear.
But it seems that he hardly touched people.
That’s “a bit of a head scratcher,” although there are several likely explanations, said Richard Webby, a flu researcher at St. Jude Children’s Research Hospital in Memphis, Tennessee. It could have something to do with how infection occurs or because a species there are differences in the microscopic entry 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, former CDC director who currently heads Resolve to Save Lives, a non-profit company that works to prevent epidemics. “I think we need to get past the ‘hope for the best and bury our heads in the sand’ approach. Because it could be bad.”
Some researchers theorize that flu viruses originating from birds were the precursors to terrible scourges in humans, including pandemics in 1918 and 1957. These viruses became fatal human infections and spread to animals and humans.
Some experts think that this virus is unlikely to become a deadly global infection, based on current evidence. But that’s not a sure bet.
Just in case, US health officials are preparing vaccines and making other preparations. But they are taking bolder steps because the virus is not causing serious disease in humans and they have no strong evidence that it is spreading from person to person.
The flu currently circulating – known as H5N1 – was first identified in birds in 1959. It didn’t really start to worry health officials until an outbreak in Hong Kong in 1997 caused serious illness and death. human.
It has caused hundreds of deaths worldwide, most of which involve direct contact between humans and infected birds. When the spread was clear between people, it involved very close and extended contact within families.
Like other viruses, however, the H5N1 virus has mutated over time. In recent years, a particular strain has spread alarmingly quickly and widely.
In the United States, animal outbreaks have been reported at dozens of dairy 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 on US poultry and dairy farms, although that may be an undercount.
Around the world, 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 or mild symptoms, according to the US Centers for Disease Control and Prevention.
There is no way of knowing how many animals are infected, but some creatures seem to be getting more severe illnesses.
Take cats, for example. Flu is commonly thought to be a disease of the lungs, but the virus can also attack and multiply in other parts of the body. In cats, scientists have found that the virus attacks the brain, damages and clots blood vessels and causes seizures and death.
Similarly, gruesome deaths have been reported in other animals, including foxes that have eaten dead infected birds.
The ability of the flu strain to lodge in the brain and nervous system is one possible reason for “a higher mortality rate in some species,” said Amy Baker, a US Department of Agriculture scientist based in Iowa who studies bird flu in animals. But scientists “but I don’t know what properties of the virus or properties of the host cause these differences,” said Baker.
Unlike cats, most are spared cows. Illnesses have been reported in less than 10% of cows in affected dairy herds, according to the USDA. Those who developed symptoms had fever, lethargy, decreased appetite and increased respiratory secretions.
Bovine infections are mostly concentrated in the udders of lactating animals. Researchers investigating the death of cats on dairy farms with infected cows concluded that the cats contracted the virus from drinking raw milk.
Researchers are still unraveling how the virus is spreading from cow to cow, but studies suggest that the main route of exposure is not the type of airborne droplets associated with coughing and sneezing. Instead it is thought to be direct contact, perhaps through shared milking equipment or spread by the workers who milk them.
Then there is the question of susceptibility. Influenza virus must be able to latch onto cells before they can invade them.
“If it doesn’t go into a cell, nothing happens. … The virus is floating around,” explained Juergen Richt, a researcher at Kansas State University.
But those docking spots – sialic acid receptors – are not found uniformly throughout the body, and differ between species. One recent study documented the presence of receptors supporting bird flu in the mammary glands of dairy cattle.
Eye redness is a common symptom among people infected with the current strain of bird flu. People who milk cows have their eyes at eye level, and splashes are common. Some scientists also note that the human eye has receptors that the virus can attach to.
A study published this month found that ferrets infected in the eyes were dying, as the researchers showed that the virus could be just as deadly entering through the eyes as it is through the respiratory tract.
Why didn’t the same thing happen in US farmers?
Some experts wonder if people have some level of immunity, due to exposure to other types of flu or vaccinations in the past. However, a study in which human blood samples were exposed to the virus showed that there is little immunity to this version of the virus, including among people who have had seasonal flu.
A more threatening question: What if the virus mutates in a way that makes it more lethal to humans or allows it to spread more easily?
Pigs are a concern because they are considered ideal mixing vessels for bird flu to combine with other flu viruses to create something more dangerous. Baker is studying the current strain in pigs and found that it can be replicated in the lungs, but the disease is very mild.
But all that could change, which is why the scientific community is pushing to increase animal testing.
Frieden, of Resolve to Save Lives, noted that public health experts have long been concerned about the new deadly flu pandemic.
“The only thing that can be predicted about the flu is that it is unpredictable,” he said.
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The Associated Press Health and Science Section is supported by the Howard Hughes Medical Institute Science and Media Education Group. The AP is solely responsible for all matters.
