NEW YORK (AP) — Over the past two years, bird flu has been blamed for the deaths of millions of wild and domestic birds worldwide. It has killed legions of seals and sea lions, wiped out mink farms and sent out cats, dogs, skunks, foxes and even a polar bear.
But it hardly seems to have affected people.
That’s “a bit of a head-scratcher,” although there are some likely explanations, says Richard Webby, a flu researcher at St. Jude Children’s Research Hospital in Memphis, Tennessee. It could have to do with the way infections happen or because types There are differences in the microscopic attachment points that flu viruses need to take root and multiply in cells, experts say.
But what keeps scientists awake at night is the question of whether that situation will change.
“There’s a lot we don’t understand,” said Dr. Tom Frieden, a former CDC director who currently leads Resolve to Save Lives, a nonprofit organization working to prevent epidemics. “I think we need to get over the ‘hope for the best and bury our heads in the sand’ approach. Because it can be very bad.”
Some researchers theorize that flu viruses that originated in birds were the precursors to terrible plagues in humans, including pandemics in 1918 and 1957. Those viruses became deadly human infections and spread among animals and humans.
Based on current evidence, a number of experts believe this virus is unlikely to become a deadly global contagion. But that is not a certainty.
Just in case, U.S. health officials are readying vaccines and making other preparations. But they’re holding off on bolder steps because the virus doesn’t cause serious illness in people and they don’t have strong evidence that it spreads from person to person.
The currently spreading flu – known as H5N1 – was first identified in birds in 1959. Health officials only became really concerned when an outbreak occurred in Hong Kong in 1997, causing serious illness and deaths in humans .
It has caused hundreds of deaths worldwide, the vast majority of which involve direct contact between humans and infected birds. If there was apparent spread between people, it involved very close and extensive contact within households.
However, like other viruses, the H5N1 virus has mutated over time. In recent years, one particular species has spread alarmingly quickly and widely.
Animal outbreaks have been reported in the United States on dozens of dairy cow farms and more than a thousand poultry farms, according to the U.S. Department of Agriculture. Four human infections have been reported among the hundreds of thousands of people who work on U.S. poultry and dairy farms, although that may be an undercount.
Doctors worldwide have discovered 15 human infections caused by the widespread 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 only mild symptoms, according to the U.S. Centers for Disease Control and Prevention.
There is no way to know how many animals are infected, but certain creatures seem to get more serious diseases.
Take cats, for example. Flu is often thought of as a lung disease, 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, damaging and clotting blood vessels and causing seizures and death.
Similar gruesome deaths have been reported in other animals, including foxes that ate dead, infected birds.
The flu strain’s ability to take hold in the brain and nervous system is a possible reason for “the higher mortality rate in some species,” said Amy Baker, a U.S. Department of Agriculture scientist who studies avian flu in animals. I don’t know what the properties of the virus or the properties of the host are that lead to these differences,” Baker said.
Unlike cats, cows have been largely spared. According to the USDA, diseases have been reported in less than 10% of cows in affected dairy herds. Those who did develop symptoms experienced fever, lethargy, decreased appetite and increased respiratory secretions.
Cow infections are largely concentrated in the udders of lactating animals. Researchers investigating cat deaths on dairy farms with infected cows concluded that the cats had contracted the virus from drinking raw milk.
Researchers are still trying to figure out how the virus 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 involve direct contact, perhaps through shared milking equipment or spread by the workers who milk them.
Then there is the matter of sensitivity. The flu virus must be able to attach to the cells before they can enter them.
“If it doesn’t end up in a cell, nothing happens. … The virus is just swimming around,” explains Juergen Richt, a researcher at Kansas State University.
But these docking points – sialic acid receptors – are not found uniformly everywhere in the body and differ per species. A recent study documented the presence of bird flu-friendly receptors in the mammary glands of dairy cattle.
Redness of the eyes is a common symptom in people infected with the current bird flu. People who milk 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 study published this month found that ferrets infected in the eyes ultimately died, as researchers showed the virus could be just as deadly entering through the eyes as through the respiratory tract.
Why didn’t the same thing happen to America’s farm workers?
That’s difficult to answer, experts say. Perhaps people have some degree of immunity, due to previous exposure to other forms of flu or vaccinations, Richt suggested.
A more threatening question: What happens if the virus mutates in a way that makes it more deadly to people or makes it spread more easily?
Pigs are of 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 multiply in the lungs, but the disease is very mild.
But that could all change, which is why there is a push in the scientific community to ramp up 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 thing that’s predictable 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 Educational Media Group. The AP is solely responsible for all content.