NEW YORK (AP) – Bird flu has been blamed for the deaths of millions of wild and domestic birds worldwide in the past two years. It killed hordes of seals and sea lions, destroyed mink farms, and wiped out cats, dogs, skunks, foxes, and even a polar bear.
But it doesn’t seem to have touched people much.
St. in Memphis, Tennessee. Richard Webby, an influenza researcher at St. Jude Children’s Research Hospital, said this is “a little confusing,” but there are some possible explanations. Experts say differences arise in the microscopic docking points that flu viruses need to take root and multiply in cells.
But what keeps scientists awake at night is whether this situation will change.
Former CDC director Dr. D., who heads Resolve to Save Lives, a nonprofit organization that works to prevent epidemics. “There’s a lot we don’t understand,” Tom Frieden said. “I think we need to put aside the ‘burying our heads in the sand hoping for the best’ approach. Because it could be really bad.”
Some researchers suggest that bird-borne influenza viruses were harbingers of dire scourges in humans, including epidemics in 1918 and 1957. These viruses evolved into deadly human contagions and spread in animals and humans.
Some experts think it is unlikely, based on current evidence, that this virus will develop into a deadly global contagion. But it’s not a sure bet.
Just in case, US health officials are preparing vaccines and making other preparations. But because the virus does not cause serious illness in humans and there is no strong evidence that it spreads from person to person, they are reluctant to take bolder steps.
The flu that is currently spreading, known as H5N1, was first detected in birds in 1959. It wasn’t until the Hong Kong outbreak in 1997, which caused serious human illness and death, that it really started to worry health officials.
It has caused hundreds of deaths worldwide, the vast majority of which are due to direct contact between humans and infected birds. When there was significant spread between people, this involved very close and prolonged contact within the household.
However, like other viruses, the H5N1 virus has mutated over time. In the last few years a particular species has spread alarmingly quickly and widely.
Animal outbreaks have been reported at dozens of dairy cow farms and more than 1,000 poultry herds in the United States, according to the U.S. Department of Agriculture. Four human infections have been reported among hundreds of thousands of people working on poultry and dairy farms in the United States, but this may be an undercount.
Doctors worldwide have identified 15 human infections caused by a widely circulating strain of bird flu. This number includes the death of a 38-year-old woman in southern China in 2022; but most people had either no symptoms 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 some creatures appear to develop more serious illnesses.
Take cats, for example. Flu is generally thought of as a disease of the lungs, but the virus can also attack and multiply in other parts of the body. Scientists have found that in cats, the virus attacks the brain, damaging blood vessels and causing clots, causing seizures and death.
Similarly, gruesome deaths have been reported in other animals, including foxes that eat dead and infected birds.
Amy Baker, an Iowa-based U.S. Department of Agriculture scientist who studies bird flu in animals, said the flu strain’s ability to establish itself in the brain and nervous system is a possible reason for the “higher mortality rate in some strains.” But scientists “just don’t know what the characteristics of the virus or the host are that lead to these differences,” Baker said.
Unlike cats, cows are largely protected. According to the USDA, diseases have been reported in less than 10% of cows in affected dairy herds. Those who developed 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 in dairies with infected cows concluded that felines contracted the virus by drinking raw milk.
Researchers are still trying to figure out how the virus spreads from cow to cow, but studies suggest the main route of exposure is not airborne droplets associated with coughing and sneezing. Instead, it is thought to be spread by direct contact, perhaps through shared milking equipment or by workers milking them.
Then there is the issue of sensitivity. The flu virus must be able to attach to cells before it can invade cells.
“If it doesn’t get into the cell, nothing happens. … The virus is floating around,” explained Kansas State University researcher Juergen Richt.
However, these docking points (sialic acid receptors) are not found in the same way throughout the body and vary between species. A recent study documented the presence of bird flu-friendly receptors in the mammary glands of dairy cattle.
Eye redness is a common symptom among people affected by the current strain of bird flu. People milking cows have their udders at eye level and splashing is common. Some scientists state that the human eye also has receptors to which the virus can bind.
A study published this month found that ferrets infected through the eyes died, as researchers showed that the virus can be fatal through the eyes as well as through the respiratory tract.
Why didn’t the same thing happen with US farmworkers?
Some experts wonder whether people have some level of immunity due to past exposure to other strains of flu or due to vaccinations. However, a study in which human blood samples were exposed to the virus showed that there was little or no existing immunity to this version of the virus, including in people who had received a seasonal flu vaccine.
A more threatening 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 concerning because they are considered ideal mixing vessels for bird flu to combine with other flu viruses, potentially creating something more dangerous. Baker studied the current strain in pigs and found that it could replicate in the lungs, but the disease was very mild.
But all of this is subject to change, which is why there is pressure 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 epidemic.
“The only predictable thing about the flu is that it is unpredictable,” he said.
___
The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Education Media Group. AP is solely responsible for all content.