Scientists are trying to understand why bird flu is so deadly to some animals but not others.

NEW YORK (AP) — Over the past two years, millions of wild and domestic birds around the world have been blamed for dying from avian influenza, which has killed large numbers of seals and sea lions, devastated mink farms and killed cats, dogs, skunks, foxes and even polar bears.

But it seems to have had little effect on people.

That's “a bit of a puzzle,” said Richard Webby, an influenza researcher at St. Jude Children's Research Hospital in Memphis, Tennessee, but there are several possible explanations. Experts say it could have to do with differences in how infection occurs, or in the microscopic junctions that influenza viruses need to attach to cells and multiply.

But what keeps scientists up at night is whether that will change.

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“There's a lot we don't know,” said Dr. Tom Frieden, a former CDC director who is now president of the nonprofit Resolve to Save Lives, an infectious disease prevention group. “We need to let go of the 'hope for the best and be skeptical' attitude, because things could get really bad.”

Some researchers theorize that influenza viruses originating from birds were the precursors to terrible human tragedies, including the 1918 and 1957 pandemics, which became deadly human epidemics and spread to animals and humans.

Many experts believe that based on current evidence, the virus is unlikely to become a deadly global epidemic, but that is not a certainty.

U.S. health officials are readying vaccines and making other preparations just in case, but they are refraining from more drastic measures because the virus is not causing severe illness in people and there is no solid evidence that it can be transmitted from person to person.

The influenza currently circulating, known as H5N1, was first identified in birds in 1959. Health authorities were not particularly concerned until an outbreak in Hong Kong in 1997 caused severe human illness and death.

It has caused hundreds of deaths worldwide, most of which have been due to direct human contact with infected birds, and in cases where human-to-human transmission has been evident, it has been through very close and prolonged contact within households.

But like any virus, the H5N1 virus has mutated over time, and in the past few years, one particular strain has spread widely and with alarming rapidity.

Dozens of animal outbreaks have been reported in the United States. Dairy Farm and More than 1,000 poultryAccording to the USDA, there are four reported cases among the hundreds of thousands of people working on U.S. poultry and dairy farms, but the actual number may be underestimated.

clock: Outbreaks of avian influenza on dairy farms continue to raise concerns about the spread of the virus.

Worldwide, doctors have confirmed 15 human infections with the widespread bird flu strain, including one 38-year-old woman who died in southern China in 2022, though most had no or mild symptoms. according to Centers for Disease Control and Prevention.

It is not known how many animals are infected, but certain animals appear to be more severely ill.

Take cats, for example. Although influenza is generally thought of as a lung disease, the virus can attack and multiply in other parts of the body. In cats, scientists found The virus attacks the brain, damaging and clotting blood vessels, causing seizures and death.

Similar brutal deaths have been reported for other animals, Fox Eating dead infected birds.

Amy Baker, an Iowa-based USDA scientist who studies avian influenza in animals, said the ability of influenza virus strains to persist in the brain and nervous system could be one reason “we see higher mortality in some species.” But scientists “just don't know what properties of the virus or the host are that are causing these differences,” she said.

Unlike cats, cows have suffered little damage. Less than 10 percent of cattle According to the USDA, affected dairy herds showed symptoms including fever, lethargy, loss of appetite and increased respiratory secretions.

In cattle, infections are mainly localized in the udders of lactating animals. Researchers investigating cases of cat deaths on dairy farms with infected cows said: Conclusion The felines contracted the virus by drinking raw milk.

Researchers are still learning how the virus spreads from cow to cow, but studies suggest it isn't primarily transmitted through airborne droplets from coughing or sneezing, but rather through direct contact, including sharing milking equipment and spread by milking workers.

Then there's the issue of susceptibility: the influenza virus needs to be able to attach to a cell before it can enter it.

“Without getting inside the cell, nothing happens; the virus just swims around,” explained Jurgen Licht, a researcher at Kansas State University.

However, these docking sites, or sialic acid receptors, are not uniformly distributed throughout the body and vary among species. study The presence of receptors favorable for avian influenza has been documented in the mammary glands of dairy cows.

Bloodshot eyes are a common symptom in people infected with the current strain of bird flu. Milkers are at eye level with their udders, making it common for the virus to spread. Some scientists suggest that human eyes contain receptors that the virus can bind to.

a study In a paper published this month, ferrets with eye infections ultimately died, and researchers demonstrated that the virus can be just as deadly when it enters through the eye as it is when it enters through the respiratory system.

Why hasn't the same thing happened to farmworkers in the United States?

Experts said that's a difficult question to answer, but Licht suggested that people might have some immunity, perhaps from having been infected with other types of flu in the past or having been vaccinated.

An even scarier question is what happens if the virus mutates and becomes more deadly to humans or more easily transmitted.

read more: Avian flu poses high risk to workers on farms and in food processing plants

Pigs are a concern because they are thought to be an ideal mixing vessel for bird flu to mix with other influenza viruses to create more dangerous viruses. Baker has studied the current strain in pigs and found that the virus can grow in the lungs, but causes very mild disease.

But that could change dramatically, and there are calls within the scientific community for increased animal testing.

“Determined to Save Lives” Frieden noted that public health experts have long been concerned about a deadly new influenza pandemic.

“The only thing predictable about influenza is that it's unpredictable,” he said.