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Bird Flu is Spreading Through U.S. Farms. What Are the Risks to People?

By Melody Schreiber

This past March, dairy cows in Texas stopped eating well and their milk became thick and discolored; some of the cows spiked fevers. They weren’t extremely sick, but, in the parlance of the dairy industry, they “ain’t doing right,” said Meghan Davis, an associate professor of environmental health and engineering at the Johns Hopkins Bloomberg School of Public Health and a former dairy veterinarian.

The reason soon made headlines: for the first time, H5N1, a highly pathogenic avian influenza, was identified in cows, less than a week after the first case of bird flu was discovered in livestock among baby goats at a Minnesota farm.

The news made infectious disease doctors and researchers take notice, since one warning sign that bird flu could be evolving in a dangerous way is the increased ability to infect new species. But what made them really pause was the update that a dairy worker had also contracted the virus, raising fears that the virus had mutated to become more transmissible to people—an even more worrisome step.

So, what are the risks to humans now, and what are the warning signs that risks could be shifting?

The current risks

H5N1 was first identified in 1959, but it didn’t make waves until an outbreak in China in 1996 spread to people with an eventual mortality rate of more than 50 percent. Since 2003, a total of 889 people have been sickened by H5N1, with 463 of those people dying.

Bird flu is particularly contagious and virulent among birds, but it has also been detected in scores of other animals, especially since the outbreak reignited in 2020.

Yet over the past few decades, compared to how much avian flu has spread among birds and other animals, “the number of cases that get into humans is quite small, so that’s a very rare event,” Davis said. The U.S. also randomly samples a handful of flu tests every week in order to track which flu variants are in circulation.

It’s important to note, however, that flu infections are typically examined for H5N1 in fairly limited circumstances: when people become very sick and have had close contact with animals, especially birds, or when they develop symptoms after close contact with animals known to have the virus. So it’s possible some infections are not being detected.

It’s still not clear how the virus was introduced to dairy cows and how it’s spreading—whether between cows or whether birds or other animals shared the virus with cows, which could indicate the virus mutated to become better at infecting cows. Recent genetic analysis indicates the virus may have begun spreading in cows in late 2023.

Officials in the U.S. have said that the risk to most people at this point is low, but scientists continue to sound the alarm about the risks of a continually spreading and evolving virus. “This remains, I think, an enormous concern,” Jeremy Farrar, chief scientist at the World Health Organization (WHO), told reporters recently. Every time the virus adapts to a new host, it could pick up mutations that make it more transmissible to humans—and the more sick animals there are, the more likely people are to come into contact with them.

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