Bird Flu Is Spreading in Alarming New Ways

Last Friday, a health alert from the Centers for Disease Control and Prevention pinged its way across the inboxes of clinicians and state health departments all over the US. The message described how a dairy farm worker in Texas had contracted H5N1, the highly infectious strain of avian influenza, or bird flu, that’s currently circling worldwide. The dairy worker had caught the virus, apparently, from cattle.

The CDC’s alert urged doctors to be vigilant and consider H5N1 a possibility in any patients presenting with acute respiratory symptoms or sore eyes who had recently been in contact with animals.

Around the world, virologists responded to the news with varying degrees of alarm. While this is the second confirmed human case of H5N1 infection in the US in the past two years—in 2022, a worker at a poultry farm in Colorado contracted it from infected birds—it appears to be the first known case of the virus jumping from another mammal to humans.

According to Raina MacIntyre, who heads the Biosecurity Program at the Kirby Institute in Sydney, the biggest concern is that as the virus adapts more and more to mammalian species, it will gain mutations that make it better equipped to spread between humans. So far, the public health threat from H5N1 has been minimal because it cannot easily enter the cells of the human nose and mouth, though when the virus does infect people, it can be deadly.

“The virus is not one that can transmit easily between humans,” says MacIntyre. “The key event that could result in a human pandemic is a mutation that switched the affinity of the virus to certain receptors in the human respiratory tract.”

H5N1 is not a new virus. It was first detected in chickens in Scotland way back in 1959, and between January 2003 and December 2023, 882 human cases were reported across 23 countries, with a case fatality rate of 52 percent. The subtype that infected the Texan farm worker, known as 2.3.4.4b, has been spreading in the wild on an unprecedented scale over the past four years. Reports suggest that it has killed hundreds of millions of domestic and wild birds since 2020, while at least 48 mammal species across 26 countries have also perished from