Will bird flu spark a human pandemic? Scientists say the risk is rising

Ten months on from the shocking discovery that a virus usually carried by wild birds can readily infect cows, at least 68 people in North America have become ill from the pathogen and one person has died.

Why a teenager’s bird-flu infection is ringing alarm bells for scientists

Although many of the infections have been mild, emerging data indicate that variants of the avian influenza virus H5N1 that is spreading in North America can cause severe disease and death, especially when passed directly to humans from birds. It is also adapting to new hosts — cows and other mammals — raising the risk that it could spark a human pandemic.

“The risk has increased as we’ve gone on — especially in the last couple of months, with the report of [some] severe infections,” says Seema Lakdawala, an influenza virologist at Emory University School of Medicine in Atlanta, Georgia.

Last week, US President Donald Trump took office and announced that he will pull the United States — where H5N1 is circulating in dairy cows — out of the World Health Organization, the agency that coordinates the global response to health emergencies. This has sounded alarm bells among researchers worried about bird flu.

Here, Nature talks to infectious-disease specialists about what they’re learning about how humans get sick from the virus, and the chances of a bird-flu pandemic.

Does how ill a person gets depend on whether they are infected by a cow or a bird?

There are two main variants of H5N1 that researchers are monitoring: one, called B3.13, is spreading mainly in cows; the other, called D1.1, is found mostly in wild and domesticated birds, including chickens raised for poultry.

Huge amounts of bird-flu virus found in raw milk of infected cows

B3.13 has spread rapidly in cattle across the United States, infecting more than 900 herds across 16 states, and has also infected other animals, such as cats, skunks and poultry. Infected cows and their milk contain high levels of the virus, making it easy for the pathogen to be transmitted between animals and workers on dairy farms, where milking equipment can spray liquid into the air and milk can coat surfaces.

At least 40 people have been infected by sick cows in North America, but in these cases, the virus has caused only mild respiratory illness and an inflammatory eye condition known as conjunctivitis. At least 24 people have become ill after exposure to sick birds, and 2 of these infections, caused by D1.1, were severe — one person was in hospital for months and the other died.

These numbers are too small to enable researchers to determine whether one variant of the virus is more dangerous than the other, Lakdawala says. Factors such as underlying health conditions in the people infected and the route of exposure to the virus can affect outcomes, she says.

So can an infection’s severity depend on whether a person ingests or breathes in the virus?

Dairy workers are vulnerable to infection because, during the milking process, they can inhale airborne milk particles and milk droplets can splash into their eyes. Some data suggest that if the virus enters the lungs directly, it could cause a severe infection. In a study published in Nature on 15 January¹, a research group including Heinz Feldmann, head of the US National Institute of Allergy and Infectious Diseases’ virology laboratory in Hamilton, Montana, infected cynomolgus macaques (Macaca fascicularis) with B3.13 virus.

Is bird flu spreading among people? Data gaps leave researchers in the dark