Bird flu viruses are resistant to fever which makes them a major threat to humans

Bird flu viruses are a particular threat to humans because they can replicate at temperatures higher than a typical fever, one of the body's ways of stopping viruses in their tracks, according to new research.

  In a study published this week in Science, researchers identified a gene that plays an important role in setting the temperature sensitivity of a virus. In the deadly pandemics of 1957 and 1968, this gene transferred into human flu viruses, and the resulting virus thrived. 

Human flu viruses cause millions of infections every year. The most common types of these viruses, which cause seasonal flu, are known as influenza A viruses. They tend to thrive in the upper respiratory tract, where the temperature is around 33°C, rather than deep in the lungs in the lower respiratory tract, where the temperature is around 37°C.

Unchecked, a virus will replicate and spread throughout the body, where it can cause illness, occasionally severe. One of the body's self-defense mechanisms is fever, which can cause our body temperature to reach as high as 41°C, though until now it has not been clear how fever stops viruses—and why some viruses can survive.

Unlike human flu viruses, avian influenza viruses tend to thrive in the lower respiratory tract. In fact, in their natural hosts, which include ducks and seagulls, the virus often infects the gut, where temperatures can be as high as 40°C–42°C. 

In previous studies using cultured cells, scientists have shown that avian influenza viruses appear more resistant to temperatures typically seen in fever in humans. This new study uses in vivo models—mice infected with influenza viruses—to help explain how fever protects us and why it may not be enough to protect us against avian influenza. 

The scientists simulated in mice what happens during a fever in response to influenza infections. To carry out the research, they used a laboratory-adapted influenza virus of human origin, known as PR8, which does not pose a risk to humans.

Although mice do not typically develop fever in response to influenza A viruses, the researchers were able to mimic its effect on the virus by raising the ambient temperature where the mice were housed (elevating the body temperature of the mice).

The researchers showed that raising body temperature to fever levels is effective at stopping human-origin flu viruses from replicating, but it is unlikely to stop avian flu viruses. Fever protected against severe infection from human-origin flu viruses, with just a 2°C increase in body temperature enough to turn a lethal infection into a mild disease.

The research also revealed that the PB1 gene of the virus, important in the replication of the virus genome inside infected cells, plays a key role in setting the temperature-sensitivity. Viruses carrying an avian-like PB1 gene were able to withstand the high temperatures associated with fever, and caused severe illness in the mice. This is important, because human and bird flu viruses can "swap" their genes when they co-infect a host at the same time, for example when both viruses infect pigs. 

Matt Turnbull et al, Avian-origin influenza A viruses tolerate elevated pyrexic temperatures in mammals, Science (2025). DOI: 10.1126/science.adq4691