Bird flu viruses may be more dangerous to humans than previously thought because they can survive and grow even when the body has a high fever, according to a new study from the universities of Cambridge and Glasgow.
The research, published in the journal Science, helps explain why bird flu viruses can be so deadly and how they differ from the flu viruses that usually infect people.
When someone gets the flu, the virus typically infects the upper parts of the lungs, where the temperature is about 33°C. Human flu viruses don’t usually spread deep into the lungs, where it’s warmer—around 37°C.
A fever, which can raise the body’s temperature up to 41°C, is one way the body fights viruses. But scientists didn’t fully understand how fever stops viruses, or why some viruses keep growing even when a fever is present.
Bird flu, also known as avian influenza, behaves differently. These viruses prefer the warmer parts of the body, like the lower lungs or even the guts of birds, where temperatures can reach 40–42°C. This means they are already used to high heat, so a fever doesn’t affect them as much.
In the new study, researchers used mice to model what happens during a fever when someone is infected with the flu. They used a human flu virus that was made safe for the lab. Mice don’t normally develop a fever from the flu, so the scientists raised the temperature of the room to make the mice’s body temperature higher, similar to what happens in a fever.
They found that the fever stopped the human flu virus from growing and spreading, helping protect the mice from serious illness. Just a 2°C increase was enough to turn a deadly infection into something mild.
But the bird flu virus kept growing, even at these higher temperatures, and still caused serious illness. This helps explain why bird flu can be so dangerous in people.
The researchers also discovered that a specific gene in the virus, called PB1, plays a big role in how well the virus can survive high temperatures. This gene helps the virus make more copies of itself. Viruses with a bird flu version of the PB1 gene were better at surviving fever and caused more severe disease.
This matters because human and bird flu viruses can swap genes when they infect the same person or animal, like pigs. This gene swapping can make human viruses more dangerous, which has happened in past pandemics, like in 1957 and 1968.
Dr. Matt Turnbull, one of the study authors, said that monitoring bird flu viruses is essential because of the threat they pose. If we know which viruses can survive fever, we can better prepare for possible outbreaks.
Professor Sam Wilson from Cambridge added that although bird flu rarely infects humans, when it does, it can be deadly—sometimes with a fatality rate over 40%, like with the H5N1 strain.
The findings might also change how we think about treating flu. Right now, people often take fever-reducing medicines like ibuprofen or aspirin. But there’s some evidence that lowering a fever might not always help, and could even make it easier for the virus to spread.
The researchers say more studies are needed before making treatment changes, but their work gives new insight into how fevers fight infections—and why bird flu remains a serious threat.
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