One of the body’s key defenses against infection is a fever, raising its internal temperature to make it harder for viruses to make copies of themselves and cause severe disease.

But new research from the University of Cambridge in the UK, suggests this thermal defense mechanism does not work against bird flu.

Human flu viruses typically stay in the upper respiratory tract, where temperatures are around 91 degrees Fahrenheit (33 degrees Celsius). With a fever, that can rise to nearly 106F (41C), which slows an infection to give the immune system time to clear the disease.

Bird flu viruses, however, thrive in the lower respiratory tract, and sometimes the gut, of birds, where temperatures are around 104 to 108F (40 to 42C), mostly above the level that a human can achieve.

To investigate how this affects the severity of a bird flu infection, scientists infected mice with a modified lab-grown PR8 flu virus, a strain that scientists say is not dangerous to humans, that had the section of its genetics involved in viral replication altered to more closely match either the human or bird flu viruses.

When the mice were kept at a temperature similar to a human fever, results showed the human-like version of the virus struggled to replicate, but the bird flu-like version continued to make copies of itself and caused serious disease.

The study needs to be carried out in other animals more similar to humans to confirm the results, but the researchers warned that it suggested that a fever may not be effective against the disease.

Scientists added that their findings may affect treatment for bird flu, adding that the evidence suggests that it may not always be beneficial to treat a fever in a bird flu-infected patient. 

Humans use fevers to fight infections because the higher temperatures can cause key proteins that the virus relies on to make copies of itself to denature, or take on a different form. 

This slows virus reproduction and provides more time for the immune system to attack and clear an invading virus. 

Dr Sam Wilson, a molecular virologist at Cambridge University who led the research, said: 'Thankfully, humans don't tend to get infected by bird flu viruses very frequently, but we still see dozens of human cases a year. 

'Bird flu fatality rates in humans have traditionally been worryingly high, such as in historic H5N1 infections that caused more than 40 percent mortality.

He added: 'Understanding what makes bird flu viruses cause serious illness in humans is crucial for surveillance and pandemic preparedness efforts. This is especially important because of the pandemic threat posed by avian H5N1 viruses.'

The research comes after an American in Washington state died after being infected with a strain of bird flu never before seen in humans, marking only the second bird flu fatality recorded in the US. It also comes a day after French officials warned a bird flu pandemic would likely be more deadly than the Covid outbreak.

There have been 71 human infections with bird flu recorded in the US since January 2022, after an outbreak of bird flu began in the animal world and experts are warning over the risk of a major outbreak. All but one patient has been infected with the H5N1 strain.

In the new paper, published in Science, researchers edited the PB1 genes in the virus, the genes that help the virus make copies of itself.

These were altered either to match those found in influenza A viruses, the most common form of the flu that infects humans, or the bird flu virus.

In experiments, mice were infected with either the human-like or bird flu-like virus and then kept at either ambient temperature, around 68 to 72F (20 to 22C), or at a temperature that mimicked a human fever, 106F (41C). 

Mice do not typically develop a fever in response to an infection, but researchers were able to mimic a fever by raising the ambient temperature to that of a human fever, which also raised the body temperature of the rodents.

Results showed there was no severe disease in the rodents infected with the human-like virus that were heated to 106F (41C).

But, among those infected with the bird flu-like virus, they still developed severe disease at this temperature. 

Dr Matt Turnbull, a virologist at the University of Glasgow and first author in the study, said: 'It's crucial that we monitor bird flu strains to help us prepare for potential outbreaks. 

'Testing potential spillover viruses for how resistant they are likely to be to fever may help us identify more virulent strains.'

CDC officials say that the risk of the bird flu virus being transmitted to humans and causing a new outbreak is 'low'.

Since the bird flu outbreak began in 2022, most patients in the US have been in either California, 39 confirmed or probable bird flu cases, or Washington, 15 confirmed or probable bird flu cases.

In most cases, infections have been linked either to exposure to infected domestic poultry or dairy cows, which have also been infected by the virus.

In wild and domestic birds, bird flu has now been detected in every state in the US since January 2022.

An estimated 174million wild and domestic birds are estimated to have been affected, while more than 1,000 dairy herds have also been infected.

Since September this year, it has struck 7million farmed birds nationwide, including 1.3million turkeys. 

Experts warn that infections with bird flu are more common in the fall months because wild birds, which may be carrying the virus, migrate, potentially spreading the virus to new areas.

There remain concerns that the virus could spread to and learn how to spread between humans, which could spark a new outbreak similar to the Covid pandemic.