Fever serves as one of the body’s primary defenses against infections by elevating internal temperatures, making it harder for viruses to replicate and cause significant harm.

However, recent findings from researchers at the University of Cambridge in the UK indicate that this natural thermal defense is ineffective against bird flu.

Human influenza viruses generally inhabit the upper respiratory tract, where the temperature is typically around 91 degrees Fahrenheit (33 degrees Celsius). A fever can raise this temperature to nearly 106F (41C), impeding viral replication and allowing the immune system to combat the infection effectively.

In contrast, bird flu viruses tend to thrive in the lower respiratory tract and sometimes the gut of birds, where temperatures range from 104 to 108F (40 to 42C), exceeding what the human body can typically produce.

To explore the impact of this on the severity of bird flu infections, scientists conducted experiments on mice using a modified PR8 flu virus. This lab-engineered strain, considered safe for humans, had its genetic makeup adjusted to closely resemble either human or avian flu viruses.

The study revealed that when the mice were exposed to temperatures akin to a human fever, the human-like virus struggled to reproduce. In contrast, the bird flu-like virus continued to replicate efficiently, leading to severe 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. 

The above image shows scientists in Paris, France, working to monitor the spread of bird flu. French authorities have warned that a bird flu outbreak could be deadlier than that for Covid

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.

The above shows bird flu particles, in yellow color, that were grown in cells in a lab (stock image)

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.