New research from the University of Cambridge in the UK suggests that one of the body’s primary defenses against infection, fever, may not be effective against bird flu viruses. Normally, the body raises its internal temperature to make it harder for viruses to replicate, thereby giving the immune system more time to clear an infection. For typical human flu viruses, which reside in the upper respiratory tract where temperatures are around 91 degrees Fahrenheit (33 degrees Celsius), a fever raising the temperature to nearly 106F (41C) significantly slows down the infection.

However, bird flu viruses are adapted to thrive in the warmer environments of the lower respiratory tract and sometimes the gut of birds, where temperatures range from 104 to 108F (40 to 42C). These temperatures are often above what a human body can achieve during a fever, raising concerns about the efficacy of this natural defense mechanism against avian influenza.

To investigate this, scientists, led by Dr. Sam Wilson, a molecular virologist at Cambridge University, infected mice with a modified lab-grown PR8 flu virus. This strain, considered safe for humans, had its genetic section responsible for viral replication altered to mimic either human or bird flu viruses. The mice were then kept at an ambient temperature, or a temperature that simulated a human fever, specifically 106F (41C), to raise their body temperature accordingly since mice do not typically develop fevers.

The findings, published in *Science*, were stark: the human-like version of the virus struggled to replicate and caused no severe disease in mice kept at the fever-mimicking temperature. In contrast, the bird flu-like version continued to make copies of itself and caused serious disease even at this elevated temperature. This suggests that the higher temperatures associated with a fever, which typically denature key viral proteins and slow reproduction, do not hinder bird flu viruses in the same way.

The implications of this research are significant. Researchers warned that fever may not be an effective defense against bird flu and that, consequently, treating a fever in a bird flu-infected patient might not always be beneficial. Dr. Matt Turnbull, a virologist at the University of Glasgow and the study’s first author, emphasized the importance of monitoring bird flu strains to assess their resistance to fever, which could help identify more virulent strains that pose a higher risk of spillover to humans.

This research comes amidst growing concerns about bird flu. An American in Washington state recently died from a previously unseen strain of bird flu in humans, marking only the second bird flu fatality recorded in the US. French officials have also warned that a bird flu pandemic could potentially be more deadly than the Covid outbreak. Dr. Wilson highlighted the worryingly high fatality rates of historic H5N1 infections in humans, which caused over 40 percent mortality, and stressed that understanding what makes bird flu viruses cause severe illness in humans is crucial for pandemic preparedness, especially given the ongoing threat posed by avian H5N1 viruses.

Since January 2022, an outbreak of bird flu in the animal world has led to 71 recorded human infections in the US, with all but one patient infected with the H5N1 strain. Most cases have been in California and Washington, linked to exposure to infected domestic poultry or dairy cows. The virus has been detected in wild and domestic birds in every US state since January 2022, affecting an estimated 174 million birds and infecting over 1,000 dairy herds. Since September of this year alone, 7 million farmed birds, including 1.3 million turkeys, have been affected.

Experts warn that bird flu infections are more common in the fall months due to the migration of wild birds, which can spread the virus to new areas. While CDC officials currently state that the risk of the bird flu virus transmitting between humans and causing a new outbreak is “low,” there remain significant concerns that the virus could adapt to spread efficiently between people, potentially sparking a new pandemic similar to Covid.