New research led by the universities of Cambridge and Glasgow shows that bird flu viruses pose a unique threat to humans because they can replicate at temperatures higher than those generated during fever. Fever is one of the body’s primary defenses against viral infections, yet avian influenza strains can continue to thrive even when body temperatures rise significantly. The findings, published in Science, shed new light on how temperature sensitivity influences viral survival.
How Temperature Shapes Flu Virus Behavior
Seasonal human influenza A viruses typically infect the upper respiratory tract, where temperatures remain around 33°C. They replicate less efficiently in the warmer environment of the lower respiratory tract, which averages 37°C. Fever, which can push body temperature up to 41°C, normally slows or stops viral replication.
However, avian influenza viruses behave differently. They flourish in the lower respiratory tract and, in their natural hosts—such as ducks and seagulls—they often infect the gut, where temperatures reach 40–42°C. This adaptability allows bird flu strains to withstand temperature conditions that would halt human flu viruses.
Study Design and Key Findings
To explore these differences, the research team used mice infected with influenza viruses. Although mice do not naturally develop fever in response to flu, scientists mimicked fever by raising the ambient temperature to elevate body temperature. They used a laboratory-adapted human influenza strain (PR8), which is safe for humans.
As reported by medicalxpress, the team demonstrated that fever-level temperatures effectively stopped human flu viruses from replicating. Remarkably, just a 2°C rise converted a lethal infection into a mild illness.
In contrast, avian-origin viruses continued to replicate even at elevated temperatures. The study identified the PB1 gene—responsible for viral genome replication—as a key factor in temperature resistance. Viruses carrying an avian-like PB1 gene survived fever conditions and caused severe disease. This finding is significant because human and avian viruses can exchange genes when they co-infect a host, as documented in the 1957 and 1968 pandemics.
Expert Insights on Pandemic Risks
Dr. Matt Turnbull from the University of Glasgow highlighted the ongoing threat of gene swapping among flu viruses. He noted that previous pandemics emerged when human viruses acquired avian PB1 genes, which increased severity. He emphasized the importance of monitoring bird flu strains for their temperature resilience to identify potentially dangerous variants.
Professor Sam Wilson from the University of Cambridge added that although human infection with bird flu remains rare, case fatality rates are extremely high. He stressed that understanding why avian viruses cause severe illness is essential for global surveillance and pandemic preparedness, especially given the persistent threat from H5N1 strains.
Implications for Treatment and Future Research
The study’s findings may influence future treatment strategies. While fever-reducing medications such as ibuprofen and aspirin are widely used, evidence suggests that lowering fever could sometimes worsen outcomes or increase viral transmission. Nevertheless, the researchers caution that additional studies are needed before any changes to treatment guidelines are considered.