Avian influenza can tolerate higher temperatures than other viral infections, meaning they can survive high fevers that wipe out the others.
This has worrisome implications should avian influenza mutate to become a human pandemic. It has already infected a few people, but has not spread from them.
Research led by the universities of Cambridge and Glasgow and published Science journal identified a gene that plays a key role in how well flu viruses tolerate heat. During the 1957 and 1968 flu pandemics, that PB1 gene moved from bird flu viruses into circulating human strains, which helped those strains spread more easily.
Seasonal human influenza A viruses typically replicate most effectively in the cooler upper airways, around 33 degrees celsius and spread less efficiently in the warmer lower respiratory tract. Fever, which can raise body temperature to as high as 41 degrees is one of the body’s primary defenses against viral infection.
The study used mice to examine why fever slows human flu but does little to stop avian strains. Because mice do not develop fever when infected with influenza A, researchers simulated one by increasing the animals’ environmental temperature. They found that fever-range temperatures significantly reduced replication of a human-origin strain but had little effect on avian viruses. A two-degree temperature increase was enough to turn a normally lethal human-origin infection into a mild one.
Investigators reported that viruses containing an avian-like PB1 gene withstood high temperatures and caused severe disease in mice. Scientists have long noted that bird and human flu viruses can swap genetic material when they infect the same host.
Although human infections with bird flu remain rare, some historic avian influenza strains have caused fatality rates above 40 per cent. Professor Sam Wilson of the University of Cambridge said understanding why bird flu causes severe illness in humans is important for surveillance and pandemic planning, especially given the ongoing threat posed by H5N1.
Researchers said the findings may eventually influence treatment guidance, noting that some evidence suggests reducing fever could aid influenza spread, though more study is needed. The project received funding from several UK and international agencies, including the Medical Research Council and the US Department of Agriculture.
