Influenza viruses are common, and the seasonal flu tends to infect many people every year. Influenza A viruses are the most common kind of human influenza virus, and they usually infect cells in the upper respiratory tract. The temperature in that part of the body is typically about 33ºC, unlike the 37ºC that is typical of the lower respiratory tract. This temperature selectivity can actually help the body defend against these viruses, with a fever. Fevers can raise the human body temperature to as much as 41ºC, helping the body fight upper respiratory viruses.
But scientists have now found a gene that helps viruses tolerate fevers, and continue replicating even when the body temperature is higher than usual. This gene was also found in the viruses that causes deadly pandemics in 1957 and 1968.
The research, which was reported in Science, also showed that avian influenza viruses, which usually infect the lower respiratory tract, are also unaffected by fevers. In the natural hosts of avian influenza viruses like ducks, the virus can infect gut cells, where temperatures may reach 40ºC.
This work used a mouse model of influenza infection to show how fever can shield against the flu. But avian influenza doesn’t seem to be affected by this important, natural protective mechanism.
A virus gene called PB1 was found to be relevant to temperature sensitivity. When viruses carried PB1, they withstood temperatures that would be reached during a fever, and led to serious illness in the mouse model. Human and bird influenza viruses can also share genes when they both infect a host such as a pig.
"The ability of viruses to swap genes is a continued source of threat from emerging flu viruses. We've seen it happen before during previous pandemics, such as in 1957 and 1968, where a human virus swapped its PB1 gene with that from an avian strain. This may help explain why these pandemics caused serious illness in people,” noted first study author Dr. Matt Turnbull of the Medical Research Council Center for Virus Research at the University of Glasgow.
"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% mortality,” added senior study author Professor Sam Wilson of the University of Cambridge. "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."
Sources: University of Cambridge, Science