Colorised transmission electron micrograph of Avian influenza A H5N1 viruses (seen in gold). But is out nose too cold for bird flu?

There are 16 subtypes of avian influenza and some can mutate into forms that can infect humans, by swapping proteins on their surface with proteins from human influenza viruses. The study shows that normal avian influenza viruses do not spread extensively in cells at 32°C, the temperature inside the human nose. The researchers say this is probably because the viruses usually infect the guts of birds, which are warmer, at 40°C.

This means that avian flu viruses that have not mutated are less likely to infect people, as the first site of infection in humans is usually the nose. If a normal avian flu virus infected a human nose, the virus would not be able to grow and spread between cells, so it would be less likely to damage cells and cause respiratory illness.

Professor Wendy Barclay, one of the authors of the study from the Division of Investigative Science at Imperial College London, said: “Bird viruses are out there all the time but they can only cause pandemics when they undergo certain changes. Our study gives vital clues about what kinds of changes would be needed in order for them to mutate and infect humans, potentially helping us to identify which viruses could lead to a pandemic.”

The researchers also found that when they created a mutated human influenza virus by adding a protein from the surface of an avian influenza virus, this mutated virus struggled to thrive at 32 degrees Celsius. This suggests that if a new human influenza strain evolved by adopting proteins from an avian influenza virus, this would need to undergo further changes in order to adapt to the conditions in the human body.

“It would be impossible to develop vaccines against all 16 subtypes of avian flu, so we need to prioritise. By studying a range of different viruses in systems like this one we can look for warnings that they are already beginning to make the kinds of genetic changes in nature that mean they could be poised to jump into humans; animal viruses that spread well at low temperatures in these cultures could be more likely to cause the next pandemic than those which are restricted,” added Professor Barclay.