Healthy living

Why swine flu can be more severe than seasonal flu

swine flu can infect deep into the lungs Virus can also infect lung cells

For most people, swine flu causes mild symptoms, but for others it can cause a more severe illness than ordinary seasonal flu. New research by scientists at Imperial College London suggests the reason for this is that the swine flu virus can infect cells situated deep in the lungs.

Influenza viruses infect cells by attaching to compounds called "receptors" on the outside of the cell. Different viruses have different host cells. If the virus fails to come into contact with it's matching cell receptor it cannot attach to and infect the cell. But if it does attach to the cell, it can gain entry into the cell and use the cell's own machinery to make new copies of itself. Eventually these will burst out of the cell and spread to other cells.

Seasonal flu H1N1 virus attaches to cells in the nose, throat and upper airway. The new research shows that pandemic H1N1/2009 swine flu virus can also attach to a receptor found on cells deep inside the lungs, and this can result in a more severe lung infection.

It may also help the swine flu virus spread more quickly, as it will have more cell types to attach to than seasonal flu viruses.

One of the authors of the study, Professor Ten Feizi, said: "Most people infected with swine-origin flu in the current pandemic have experienced relatively mild symptoms. However, some people have had more severe lung infections, which can be worse than those caused by seasonal flu. Our new research shows how the virus does this - by attaching to receptors mostly found on cells deep in the lungs. This is something seasonal flu cannot do."

The good news is that swine flu virus's attachment to lung cells is weaker that to other cells - this is why most people infected with the virus have experienced mild symptoms. However, the researchers are concerned that the virus could mutate enabling it to bind more strongly to the receptors on cells in the lung.

"If the flu virus mutates in the future, it may attach to the receptors deep inside the lungs more strongly, and this could mean that more people would experience serious symptoms. We think scientists should be on the lookout for these kinds of changes in the virus so we can try to find ways of minimising the impact of such changes," added Professor Feizi.

The study is reported in Nature Biotechnology.

This article was published on Fri 11 September 2009

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