London, September 11 : Scientists at Imperial College London have warned that pandemic swine flu can infect cells deeper in the lungs than seasonal flu can.

They write in a research paper that this may help understand why people infected with the pandemic strain of swine-origin H1N1 influenza are more likely to suffer more severe symptoms than those infected with the seasonal strain of H1N1.

The researchers have also stressed the need for monitoring the current pandemic H1N1 influenza virus for any changes in the way it infects cells, which may make infections more serious.

Generally, influenza viruses infect cells by attaching to bead-like molecules on the outside of the cell, known as receptors. If a virus cannot find its specific receptors, it cannot get into the cell.

Seasonal influenza viruses attach to receptors found on cells in the nose, throat and upper airway, enabling them to infect a person''s respiratory tract.

In the current study, the researchers have found that pandemic H1N1 swine flu can also attach to a receptor found on cells deep inside the lungs, which can result in a more severe lung infection.

They say that the pandemic influenza virus''s ability to stick to the additional receptors may explain why the virus replicates, and spreads between cells more quickly.

"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," Nature Biotechnology quoted Professor Ten Feizi, from the Division of Medicine at Imperial College London, as having writte in the research paper.

The researchers found that pandemic H1N1 influenza bound more weakly to the receptors in the lungs than to those in the upper respiratory tract, which is why most people infected with the virus have experienced mild symptoms.

However, the researchers are concerned that the virus could mutate to bind more strongly to these receptors.

"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," said Prof. Feizi.

"Receptor binding determines how well a virus spreads between cells and causes an infection. Our new study adds to our understanding of how swine-origin influenza H1N1 virus is behaving in the current pandemic, and shows us changes we need to look out for," added Prof. Feizi.

The financial assistance for the study came from the Wellcome Trust, the Medical Research Council and the Engineering and Physical Sciences Research Council. (ANI)