Researchers here have uncovered an affinity the Sars virus has for white blood cells, particularly in some patients, and they say this could go some way towards explaining the emergence of 'super-spreaders' of the disease, and point the way towards better treatments.

While the virus has been known to invade various cells in such organs as the lungs and intestines, the white blood cells are the first places it has been seen thriving and multiplying, said the study's principal investigator, Associate Professor Ren Ee Chee.

'Most viruses are picky,' said the deputy director of the Genome Institute of Singapore. 'They have a very specific host range.

'For the Sars virus to be replicating within 48 hours of entry into a white blood cell indicates that this is a very important site for the virus.

'When it reproduces very rapidly in some people, coupled with a wet cough, this could be what it takes to create a super-spreader.'

At the same time, a patient whose white blood cells form the ideal nurturing environment for Sars is likely to have a harder time recovering, he pointed out.

Once infected, the white blood cells in the lungs - the first organ where the virus takes hold - call for reinforcements via chemical messages called chemokines, which in turn attract even more white blood cells to the attack site, explained Prof Ren.

Rather than helping, this excessive immune response clogs up the lungs, and that leads to inflammation and other complications.

The congestion that results is so severe in some patients that it shows up as dense white patches on X-rays, which doctors initially thought was scar tissue, he added.

The virus also causes the body to produce proteins that encourage the blood to clot, which can be fatal in the lungs.

By understanding how Sars wreaks havoc in the body, doctors can develop more effective treatments, said Prof Ren.

'There are known drugs which can dampen the damaging effects of this virus, such as by preventing the patient's blood from clotting.'

Dr Ooi Eng Eong, a research clinician at Singapore General Hospital who was also part of the group, said that the anti-malarial drug chloroquine, for example, had shown in lab studies that it could potentially knock out the virus.

More research will show if stopping the virus growth in this way will also stop the excessive chemokine production.

'Those in high-risk groups, such as doctors and nurses, could conceivably take it as a preventive measure during an outbreak,' said Dr Ooi.

The work was done by infecting blood taken from healthy donors with the virus, and looking at the genes which were turned on or off by the infection.

The work of the group from GIS, the National University of Singapore and the Environmental Health Institute has been published in the online journal BioMed Central.

The findings are seen in medical circles as particularly significant in the wake of a recent World Health Organisation warning of a possible re-emergence of Sars, which infected more than 8,400 people and killed about 800 worldwide last year.

Researchers worldwide have been working on solutions. Hong Kong scientists have announced a new way to identify chemicals that can counter new and dangerous viruses like Sars, by using chemicals to thwart viruses from replicating.