Severe immune response kills SARS victims
15:20 02 May 03
NewScientist.com news service
An excessive immune reaction appears to be the fatal factor in patients who die of SARS, according to medical data from Hong Kong. The best estimate of the fatality rate of SARS is rising steadily and so understanding how the disease causes death is critical to finding the best treatments.
Scientists have also discovered that the SARS virus can remain viable for at least 24 hours after being deposited in a droplet on a plastic surface - a simulation, for example, of an infected person coughing on to the wall of a lift.
The new information was revealed by Klaus Stöhr, the World Health Organization's chief scientist for SARS in a briefing earlier this week. "In the first week or so of disease the virus is replicating mainly in the upper respiratory tract, causing high fever and dry cough. But in the second week we see an over-reaction of the immune response, and in 20 per cent of cases that leads to very severe disease."
Mark Salter, WHO's coordinator for the clinical management of SARS, told New Scientist that this excessive response seems to be destroying the alveoli, the tiny sacs at the end of the lung tubing where the oxygen is absorbed. It is like stripping all the leaves from a tree, he says, making breathing extremely difficult.
"The immune response develops, but then continues growing unchecked" says Salter. "You then get cytokines, tumour necrosis factor and so on released not just at the virally infected cells they are supposed to kill, but throughout the tissue". This also happens in other diseases but the mechanism is not entirely understood, and certainly not in the case of SARS, he says.
Nevertheless the observation significantly strengthens the basis on which doctors can select the treatments given to patients. In particular, it suggests the use of corticosteroids, alongside other drugs that reduce the viral load such as interferon.
Corticosteroids suppress the immune system and are normally used against asthma, rheumatoid arthritis and in transplant patients to reduce the risk of rejection.
Their use does raise the risk of further infection, but Hong Kong hospitals are claiming success with combinations of the antiviral ribavirin and pulsed steroids, says Salter: "And once steroids are stopped, they see a flare up of the condition suggesting there is a significant immune component to the disease."
The issue of how long the virus survives outside the body was also raised by Stöhr. Some scientists have argued that long survival could explain unusual cases of transmission in Hong Kong and Canada.
In a study in Germany reported by Stöhr, the virus was dropped in liquid on to a plastic surface, the drop was dried, and examined after 24 hours. "There were 10,000 virus units per millitre before, and after 24 hours there were 1000 - and that was viable viruses."
Robert Walgate
newscientist.com |
