As the summer of 2003 approaches, newspapers and television news programs are reporting that public health measures are succeeding in bringing the SARS epidemic under control. This is welcome news to a worried world. In every country, people have been seeking ways to avoid a disease about which they know little but which scares them very much. Even in Saint Louis, where no case of SARS has been reported, everyone seems to be talking about it. This spring Washington University held a graduation ceremony to which it had urged parents of students from China, the center of the SARS outbreak, not to attend. This level of worldwide anxiety over a disease no one had heard of six months ago is remarkable. The achievement of controlling the disease in this short period of time is even more remarkable.
SARS is a viral infection of the lungs (the letters SARS stand for Severe Acute Respiratory Syndrome) which produces severe pneumonia-like symptoms, and often death. Between the time the first cases of SARS surfaced in China in November 2002 and mid-June 2003, there have been 8,445 SARS cases, and 790 deaths. The W.H.O. estimates that worldwide from 10% to 15% of those infected eventually die of the disease, a very high mortality rate. The devastating Spanish flu of 1918-19 had a mortality rate of less than 3% but killed something like 50 million people because infected individuals spread the virus so widely.
The W.H.O. issued its global SARS alert on March 15, announcing that an unknown and deadly virus had appeared in mainland China in Guangdong province opposite Hong Kong, and that immediate steps needed to be taken to control its spread, lest the world repeat the horror of last centurys worldwide transmission of Spanish flu.
Scientists soon isolated the SARS culprit, which proved to be a new sort of corona virus, a kind of cold virus that, while causing diseases in farm animals, had never in the past been a danger to humans. This new kind of corona virus is found naturally in civets (a cat-sized relative of the mongoose) and other small Asian mammals. It does not make these mammals sick, and appears to have lived in them for a very long time before infecting humans and starting the SARS epidemic.
The 30,000 nucleotide SARS genome (composed of RNA rather than DNA, as is the AIDS virus HIV) is being sequenced repeatedly in labs all over the world. There seems to be very little difference in isolates collected around the world. The genomes of strains from Toronto and Veitnam differ by just eight nucleotides. This is good news, as it indicates that the virus genome, unlike HIV, is stable as it passes from one human to the next, and thus is a good candidate for a vaccine.
The first need was for a clear diagnostic test for the SARS virus. It took scientists at the Center for Disease Control only ten days to isolate the virus, an astonishing tour-du-force. It took virologist Christian Drosten working at the Bernhard Nocht Institute for Tropical Medicine in Hamburg, Germany, only one day more to devise a proper test. On March 26 he unveiled an effective SARS test, based on the polymerase chain reaction (PCR). His test consists of a solution containing all the enzymes and other chemicals needed to copy DNA, and takes advantage of a special requirement of the copying process — the DNA-copying enzyme requires a double-stranded bit of DNA on which to sit before it begins its job. Drostens test solution contains bits of single-stranded SARS virus DNA.
Heres how the test works: first isolate a potential virus from the patient, convert its RNA genome to single-stranded DNA, then add the test solution. If the virus is SARS, then the bits of SARS DNA in the Drosten test solution will be able to sit on their complementary sequences on the virus DNA, and DNA copying will commence at these now-double-stranded sites and be easily detected.
With a sensitive accurate SARS test in hand, public health officials immediately set out to quarantine infected individuals, and, for ten days, every person who had had contact with them since their symptoms first developed. SARS spreads primarily in tiny water droplets spread by sneezing, and so requires close contact for transmission. The virus has a ten day incubation period, so any exposed person healthy after ten days has escaped infection.
This Draconian approach, applied over a period of months, and involving unprecedented global cooperation among laboratory scientists, epidemiologists and health officials in many countries around the world, has proven quite effective in controlling the SARS outbreak in China, Taiwan, and Toronto. By June of 2003 few new cases were being reported.
It is far too soon to breath a sigh of relief, however. Flu-like respiratory illnesses typically decline steeply in frequency as warm weather arrives, only to roar back in the fall when cooler temperatures return. We do well to remember that the great influenza pandemic of 1918-1919 killed only a few thousand when it first appeared in the fall of 1917, before returning to kill millions the following two years.
The key question is whether the SARS caronavirus has a natural resevoir among small mammals in China, and whether SARS will re-emerge there in the fall. If there is no further outbreak by Christmas, we can begin to hope the SARS outbreak is under control.
©2003 Txtwriter Inc.
