Coronaviruses cause disease in both humans and animals. Although there are only two coronaviruses that infect humans, they are responsible for an estimated 1/3 of all colds. Coronaviruses also cause diseases, some of which are serious, in animals.

The first clinical cases of SARS appeared in China in November of 2002, and only in the past few weeks has SARS attracted international attention. Yet scientists have not only identified the new pathogen, but its genome has been independently sequenced at the University of Hong Kong (report from People's Daily) and at the Centers for Disease Control in Bethesda, MD (CDC SARS information page). The complete SARS virus sequence was published in GeneBank on Tuesday (April 15, 2003). It's astonishing how quickly the research community has been able to identify and characterize this new pathogen.

The SARS coronavirus is formed of 29736 bases of (+)-sense, single-stranded RNA (ssRNA). The virus enters the cell through endocytosis and membrane fusion, and releases mature mRNA into the cytosol, where its first 20 kb is translated into a viral polymerase. The polymerase then creates a full-length (-)-sense strand, that codes for several proteins. The (-)-sense mRNA is then used as a template to create several other monocistronic mRNAs that code for viral proteins. The proteins assemble in Golgi and are exocytosed through the secretory pathway.

Sequencing the genome for SARS has several benefits:

• Although the origin of SARS is as yet unknown, phylogenetic analysis of the sequence indicates that it is a new virus that has not been seen in humans before. Scientists believe that the virus has probably been in an animal population for quite some time, but the original host has not yet been identified.
• Studying the sequence has already lead scientists to believe that there is little or no evidence that SARS is a bioweapon.
• In the short term, having the SARS sequence allows design of sensitive polymerase chain reaction (PCR) tests that can rapidly identify presence of the SARS virus itself. Existing serological tests look for antibodies to the virus, but by the time the antibody titers are high enough to be detected (typically three weeks following the onset of fever), it is often too late for the patient. By contrast, the SARS RT-PCR test should work within the first ten days of onset of fever. A positive PCR test for SARS indicates positively that a patient is infected. A negative test cannot rule out exposure or infection for a variety of reasons. For example, the sample taken from the test may not have contained the virus, or the virus may be suppressed by the host's immune system.
• Knowing the sequence may help researchers to study how the virus is transmitted, to detect the viremia and virus shedding timeline, and to develop effective quarantine strategies.
• In the long term, understanding SARS may help researchers discover a way to combat the disease, or at the very least, to predict who is likely to be at greatest risk. While 96% of SARS patients recover spontaneously, those over 40 or with existing heart or liver disease are most at risk for SARS.

A strange yet oddly entertaining report on coronaviruses is available at Stanford.