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A new coronavirus variety causes SARS

By N. Gopal Raj

THIRUVANANTHAPURAM May 3. <150>Sequencing of the entire genome of the SARS virus has confirmed that the germ is indeed a new variety of coronavirus. But questions about its origin remain. The SARS coronavirus (SARS-CoV) was first sequenced by a Canadian team and shortly thereafter by group headed by scientists from the Center for Disease Control and Prevention (CDC) in the United States. The journal, Science, has rushed through the publication of the two genome sequences. The two sequences are quite similar. In addition, research teams in Singapore and Beijing have also sequenced strains of the virus.

The World Health Organisation (WHO) had created an extraordinary network of 13 laboratories in 10 countries which identified the virus associated with the SARS in just two weeks and had its entire genome sequenced in two more weeks, observes Barry R. Bloom, Dean of the Harvard School of Public Health, in an editorial in Science.

Coronaviruses infect humans and other animals. In humans, these viruses have hitherto only caused mild respiratory illnesses. Coronaviruses are divided into three groups. Groups 1 and 2 contain mammalian viruses, while group 3 is composed of those infecting birds. Human coronaviruses are found in both groups 1 and 2.

``The completion of the genomic sequence of SARS-CoV provides a first look at the molecular characteristics of this virus and clearly demonstrates that it has features typical of a coronavirus, while it also has features that distinguish it from all previously sequenced coronaviruses,'' remarked the CDC group. The virus was so distinct and genetically distant from other known coronaviruses that no large part of its genome could have come from these viruses, they added. The Canadians have proposed that SARS-CoV should be accepted as belonging to a fourth class of coronaviruses (Group 4).

But a major disappointment will be that the genome sequences of the SARS virus does not contain any obvious clue to its origins. It does not appear to be a mutant of a known coronavirus. Neither did the genomic sequence indicate its potential animals origins, says the CDC team. It might be an animal virus for which the normal host is unknown and which mutated, acquiring the ability to effectively infect humans, observed the Canadians.

China, where the SARS infection began, has been implicated in the origin of influenza pandemics. A prime reason is that humans, pigs and birds (ducks and chicken) live in close proximity. As pigs are susceptible to both human and bird flu viruses, the genes of the two viruses can mix and create lethal new varieties.

However, in 1997 and in February this year, a bird flu made the transition directly from birds to human hosts. Mercifully, this strain of bird flu could not easily transmit itself to other humans.

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