Decoding SARS

Severe Acute Respiratory Syndrome (SARS) was formed when a mammalian and an avian virus combined, according to a U of T study published in the Journal of Virology.

The study sheds light on the SARS coronavirus, which causes a potentially fatal form of pneumonia and is in the same family of viruses responsible for the common cold. By tracing its evolutionary history, David Guttman, a co-author of the study and professor of evolutionary genomics in the botany department, proposed that SARS is likely the result of a rare recombination between viruses from two hosts – mammalian (such as cats, cows and mice) and avian (such as chickens and ducks).

Guttman and John Stavrinides, a PhD student at the University of Toronto, found that a large protein encoded on the genome’s left side is more closely related to mammalian viruses, while two smaller proteins on the right side are more closely related to avian viruses. The middle gene – the S gene – encodes a protein that is a mix of mammalian- and avian-like viruses. “Recombination events such as this can bring together two distant viruses and instantaneously make a completely new molecule. It is possible that this new protein was different enough from its parents that our immune system was not able to respond to it in an effective manner,” says Guttman.

The SARS virus is believed to be transmitted to humans by masked palm civets (an animal related to ferrets and cats) in South China markets, he says. “A civet may have picked up the virus from a bird, and the new recombinant virus may have then spread to humans due to poor hygiene and close quarters in the food markets of Southern China.”

Understanding the evolution of SARS is a crucial step toward managing future viral outbreaks, adds Guttman. Similar genetic exchanges are believed to be responsible for past viral epidemics and pandemics, such as the 1918 Spanish influenza pandemic that killed more than 20 million people around the globe.