Severe Acute Respiratory Syndrome - Cause

Cause

Coronaviruses are positive-strand, enveloped RNA viruses that are important pathogens of mammals and birds. This group of viruses cause enteric or respiratory tract infections in a variety of animals, including humans, livestock and pets.

Initial electron microscopic examination in Hong Kong and Germany found viral particles with structures suggesting paramyxovirus in respiratory secretions of SARS patients. Subsequently, in Canada, electron microscopic examination found viral particles with structures suggestive of metapneumovirus (a subtype of paramyxovirus) in respiratory secretions. Chinese researchers also reported a Chlamydophila-like disease may be behind SARS. The Pasteur Institute in Paris identified coronavirus in samples taken from six patients, as did the laboratory of Malik Peiris and Yuen Kwok-yung at the University of Hong Kong, which in fact was the first to announce (on 21 March 2003) the discovery of a new coronavirus as the possible cause of SARS, after successfully cultivating it from tissue samples and was also amongst the first to develop a test for the presence of the virus. The CDC noted viral particles in affected tissue (finding a virus in tissue rather than secretions suggests it is actually pathogenic rather than an incidental finding). Upon electron microscopy, these tissue viral inclusions resembled coronaviruses, and comparison of viral genetic material obtained by PCR with existing genetic libraries suggested the virus was a previously unrecognized coronavirus. Sequencing of the virus genome — which computers at the British Columbia Cancer Agency in Vancouver completed at 4 a.m. Saturday, 12 April 2003 — was the first step toward developing a diagnostic test for the virus, and possibly a vaccine. A test was developed for antibodies to the virus, and it was found that patients did indeed develop such antibodies over the course of the disease, which is highly suggestive of a causative role.

On 16 April 2003, the WHO issued a press release stating a coronavirus identified by a number of laboratories was the official cause of SARS. Scientists at Erasmus University in Rotterdam, the Netherlands demonstrated that the SARS coronavirus fulfilled Koch's postulates thereby confirming it as the causative agent. In the experiments, macaques infected with the virus developed the same symptoms as human SARS victims.

An article published in The Lancet identifies a coronavirus as the probable causative agent.

In late May 2003, studies from samples of wild animals sold as food in the local market in Guangdong, China found the SARS coronavirus could be isolated from palm civets (Paguma sp.), but the animals did not always show clinical signs. The preliminary conclusion was the SARS virus crossed the xenographic barrier from palm civet to humans, and more than 10,000 masked palm civets were destroyed in Guangdong Province. Virus was also later found in raccoon dogs (Nyctereuteus sp.), ferret badgers (Melogale spp.) and domestic cats. In 2005, two studies identified a number of SARS-like coronaviruses in Chinese bats.

Phylogenetic analysis of these viruses indicated a high probability that SARS coronavirus originated in bats and spread to humans either directly, or through animals held in Chinese markets. The bats did not show any visible signs of disease, but are the likely natural reservoirs of SARS-like coronaviruses. In late 2006, scientists from the Chinese Centre for Disease Control and Prevention of Hong Kong University and the Guangzhou Centre for Disease Control and Prevention established a genetic link between the SARS coronavirus appearing in civets and humans, bearing out claims that the disease had jumped across species.