Saturday, September 10, 2011

Severe acute respiratory syndrome (SARS)

nullSARS belongs to Coronaviruses which are large, enveloped, positive-stranded RNA viruses with a diameter of 60–220 nm. Most but not all viral particles display the characteristic appearance of surface projections, giving rise to the virus family’s name (corona, Latin, = crown). They have the largest genomes of all RNA viruses (Ofner et al., 2006).
Nosocomial transmission to HCWs (with some fatal results) was a prominent feature in virtually every country experiencing the disease. Much of the nosocomial spread of SARS was attributed to inconsistent observance of strict airborne precautions and inconstant use of personal protective equipment, particularly during aerosol-generating activities (Chen et al., 2009).
Throughout the outbreak, the primary mode of transmission appears to be direct mucous membrane (eye, nose and mouth) contact with infectious respiratory droplets and/or through exposure to fomites. Cases have occurred primarily in persons with close contact with those very ill with SARS in healthcare settings. Under certain circumstances, such as in healthcare settings or other closed environments, contamination of inanimate materials or objects by infectious respiratory secretions or other body fluids (saliva, tears, urine and faeces have been found to contain virus) seems to occasionally play a role in disease transmission (Chan., 2004).

The laboratory diagnosis:
Respiratory, stool and rectal swab specimens are collected in viral transport medium and urine samples are transported in sterile containers. Specimens collected are refrigerated (approximately 10°C) until delivery. Any procedure with the potential to generate fine particulate aerosols (e.g., vortexing or sonication of specimens in an open tube) should be performed in a biologic safety cabinet and consideration should be given to use of respiratory protection using a properly fitted respirator (N-95 or higher). Work surfaces
and equipment should be decontaminated using agents that are effective against lipid-enveloped viruses (Chen et al., 2009).
The diagnosis of SARS is based on either detection of the virus in clinical specimens or the finding of antibodies directed against the virus in serum. Isolation of the virus in Vero E6 cells and morphologic characterization of the virus by electron microscopy provided the initial evidence for the role of a previously unrecognized coronavirus in SARS. However, viral culture is not recommended for routine diagnosis because of low sensitivity and because isolation of the virus requires biosafety level (BSL)-3 facilities and work practices (Chan., 2004).
All positive tests should be confirmed by testing of a second specimen and by testing in a reference laboratory. Shedding of virus in respiratory secretions is greatest early in the second week of illness therefore, specimens collected in the first few days after onset may be falsely negative because of low numbers of virions (Varia et al 2003).
RT-PCR has provided a powerful tool for the diagnosis of SARS. A highly sensitive real-time RT-PCR assay that can detect as few as one to ten copies is available. The assay targets three sites in the nucleocapsid and polymerase genes and amplification of at least two targets is required for a test to be considered positive (Susan et al., 2004).
Antibodies to SARS may be detected by enzyme immunosorbent assay (EIA) as early as the end of the first week of illness. However, delayed seroconversions have been reported therefore, serologic assays for SARS should not be considered negative until a specimen collected at least 28 days after onset has been tested. The EIA for SARS appears to be highly specific (Shigayeva et al., 2007).

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