Diagnosis

Differentiating SARS from other respiratory tract infections is difficult, especially during winter flu months. Serological assays involve detecting, in patient blood, antibodies specific to SARS-CoV that do not cross-react with other coronaviruses. IgM can be detected in the blood as early as 3-4 days post infection, whereas IgG is not produced till at least 10 days after infection.

RT-PCR assays are also used to detect viral mRNA specific to SARS in the nasopharyngeal aspirates of patients within three days of infection. This method provides a quick and sensitive way of detecting the presence of SARS-CoV in the patient.

SARS-CoV specific monoclonal antibodies may be manufactured by inoculating animal models with SARS-CoV antigens and isolating the antibodies from the animal. These antibodies can then be used to detect the presence of SARS-CoV in patients' blood (70).

Serological Assays

Serological assays developped for the diagnosis of SARS would be useful in the event of future outbreaks. Current PCR techniques are time-consuming and difficult to perform. Additionally, serodiagnosis of SARS would circumvent the need for BSL-3 facilities.

Tan et al. (120) have characterized antibody profiles against proteins expressed by SARS-CoV. Proteins were expressed and individually tested for reactivity with sera samples from 81 patients. All samples showed reactivity toward N protein, while 73% were reactive toward U274, which is a protein unique to SARS-CoV. Additionally, it was found that while all convalescent-phase sera were reactive toward S protein, acute phase sera (2-9 days after the onset of illness) were not, suggesting that anti-N antibodies appear earlier than anti-S antibodies during in the humoral immune response. None of the 100 control sera from healthy donors reacted, attesting to the high specificity of the antibodies. As such, serological assays could be devloped using recombinant N, S, and U274 proteins.

In another study by Timani et al. (124), N protein was cloned, sequenced, and expressed in E. coli . It was found that at 10, 20, and 30 days after the onset of disease, 13/16 (81.3%), 16/16 (100%), and 16/16 (100%) of SARS patients' sera exhibited significant positive responses to N protein, respectively, implying that anti-N IgG is commonly present at 10 days after disease onset. Additionally, none of the 131 control samples reacted, indicating the possibility of developping highly specific and sensitive N protein ELISAs for use in serodiagnosis. It hsould be cautioned, however, that cross-reactivity with the other known human coronaviruses (HCoV's) would have to be investigated, since conserved regions of N protein across different coronaviruses could lead to incorrect diagnoses.

Finally, it should be noted that similar approaches using N protein in DNA vaccination have been studied in related coronaviruses. For example, Liu et al. (78) found that DNA immunization with N protein from porcine transmissible gastroenteritis virus (TGEV) induced immunity via T cell activation and memory.

Clinical Presentation

• Fever
• Chills/rigors
• Headache
• Malaise
• Myalgia (muscle ache)
• Cough
• Dyspnea (shortness of breath)
• May progress to acute respiratory distress syndrome (inflammation of the lungs and accumulation of fluid in the alveoli): leads to low blood oxygen levels - shortness of breath, rapid labored breathing, low blood pressure
• Leukopenia (low white blood cell count)
• T-Lymphopenia (low T cell count)
• Hypoxemia (<90 mmHg of O₂ in blood)

Treatment Strategies

Antiviral drug screening & design
Drug targets can be proteins in the viral infection pathway that are specific to the virus. A drug that associates irreversibly with the spike proteins and/or other membrane glycoproteins can prevent viral binding and fusion to the host membrane. Viral replication can be inhibited by interference with virus-specific polymerases and proteases (70). Viral assembly may be hindered by drugs which block assembly proteins.

Immunomodulation
Cytokine therapy via the administration of IFN-gamma was shown to be an effective inhibitor of SARS-CoV replication. (60) Furthermore, since SARS-CoV is known to suppress immune function in patients (link to immune evasion), cytokine therapy that upregulates the arms of immunity which have been depressed would be useful treatment. Passive immunization with neutralizing antibodies (e.g. IgG) to S protein may block receptor association of the virus with the host cell membrane and thus prevent infection of cells. Administration of corticosteroids would down modulate the pro-inflammatory response in lung tissue and reduce some of the lung pathology.

siRNA technology
Small interfering RNA (siRNA) is used to inhibit gene expression. It is complementary to the mRNA of the gene of interest and thus binds to it, triggering degradation of the double stranded mRNA. siRNA could be used to inhibit viral gene expression and thus viral replication (138).