Covid-19 Fundaments

Introduction: Corona viruses are important respiratory pathogens to humans and animals. The viruses are widespread among birds and mammals, with bats having largest variety of genomes. They are the cause of community-acquired upper respiratory tract infections in adults and probably also play a role in severe respiratory infections in all age groups.

A novel corona virus was identified as the cause of a cluster of pneumonia cases in Wuhan, a city in the Hubei province of China, at the end of 2019. The 2019 novel corona virus (nCoV-19) is a new virus that causes respiratory illness in people and can spread from person to person. The cases are characterized primarily as fever, dyspnea, and bilateral infiltrates on chest imaging, but full clinical information is still under study.

Virology: Corona viruses are enveloped non-segmented positive sense RNA viruses belonging to the family Coronaviridae. The corona virus subfamily is further classified into four genera; alpha, beta, gamma and delta corona viruses. The human corona viruses (HCoVs) are in two of these genera: alpha corona viruses (HCoV-229E and HCoV-NL63) and beta corona viruses (HCov-OC43, HCov-HKU1, Middle East Respiratory Syndrome Corona Virus (MERS-CoV), and the Severe Acute Respiratory Syndrome Corona Viruses (SARS-CoV). Corona viruses are medium-sized enveloped RNA viruses whose name derives from their crown-like appearance in electron micrographs. The genome encodes four or five structural proteins; S, M, HE and E.

The corona virus infection is initiated by the attachment to specific host cell receptors in respiratory system via the spike (S) protein. The host receptor is a major determinant of pathogenicity, tissue tropism, and host range of the viruses. The interaction between the spike protein S1 triggers conformational changes in the S protein, which then promotes membrane fusion between the viral and cell membrane through the S2 domain protein. Ultimately, such phenomenon is responsible for viremia and respiratory problems including cough, chest pain and breathing difficulty, followed by pneumonia and severe acute respiratory infection (SARI).

Laboratory: Infections of the corona virus could be detected by various methods such as 1) viral antigen or antibody test by rapid diagnostic method, 2) detection of viral antigen by polymerase chain reaction (PCR) method, and 3) virus isolation. Rapid detection of viral antigen is useful in early stages of infection with the presence of clinical symptoms. However, rapid detection of antibodies against the novel corona virus can be done after 7 to 10 days of infection with clinical symptoms. In this context, antibody detection tests should be repeated after one to two-weeks later because of the long incubation period and other confounding factors such as immune status, exposure or contact history as well as, asymptomatic condition of individuals.

The Real-Time RT-PCR diagnostic test is intended for the qualitative detection of nucleic acid of nCoV in upper and lower respiratory specimens (such as nasopharyngeal or oropharyngeal swabs, sputum, lower respiratory tract aspirates, bronchoalveolar lavage and nasopharyngeal wash/aspirate or nasal aspirate) collected from individuals who meet 2019-nCoV clinical and/or epidemiological criteria (for example, clinical sign and symptoms associated with 2019-nCoV infection, contact with probable or confirmed case, history of travel to geographic locations where novel corona virus cases were detected, or other epidemiological links such a community transmission for which novel corona virus testing may be indicated as part of a public health investigation).The Real time PCR method is considered as the gold standard test for early detection and confirmation of the novel corona virus infection. However, detection limits, sensitivity and specificity of the real time PCR test kit are crucial factors for a confirmatory diagnosis of the novel corona virus 2019 infection.

@ Courtesy of Bishnu P Upadhyay, PhD Scholar, Pokhara University & Dr. Shyam P. Lohani, Professor and Founder, Nobel College  (Affiliated to Pokhara University), Kathmandu, Nepal