COVID-19: THE SERIES
7.03.2020
CORONA VIRUS: WHAT IT IS AND HOW IT IS TRANSMITTED
In general, viruses are inactive, microscopic and parasitic infectious agents that cannot reproduce by themselves outside a host cell. However, once they infect a susceptible cell, viruses have the capacity to direct the machinery of the infected cell to reproduce and multiply itself, thereby causing diseases. The whole infectious virus particle is called a virion and it consists of the nucleic acid and an outer shell of protein. Viruses contain either RNA or DNA as their genetic material and their nucleic acid may be single or double-stranded. The simplest virus particles contain only enough RNA or DNA to encode four proteins while the most complex viruses encode about 200 proteins.
Now that we understand what viruses are in general, let us then focus on and discuss the virus of significance at the moment- corona virus. The most recent outbreak is reported to have begun in Wuhan (China) in late 2019, infecting more than 90,000 people and claiming over 4,000 lives. Corona viruses are crown-like shaped infectious agents or a group of agents which can cause diseases in birds and mammals. The virus, which was first identified in the 1960’s, is a common virus that predominantly infects the nose, upper throat and sinuses. While most strains aren’t dangerous, some strains and less common forms such as SARS, MERS, and COVID-19 cause severe symptoms and can be lethal. In humans more specifically, corona viruses cause respiratory tract infections that are characteristically mild, as in some cases of the common cold. Symptoms vary in other species, for example; in chickens, they cause an upper respiratory tract disease, while in cows and pigs they cause diarrhea. Treatment measures for viruses such as vaccines and antiviral drugs to prevent or treat human corona virus infections are yet to be developed.
Coronavirus infection, like all viral infections, begins when the virus gains access into the body of the host organism and attaches itself to the complementary host cell receptor using its spike protein. Following the viral attachment, a protease of the host cell cleaves and activates the attached spike protein. Depending on which protease is available in the host cell, cleavage and activation permits the entry of the virus into the cell through endocytosis or direct fusion of the viral envelop with the host membrane. Once in the host cell, the virus particle is uncoated, and its genome enters the host cell cytoplasm. Then, the viral RNA attaches to the host cell’s ribosome for translation and transcription while the host ribosome begins to replicate and reproduce the viral genome. Establishing an infection, determining tissue tropism, infectivity and species range of the virus all depend on the interaction of the corona virus spike protein with its complementary host cell receptor.
Similar to most flu-like viruses, human to human transmission of corona virus occurs predominantly through respiratory droplets generated by sneezing and coughing. Infection occurs when the virus gains access into the host cell following direct or close contact with these droplets released by already infected people. Infection can also occur through touching everyday objects such as doorknobs that may be harboring these droplets, and then using the infected hands to touch ones face.