Coronaviruses are microscopic infectious agents requiring a host organism to replicate. The novel coronavirus SARS-COV2 infects humans causing a respiratory disease. Let us explore how viral infection occurs, the ways to prevent the infection and how to treat the disease.
Coronaviruses belong to a family of RNA viruses known as Coronaviridae that have a lipid envelope decorated with proteins. The proteins on the surface give the viruses a characteristic crown like appearance when viewed under an electron microscope, thus giving them the name corona. Viruses need a host organism to replicate. Coronaviruses are known to infect birds and mammals including humans.
What is a virus?
Viruses are microscopic infectious agents which require a host organism to replicate and infect all living organisms from bacteria to humans. Viruses have DNA or RNA as their genetic material protected by a protein coat called a capsid and, in some cases, enveloped by a lipid shell derived from their host organism.
In humans, coronaviruses cause mild diseases like the common cold and severe respiratory illnesses like SARS and MERS. The latest pandemic gripping the world is caused by a novel coronavirus officially named severe acute respiratory syndrome coronavirus 2 (SARS-COV2). In the UK the virus is often referred to by its unofficial name coronavirus. The SARS-COV2 virus is thought to have originated in bats, and ‘jumped’ to humans spreading to 210 countries to date.
What is COVID-19?
SARS-COV2 causes a respiratory disease called COVID-19 (Coronavirus Disease 2019). In mild cases the disease in mild cases affects the upper respiratory tract causing coughs and fever. In severe cases it affects the lungs causing pneumonia which requires hospital care. According to the WHO, 80% of the infections are mild, 15% have a severe infection requiring oxygen support, and 5% need critical care. About 50% of the infections are asymptomatic, this means the person does not have symptoms. The virus spreads directly by droplets expelled when coughing, sneezing, talking and indirectly by touching contaminated surfaces like doors, doorknobs, handles, tables etc.
^ TEM of MERS-COv showing the characteristic crown like appearance (from CDC)
What is the biology behind COVID19?
^ Credit: Kanaga Vijayan Dhanabalan, Department of Biological Sciences, Purdue University USA
The shell of coronavirus is decorated with three kinds of proteins. One type is referred to as spike proteins. These spike proteins are used as a key to enter the host cell. They bind to specific receptor proteins called Angiotensin Conversion Enzyme 2 (ACE-2), which are found on the membranes of cells mostly in the lungs, intestines and kidneys.
Binding of the virus to ACE-2 receptors activates the cell to engulf the virus. Once inside, the virus takes over the host cell machinery to make more copies of the virus RNA and envelope proteins. The assembled virus then exits the cell to infect other cells.
^ Credit: Kanaga Vijayan Dhanabalan, Department of Biological Sciences, Purdue University USA
Our immune system responds to viral infections in several ways. Specialised cells called T cells and Natural Killer cells recognise infected cells and kill them with little damage to the body. Specific antibodies are secreted to block the virus from infecting other cells.
The immune system is further activated by production of molecules called interferons and cytokines. In most cases, the COVID-19 infection has mild to moderate symptoms like headache, cough and fever. However, in the case of severe infections, the virus infects tissues deep in the lungs and the body responds by releasing a battalion of inflammatory molecules called cytokines. The excessive inflammation produced damages the alveoli, air sacs responsible for oxygen exchange. In turn this causes breathlessness and can also result in secondary bacterial infections.
What makes Coronavirus deadly?
Although coronavirus causes a respiratory illness like that of flu, the rate of transmission of coronavirus is much higher. The R0 (read as R-nought) of coronavirus is about 3-6 i.e. 1 person is able to infect up to 3- 6 people when no precautions are taken. The higher R0 combined with the fact that many people are asymptomatic carriers is what makes the virus deadly.
^ Credit: Kanaga Vijayan Dhanabalan, Department of Biological Sciences, Purdue University USA
As of now, because a vaccine or treatments are unavailable, preventing the disease is the best possible way to overcome the disease. Repeated handwashing, sanitising surfaces and maintaining physical distances are the main modes for preventing transmission.
What are the tests for coronavirus?
The primary method of testing for coronavirus is a polymerase chain reaction (PCR) test that detects for the presence of the viral gene sequences. The accuracy of the test depends on several factors like when the samples were collected and how it was handled. PCR test can only detect an ongoing infection. The second method of testing is called a serological or antibody test that detects the presence of specific antibodies in the blood against the virus. This type of test tells us if an individual has already had the infection.
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