Corona Virus Part I

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Joined: 26 Feb 2013 09:59
Full Name: Kannivelu Badrinath
Name of Your College/Medical School: Madras Medical College, Madras, India

Corona Virus Part I

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With the present COVID 19 infection becoming endemic, I wanted to read a little about viruses in general and how they infect a human being. Since the start of the endemic there has been so much interest on the topic that there are now thousands of articles written on the web. Research on Covid 19 is also being conducted on a large scale all over the world. Here is a summary of what I learnt from a number of articles that I read on the web during the last 2 weeks.

Everyone knows that viruses cause a variety of infection. They vary from simple infections like the common cold to profoundly serious ones like the Ebola infection. Some of the other infections that we regularly read about are influenza, pneumonia, gastroenteritis, hepatitis, rabies, encephalitis, chicken pox, herpes etc.

Luckily some serious infections like poliomyelitis and smallpox have been eradicated by vaccines.

What are viruses?

Are they living organisms? There are many scientists who believe that they may be the precursors of life. There are many others who believe that they are non-living and represent a different type of organism.

David R Wessner, on the “The Origin of Viruses” in Nature Education indicated in 2010 that there was no clear explanation for the origin of viruses. He said they may be descendants of previously free-living organisms that adapted a parasitic replication strategy. He also said that perhaps viruses existed before, and led to the evolution of, cellular life.

Biologists generally agree that all living organisms exhibit several key properties: They can grow, reproduce, maintain an internal homeostasis, respond to stimuli, and carry out various metabolic processes. In addition, populations of living organisms evolve over time.

Viruses on the other hand are somewhat different. We know that viruses reproduce in some way. We can become infected with a small number of virus particles that replicate within our bodies causing an illness. We also know that viruses evolve over time resulting in new diseases like influenza epidemic almost every year.

Yet some of the more common questions to distinguish between living and non-living things are whether they can replicate by themselves, whether they multiply through cellular division or if they have a metabolism. Viruses do not seem to have any of these properties.

1. They steal the host cell’s “machinery” to ‘photocopy’ the genetic code of the virus and seal it inside a newly formed container called a capsid. Without a host cell, the virus simply cannot replicate.
2. Unlike other living organisms viruses cannot self-divide, they make the host cells to manufacture the viral components.
3. Unlike living organisms that consume adenosine triphosphate (ATP) through metabolic activity, viruses do not consume energy to survive. Viruses can survive on nothing and drift around until they find the right kind of cell to infiltrate and create more copies of themselves.

In summary viruses should be considered as non-living crafty organisms enslaving host cells to do the work for them to multiply.

How does the Virus spread an Infection

Most viruses have either RNA or DNA as their genetic material. The nucleic acid may be single or double-stranded. The entire infectious virus particle, called a virion, in the genetic material is embedded in the core of the virus. This is protected by the Capsid shell. The capsid is made up of several complex protein structures. A further layer of protein protects the Capsid and in some this is further enclosed by a lipid layer (as in the Covid 19 virus). The genetic material contained in the core will instruct the host cell to reproduce the virus. The Covid 19 is an RNA virus.

The functions of the capsid are to:
1. protect the genome,
2. deliver the genome
3. interact with the host

Although viruses are incapable of surviving outside a cell for long, the protective coating over the capsid enable them to be active for varying lengths of time. This will depend on the surrounding temperature, moisture, the surface on which they are deposited etc. They survive longer in cold and wet environments. They also survive longer when deposited on water resistant surfaces like steel and plastic. Enteric viruses and hepatitis “A” can survive for weeks. A cold virus can survive on a surface for a few days. Surprisingly the COVID 19 which has a lipid cover is very fragile and can survive for only 4 to 72 hours (24 hrs on cardboard). For some reason it can survive only for 4 hours on copper. Yet when it infiltrates a human cell can cause untold damage.

Mechanism of infecting an individual.
The glycoprotein spikes on the virus are responsible for entry of the virus into the host cell. They first fuse to the host cell membrane and then inject the infection into the cell. The spike is also the main target for neutralizing antibodies if they are present.

Angiotensin converting enzyme 2 (ACE2) is present on the cell membranes of cells in the lungs, arteries, heart, kidney, and intestines. (ACE2 lowers blood pressure by catalysing the cleavage of angiotensin II (a vasoconstrictor peptide) into angiotensin, a vasodilator). ACE2 also serves as the entry point into cells for some coronaviruses. The novel coronavirus latches onto ACE2 and sneaks inside. Once inside the host cell they start replicating.

The new coronavirus as it travels down the airway can infect any part from the bronchus to the alveoli. As the cells are infiltrated the lining can become irritated and inflamed. The alveoli then gets filled with fluid and inflammatory cells. Exchange of gases is therefore impaired and depending on the severity of the disease the blood oxygen tension may go down to a level that can cause death. CT scan of chest in severe cases would show ground glass opacity.

About 80% of people who have COVID-19 only get mild to moderate symptoms with a dry cough or a sore throat with or without temperature. This is probably because their immune system is able to produce enough antibodies to stop the infection from getting worse
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