Dr Kiran Kumar Mukku MD DM
Hyderabad – 22 July 2020
What is COVID 19 ?
COVID 19 derives its name from the disease caused by the virus named ‘Severe Acute Respiratory Syndrome – Corona virus -2 (SARS- CoV 2) ‘which started in the year 2019. Corona viruses are commonly found in humans, dogs, cats , chicken , cattle , pigs and birds. They are responsible for causing common colds in immunocompetent individuals. Corona viruses usually don’t cause severe disease. SARS CoV 2 is the third corona virus to cause devastating infection in the last couple of decades. The ﬁrst one was SARS virus that originated in China and caused 2002-2003 SARS pandemic. The second one was MERS – Middle East Respiratory Syndrome that originated in Arabian region in 2012.
What is origin of novel Corona virus in humans?
Corona virus with its spikes
Corona virus is so named because of the spikes on its surface giving it an appearance of solar corona. These viruses have the ability to undergo genetic changes and adapt to and infect new hosts. Bats are the natural reservoirs of SARS CoV-2 and humans got infected with this virus through an intermediate host ‘pangolin’.
How does corona virus transmit ?
The most common mode of transmission is via droplets expelled during face – to – face contact while talking, coughing, sneezing. It is said that spending 15 minutes with an infected person within 6 meters distance and briefer time with a symptomatic (eg. coughing, sneezing) person is enough for easy transmission of the virus. Touching a surface with virus on it is also one of the modes of transmission. It is being proposed that aerosols also transmit the virus. Aerosols are small droplets of respiratory secretions that suspend in the air. But , at this point in time , there is no strong proof to suggest that aerosols are signiﬁcant sources of virus. The viral particles tend to be in higher numbers on impermeable surfaces like stainless steel and plastic compared to permeable surfaces like cardboard for upto 3-4 days after settling on these surfaces.
When does corona virus start to transmit from the infected person ?
The virus starts shedding in the droplets during coughing , sneezing , talking approximately 2-3 days before appearance of symptoms. The maximum amount of viral particles appears around the time of appearance of symptoms. Infected patients with out symptoms (asymptomatic) or before the appearance of symptoms (presymptomatic) can transmit virus to other non infected persons. Around 48 to 62 % of infections were transmitted from pre symptomatic persons in China and Singapore. Studies have suggested that transmission from an infected person is very minimal 5 days after the onset of symptoms. All the above facts lead to a conclusion that using face mask , maintaining social distancing , frequent hand washing or hand sanitisation with alcohol based solutions help in a great deal in preventing transmission especially when we don’t know who is infected and who is not.
How does corona virus produce disease after entering our body ?
The novel corona virus targets cells present in the nose , upper respiratory tract (wind pipe) and the lower airways including the lungs. These cells have speciﬁc receptors called ‘Angiotensin Converting Enzyme 2 (ACE2) receptors. The spike (’S’protein) protein , which is like a spike o the virus covering and gives it a spiky appearance , attaches to these ACE2 receptors of the host cells and enter the cells. Corona virus is a RNA virus. Every living organism capable of replicating need either RNA or DNA for its replication. RNA is single stranded genetic material and cannot replicate by itself and needs some other help for self replication. DNA is double stranded and capable of self replication. Hence , once Novel corona virus enters the host cell , it will take the help of replicating apparatus of the host cell and starts multiplying. In this process , the host cells get destroyed. The infection reaches the blood circulation , destroys blood cells called Lymphocytes predominantly. The host body mounts immune attack against the virus by producing lot of chemicals called cytokines, interleukins , tumor necrosis factors. These substances when produced in excessive quantities destroys the host cells also thereby acting as a double edged sword. This is called cytokine storm. Cytokine storm is responsible for multi organ involvement seen in severe cases of COVID 19. Another possible reason for high death rates of severe COVID is blood clotting in vital organs like heart, lung, brain leading to Myocardial Infarction (heart attack), Pulmonary thromboembolism (blood clots in blood vessels to lungs), stroke ( paralysis). This blood clotting is due to activation of clotting mechanisms due to host responses in the lungs. Any viral infection always leads to super-added bacterial infections. When the body’s immunity is weakened due to viruses , the bacteria take this advantage and causes devastating infections that can lead to even death.
PS: Novels corona virus aka SARS- CoV2 and COVID 19 are brand new virus which was not known to cause human disease. The mechanisms of infection , transmission , process of causing disease , damage to various organs , symptoms , treatment , testing , treatment are still being investigated and worked upon. The above information is based on a thorough review of literature and ongoing research.