Virus: Transmission Phases
From December 2019, we suffered a mysterious viral disease which is COVID-19 caused by β-Coronavirus SARS-CoV-2. The genetic material of this virus is positive sense single stranded RNA (ssRNA +). The entry of SARS-CoV-2 into human cell is based on via ACE2 receptor. ACE2 receptor is present in many vital organs like lungs, heart, stomach, kidney, testis and other organs and involves in Renin-Angiotensin System (RAS).
The genetic variations help to evolve the viruses. There are many molecular mechanisms by which a number of viral variants originate. The molecular mechanism of viruses are same as the other living organisms like mutation, recombination and gene re-assortment. The average mutation rate in the genetic material of RNA viruses are 10–3 to 10–5 nucleotide/site, which is 300 times more as compare to DNA viruses. The genetic drift and genetic shift molecular mechanisms are also responsible for producing many viral variants.
As we know that SARS-CoV-2 is not pre-existing virus but it shows similarity with other β-Coronaviruses so, after the cases reported we put this into Coronaviridae family. Viruses have the ability of random evolution by which they affect the traditional host as well as the new host that alter the epidemiology and produce a new type of disease. The reason of new type disease is transfer of pathogens from one animal to another or from one animal to human. There are many viral diseases are known by transfer of viruses from animals to human beings like SARS virus, MERS virus, Influenza virus, Nipah virus, Ebola, and the present pandemic due SARS-CoV-2 viruses.
The introduction of any viruses into the population depends on two key points: 1. Entrance of the virus into the population, and 2. The expansion of virus into the population is sustained by the virus itself because they have capability of mutation to adopt their genetic material according to the environment.
The new virus infection into the host follows mainly four stages which is : a. Stable, in this type the virus and host shows a equilibrium stage and focused on the existance of the virus into the environment, b. Evolving, the transfer of virus from one traditional host to new host leads to mutate and provide stable genetic material to the virus, c. Dead-end, due to high mortality rate the effective transmission of the virus from natural reservoir to new host will not occur, d. Resistant, the infected host inhibit the replication of virus by which the infection into new host will not happen.
The successful transmission of a new virus from one population to another is characterize by following features: a. the number of primary infected patients reveal the distinct transfer of virus into new host, b. the high value of R0 shows the probability of virus transmission into the new population, c. the total mutation performed by the virus to adapt new environment to complete its life cycle, and d. the change in R0 per mutation i.e. genetic variability.
SARS-CoV-2, a RNA virus has much genetic variability than a DNA virus. It shows high number of replication error by the activity of RNA Polymerase and Reverse Transcriptase enzymes due to very fast replication rate. These evolutionary modification generate the mutations in the virus and helps to adapt new environment as well as new host.