Virus Life Cycle - The growth curve

The above image shows a Virus life cycle growth curve...

Now, let me explain more about it to give you a better understanding of this illustration!

The virus would first attach itself to the host and undergo it's replication cycle as explained earlier on in previous post, after which the virus would enter...

Eclispe/Latent stage

During this stage, there is a fall in virus titre as there are no infectious particles present in the infected host's bloodsteam while the virus is undergoing it's replication cycle.

Hence, the virus would not be detected by any external mediums whatsoever until it has been released.

Next, from the graph we can see a significant increase straight after the eclispe/latent stage.

This is known as the stage where the virus have burst, whereby new progeny virus has assembled and been released into the host's bloodsteam.

After being released into the person's bloodsteam, the body would begin to produce antibodies to fight the virus. After a certain period, the number of virus as can be seen from the illustration would begin to hold stable and soon after, decrease in numbers as the virus begins to die off.

After which, the infected host would recover completely and would be healthy once again!

Portals of entry

Virus can enter the body at several sites which are known as portals of entry.
There are four majoy portals of entry:

1) Skin
2) Mucuous membrane
3) Placenta
4) Parenteral route


Let's take a look at...

(1)Skin

The skin is a barrier to most pathogens if it is intact.
However, some pathogens can still enter via hair follicles, sweat glands and through cuts and bruises.

(2) Mucous membrane

There are mucous membrane linings in all of our body cavities such as...

1. Respiratory

2. Gastrointestinal
3. Urinary

4. Reproductive

5. Conjuctiva which is also known as our eyes.

Mucuous membrane, unlike our skin, are thin, moist and warm and it is also where all cells

are living.

(3) Placenta

Pathogens can also be passed from mother to fetus through the placenta. This would result in...

1. Spontanous abortion or otherwise better known as miscarriage

2. Birth defects

3. Premature births which may result in other complications

Lastly, we have...

(4) Parenteral route

This is the most common portal of entry route. Pathogens may infect host in this way via punctures, contaminated needles, bites, cuts, stab wounds or surgery.

Release

a. Naked viruses

Naked viruses are predominantly released by host cell lysis. While some viruses are cytolytic and lyse the host cell more or less directly, in many cases it is the body's immune defenses that lyse the infected cell.
For lytic viruses (most non-enveloped viruses), release is a simple process - the cell breaks open and releases the virus.

b. Enveloped viruses

With enveloped viruses, the host cell may or may not be lysed. The viruses obtain their envelopes from host cell membranes by budding. As mentioned above, prior to budding, viral proteins and glycoproteins are incorporated into the host cell's membranes. During budding the host cell membrane with incorporated viral proteins and glycoproteins evaginates and pinches off to form the viral envelope. Budding occurs either at the outer cytoplasmic membrane, the nuclear membrane, or at the membranes of the Golgi apparatus

Enveloped viruses acquire the lipid membrane as the virus buds out through the cell membrane. Virion envelope proteins are picked up during this process as the virus is extruded. Budding may or may not kill the cell, but is controlled by the virus - the physical interaction of the capsid proteins on the inner surface of the cell membrane forces the particle out through the membrane.

1. Viruses obtaining their envelope from the cytoplasmic membrane are released during the budding process

Release of an Enveloped Virus by Budding, step 1.

The virus obtains its envelope from the host cell's cytoplasmic membrane as it buds from the cell surface.

Release of an Enveloped Virus by Budding, step 2.

The virus obtains its envelope from the host cell's cytoplasmic membrane as it buds from the cell surface.


Transmission Electron Micrograph of Rubella Viruses Budding from a Host Cell

Image provided by Dr. Fred Murphy and Sylvia Whitfield.
Courtesy of the Centers for Disease Control and Prevention.
The rubella virus causes German measles. The virus gets its envelope by budding from the host cell's cytoplasmic membrane.

2. Viruses obtaining their envelopes from the membranes of the nucleus, the endoplasmic reticulum, or the Golgi apparatus are then released by exocytosis via transport vesicles.

Release of an Enveloped Virus by Fusion of a Transport Vesicle with the Host Cell's Cytoplasmic Membrane, step-1

The assembled virus is placed in a transport vesicle.


Release of an Enveloped Virus by Fusion of a Transport Vesicle with the Host Cell's Cytoplasmic Membrane, step-2.

The transport vesicle fuses with the host cell's cytoplasmic membrane releasing the vitrus from the cell.


Some viruses, capable of causing cell fusion, may be transported from one cell to adjacent cells without being released, that is, they are transmitted by cell-to-cell contact whereby an infected cell fuses with an uninfected cell.


The assembled virus is placed in a transport vesicle. Then, the transport vesicle fuses with the host cell's cytoplasmic membrane, releasing the virus from the cell.



The uninfected cells and infected cell then fuse together forming a multinucleated giant cell or syncytium.

Maturation

Maturation

The stage of the life-cycle at which the virus becomes infectious. Usually involves structural changes in the particle, often resulting from specific cleavage of capsid proteins to form the mature products, which frequently leads to a conformational change in the capsid, or the condensation of nucleoproteins with the genome. For some viruses, assembly and maturation are inseparable, whereas for others, maturation may occur after the virus particle has left the cell.

Generally, protein capsid is assembled around the viral genome

Fig. 1A: Maturation of an Enveloped Virus

Envelope glycoproteins are transported to the host cell's cytoplasmic membrane and the capsid assembles around the viral genome.



Fig. 1B: Maturation of a Naked Virus



The capsid assembles around the viral genome.