What is the Hepatitis C Virus?

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What is the Hepatitis C Virus?

What is a Virus?

A virus is an extremely tiny infectious agent, many times smaller

than bacteria, which can only live inside a cell of another organism,

like a person, animal, or plant. Each type of virus has particular

types of host creatures and host cells that it will invade

successfully.

Viruses are composed of two parts: a protective shell made of

protein, and sometimes a second protective layer, or envelope, made

of protein or lipids (fats). Inside these protective layers is the

genetic material of the virus, which is either RNA or DNA.

So, in broad terms, viruses can be described as enveloped or non-

enveloped, and as RNA or DNA viruses.

Other than RNA or DNA, viruses do not have the structures, or

organelles, necessary to reproduce. It cannot reproduce unless it

invades a cell within another organism, called the host.

In many ways, viruses do not meet criteria that we use to determine

if things are actually alive.

Viruses do not respire, that is, they do not breath in and out

Viruses do not take in nutrients or excrete waste products

Viruses do not respond to stimuli

Viruses do not move under their own power

Viruses do not grow, instead, they are assembled by the organism they

have infected.

Once within the host cell, the virus uses the host cell's organelles

to produce more viruses, by forcing the cell to replicate the virus'

own genetic material and protective shell. Once replicated, the new

viruses leave the host cell and are ready to invade others.

What Kind of Virus is the Hepatitis C Virus?

The hepatitis C virus is an enveloped virus, with a single strand of

RNA. The envelope is a double-layer of lipids (fats) with several

proteins imbedded in the envelope surface.

The hepatitis C virus is a member of the Flaviviridae family of

viruses, which can cause disease in both humans and animals. The

Flaviviridae family includes at least three distinct genera, or

subtypes:

Pestiviruses, which cause disease in cattle and pigs

Flaviviruses, the cause of diseases such as dengue fever and yellow

fever

Hepaciviruses, which causes hepatitis C.

How does HCV infect Host Cells?

The Hepatitis C virus seems to prefer to infect the cells of the

liver (hepatic cells), but it's not exactly clear how the virus

enters the cell. Some researchers have suggested that the proteins

embedded in the viral envelope engage with certain receptors on the

surface of liver cells.

These viral surface proteins and cell surface receptors fit together

like a lock and key, allowing the virus to penetrate the surface of

the cell and inject the viral RNA into the cell.

Once the virus attaches to the surface of a cell, it inserts its RNA

(the genetic material of the virus) into the cell. Inside the cell,

the viral RNA is acted upon by substances called proteases, which

begin to make copies of the RNA.

The rate at which new viruses are replicated is staggering - it is

estimated that between 100 billion (1011) and 1 trillion (1012)

viruses are replicated per day.

The process of copying the viral RNA is imperfect, and tends to

introduce variations into the RNA sequence at the rate of one error

per genome per replication cycle.

The result of this rapid, error-prone replication process is a subtle

variation in the genome, or genetic blueprint, of newly-produced HCV

viruses. These genetic mutations are called genotypes and

quasispecies.

Hepatic cells normally have a lifespan of 6 to 12 months, so the

reservoir of infected hepatic cells tends to die off over time if

they are not constantly replenished. In many patients, the hepatitis

virus can be eradicated if its replication can be sufficiently

suppressed with medications.

Genotypes and Quasispecies

The term genotype refers to the major genetic divisions among HCV at

large, and six genotypes (1 though 6) have been identified. The term

quasispecies refers to genetic diversity within a particular

genotype, and is identified by lower-case letter.

An person's genotype does not change over time, but mutations in the

virus during the course of infection can result in changes in an

individual's quasispecies. If a person is found to have more than

one genotype, it is presumed that they have been repeatedly infected

with different types of HCV.

Since the replication process rapidly generates virus particles with

many subtle genetic variations, some of the virus variants can escape

the host's immune response. This is why chronic HCV infection occurs

in approximately 85 percent of infected people.

Although the human immune system is capable of producing an effective

immune response to HCV infection, the response is highly specific,

that is, the immune cells are not able to defend against a wide

variety of slightly different HCV particles.

The strength of the host's immune response is sometimes referred to

as " immune pressure. " People who generate a stronger immune response

exert more immune pressure on the virus, and this tends to drive the

virus to develop a broader range of quasispecies than those who have

a weaker immune response.

How are Genotypes distributed?

Genotypes 1, 2, and 3 account for the majority of HCV infections in

North and South America, Europe, Russia, China, Japan, Australia, and

New Zealand.

Genotype 3 is common in the Indian subcontinent, and genotype 4 is

prevalent in Egypt and Central Africa. Genotype 5 is common in South

Africa, and genotype 6 is primarily found in Southeast Asia.