Virus. – Dr. Sayeed Ahmad

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Virus.
Dr. Sayeed Ahmad D.
I. Hom. (London)

Virus is a microscopic organism that lives in a cell
of another living thing. Although viruses are extremely small and
simple, they are a major cause of disease. Some viruses infect human
beings with such diseases as measles, influenza, and the common cold.
Others infect animals or plants, and still others attack bacteria.
Viruses produce disease in an organism by damaging some of its cells.
However, viruses sometimes live in cells without harming them.

Viruses are so primitive that many scientists
consider them to be both living and nonliving things. By itself, a virus
is a lifeless particle that cannot reproduce. But inside a living cell,
a virus becomes an active organism that can multiply hundreds of times.

Viruses are shaped like rods or spheres and range in
size from about 0.01 to 0.3 micron. A micron is 0.001 millimeter or
1/25,400 inch. Most viruses can be seen only with an electron
microscope, which magnifies them by thousands of times. The largest
virus is about 1/10 as big as a bacterium of average size.

The study of viruses began in 1898, when a Dutch
botanist named Martinus Beijerinck realized that something smaller than
bacteria could cause disease. He named this particle a virus, a Latin
word meaning poison. In 1935, Wendell M. Stanley, an American
biochemist, showed that viruses contain protein and can be crystallized.
This research and other studies led to the development, in the 1950’s,
of vaccines for measles, poliomyelitis, and other diseases. Virologists
(scientists who study viruses) demonstrated in the early 1900’s that
viruses can cause cancer in animals. In the 1980’s, research linked
viruses to a few cancers in humans.

The structure of a virus

.

Viruses, unlike other organisms, are not made up of
cells. Therefore, they lack some of the substances needed to live on
their own. To obtain these substances, a virus must enter a cell of
another living thing. It then can use the cell’s materials to live and
reproduce.

A typical virus has two basic parts, a core of a
nucleic acid and an outer coat of protein. The core consists of either
DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). The DNA or RNA
enables the virus to reproduce after it has entered a cell. Some RNA
viruses contain an enzyme called reverse transcriptase, which converts
virus RNA to a DNA copy inside cells. Such viruses are called
retroviruses. The virus that causes AIDS (acquired immunodeficiency
syndrome) is a retrovirus. The coat of a virus consists of individual
proteins that give the virus its shape. This coat protects the nucleic
acid and helps the DNA or RNA get inside a cell. Some viruses have an
additional outer membrane that provides further protection.

How a virus infects an organism

.

Most viruses reproduce in specific cells of certain
organisms. For example, viruses that cause colds reproduce in cells of
the human respiratory tract. Viruses cannot live outside their
particular cells. They must be carried into the organism by air currents
or some other means, and then transported by body fluids to the cells.

When a virus comes into contact with a cell that it
can enter, it attaches itself to the cell at areas called receptors.
Chemicals in the receptors bind the virus to the cell and help bring it
or its nucleic acid inside. The nucleic acid then takes control of the
cell’s protein-making process. Previously, the cell made only the
proteins specified by its own genes. The genes are the cell’s hereditary
structures, and they consist of nucleic acid. A cell that has been
infected by a virus begins to produce the proteins that are called for
by the nucleic acid of the virus. These proteins enable the virus to
reproduce itself hundreds or thousands of times.

As new viruses are produced, they are released from
the cell and infect other cells. The new viruses become lifeless as soon
as they are released. But they return to life after entering another
cell. The viruses then start to reproduce and thus spread infection to
more cells.

When a virus reproduces, it changes a cell’s chemical
makeup. This change usually damages or kills the cell, and disease
results if many cells are affected. Some viruses change a cell only
slightly because they do not reproduce. The DNA copy of a retrovirus may
hide inside a cell on cell DNA. Such a virus may cause no immediate
symptoms but might later damage the cell.

Virus diseases in human beings include AIDS;
chickenpox; colds; cold sores; hepatitis, a liver disease; influenza;
measles; mumps; poliomyelitis; rabies; and yellow fever. The nature of
the disease caused by a particular type of virus is determined by which
cells and tissues in the body the virus tends to invade.

The body protects itself from viruses and other
harmful substances by several methods, all of which together are called
the immune system. For example, white blood cells called lymphocytes
provide protection in two ways. Some lymphocytes produce substances
called antibodies, which cover a virus’s protein coat and prevent the
virus from attaching itself to the receptors of a cell. Other
lymphocytes destroy cells that have been infected by viruses and thus
kill the viruses before they can reproduce. However, some viruses are
able to suppress the functioning of the immune system and thus enable
themselves to reproduce more easily. Such viruses include those that
cause measles, influenza, and AIDS.

Lymphocytes do not start to produce antibodies until
several days after a virus has entered the body. However, the body has
additional methods of fighting virus infections. For example, the body
produces a high fever to combat such virus diseases as influenza and
measles. The high fever limits the ability of the viruses to reproduce.
To fight colds, the body forms large amounts of mucus in the nose and
throat. The mucus traps many cold viruses, which are expelled from the
body by sneezing, coughing, and blowing one’s nose. The body also makes
protein substances called interferons that provide some protection
against many types of viruses.

The treatment of a virus disease consists mainly of
controlling its symptoms. For example, physicians prescribe a drug
called acetaminophen to bring down a high fever. In most cases, doctors
cannot attack the cause of the disease itself, because most drugs able
to kill or damage a virus also damage healthy cells. The U. S. Food and
Drug Administration has approved a few drugs-including zidovudine
(formerly called azidothymidine and commonly known as AZT), adenine
arabinoside (ara-A), and acyclovir-for limited use against certain virus
diseases. Researchers have found other potential antiviral drugs,
including interferons. But these drugs must undergo further testing
before their safety and effectiveness are known.

A few viruses are called slow viruses because they
reproduce more slowly than the others. Some researchers believe a slow
virus causes multiple sclerosis, a disease of the brain and spinal cord.
Other viruses, such as the herpesviruses, can remain dormant in cells
for years and then become reactivated and cause sporadic outbreaks of
symptoms. Still other viruses including HIV, the virus that causes AIDS,
can cause prolonged, persistent infections in which the virus multiplies
continuously. Some viruses have been linked to human cancer. For
example, hepatitis B virus is linked to hepatoma, a type of liver
cancer. Burkitt’s lymphoma, a cancer of the lymph tissues, may be caused
by the Epstein-Barr virus. In addition, some leukemias are caused by
human retroviruses.

Virus diseases in animals

.

Viruses cause hundreds of diseases in animals. These
diseases include distemper in dogs and foot-and-mouth disease in cattle.
Most virus diseases in animals occur in certain species. But some of the
diseases spread to other species, and a few of them infect human beings.
For example, dogs can give people rabies, which destroys nerve cells.

Certain viruses can cause cancer in animals. These
viruses do not destroy all the cells they infect. Some of the infected
cells have their chemical makeup altered, which causes them to behave
abnormally. These altered cells reproduce in an uncontrolled manner,
forming masses of tissue called tumors. Cancerous tumors invade and
damage surrounding healthy tissue. Researchers have discovered a
similarity between some viruses that cause cancer in animals and certain
viruses that infect human beings.

Virus diseases in plants

.

Viruses infect all kinds of plants and can cause
serious damage to crops. Plant cells have tough walls that a virus
cannot penetrate. But insects penetrate the cell walls while feeding on
a plant and thus enable viruses to enter. Plant viruses may infect one
or two leaves or an entire plant. They produce billions of viruses,
which are then carried to other plants by insects or air currents.
Common diseases that are caused by plant viruses include tobacco mosaic
and turnip yellows mosaic.

Viruses that attack bacteria are called
bacteriophages. The word bacteriophage means bacteria eater. Bacteria,
like plants, have tough cell walls. To penetrate these walls, most
bacteriophages have a structure that works like a hypodermic needle.
This structure consists of a sphere-shaped head that contains a nucleic
acid, and a hollow, rod-shaped tail made of protein. When a
bacteriophage enters a bacterium, the tail first penetrates the cell
wall. Then the nucleic acid in the head moves through the tail and into
the cell.

How viruses are used

.

Virologists study viruses chiefly to learn how they
cause disease and how to control these organisms. Scientists also use
viruses for such purposes as (1) insect control, (2) cell research, and
(3) development of vaccines and other drugs.

Insect control

.

Certain viruses cause fatal diseases in insects.
Virologists are seeking ways to use these viruses to kill insects that
damage crops. The use of such viruses may someday replace insecticides,
which kill insects but also may harm plants as well as other animals.

Cell research.

Viruses are such simple organisms that scientists can
easily study them to gain more knowledge about life itself. Research on
bacteriophages has helped biologists understand genes, DNA, and other
basic cell structures. Future research may provide further knowledge of
how cells function and reproduce.

Development of vaccines and other drugs

.

Scientists produce vaccines from either dead or live
viruses. Those used in dead-virus vaccines are killed by chemicals and
injected into the body. For live-virus vaccines, virologists select very
mild forms of living viruses.

Following is the table of some of the viruses which
are responsible for many common human diseases such as colds, flu,
diarrhœa, chicken pox, measles, and mumps. Some viral diseases such as
rabies, hæmorrhagic fevers, encephalitis, polio, yellow fever, and
acquired immunodeficiency syndrome (AIDS) can result in death. German
measles and cytomegalovirus can cause serious abnormalities or death in
unborn infants. Of the estimated 1000 to 1500 types of viruses,
approximately 250 cause disease in humans.

FAMILY —– VIRUS —– DISEASE

Adenovirus

Common cold

Bunyavirus

Hantaan
La Crosse
Sin Nombre

Kidney failure
Encephalitis (brain infection)
Lung syndrome

Calicivirus

Norwalk

Gastroenteritis (diarrhœa, vomiting)

Coronavirus

Corona

Common cold

Filovirus

Ebola
Marburg

Hemorrhagic fever
Hemorrhagic fever

Flavivirus

Hepatitis C (non-A, non-B)
Yellow fever

Hepatitis
Hepatitis, hemorrhage

Hepadnavirus

Hepatitis B virus (HBV)

Hepatitis, liver carcinoma

Herpesvirus

Cytomegalovirus
Epstein-Barr virus (EBV)
Herpes simplex type 1
Herpes simplex type 2
Human herpesvirus 8 (HHV8)
Varicella-zoster

Birth defects
Mononucleosis, nasopharyngeal carcinoma
Cold sores
Genital lesions
Kaposi’s sarcoma
Chicken pox, shingles

Orthomyxovirus

Influenza types A and B

Flu

Papovavirus

Human papillomavirus (HPV)

Warts, cervical carcinoma

Picornavirus

Coxsackie virus
Echovirus
Hepatitis A
Poliovirus
Rhinovirus

Myocarditis (heart muscle infection)
Meningitis
Infectious hepatitis
Poliomyelitis
Common cold

Paramyxovirus

Measles
Mumps
Parainfluenza

Measles
Mumps
Common cold, ear infections

Parvovirus

B19

Fifth disease, chronic anemia

Poxvirus

Orthopoxvirus

Smallpox (eradicated)

Reovirus

Rotavirus

Diarrhea

Retrovirus

Human immunodeficiency virus (HIV)
Human T-cell leukemia virus (HTLV-I)

Acquired immunodeficiency syndrome (AIDS)
Adult T-cell leukemia, lymphoma, neurologic disease

Rhabdovirus

Rabies

Rabies

Togavirus

Eastern equine encephalomyelitis
Rubella

Encephalitis
Rubella, birth defects

Antiviral drug

is the term for a group of chemical
compounds used to treat diseases caused by viruses. Antiviral drugs
usually do not cure viral diseases but can shorten the duration of the
disease and lessen the severity of symptoms. Many antiviral drugs,
however, can cause side effects, such as anemia or kidney damage.

Many viral diseases can be prevented through the use
of vaccines. But vaccines have not been developed for all viral
diseases, and vaccines are not useful for treating people once they
become ill. Other drugs, including antibiotics such as penicillin, have
no effect on viruses.

Viruses use substances within the cells of living
organisms, called host cells, to manufacture enzymes and other materials
they need to reproduce. Antiviral drugs work by interfering with parts
of the viral life cycle that are different from steps completed by the
hosts. This allows drugs to attack viruses while not harming host cells.
However, since viruses rely on substances made by hosts to carry out
many steps in their life cycles, there are only a few viral materials
that the drugs can target. Getting drugs into infected cells is also an
obstacle. In spite of these difficulties, researchers have developed
many successful antiviral drugs.

Many antiviral drugs are chemical compounds that bind
to viral enzymes, changing the structure so the enzymes cannot be used
by the viruses. The drugs bind only to viral enzymes, while enzymes used
by the host cell are not impaired. The virus, however, cannot reproduce
to infect other host cells. The progression of the disease is thus
slowed or halted by the drugs.

The first antiviral drugs were developed in the
1960’s. One of the earliest, acyclovir, is widely used to treat
infections of herpesviruses, a group of viruses that cause chickenpox,
mononucleosis, shingles, and cold sores. This drug mimics a building
block of the genetic material DNA (deoxyribonucleic acid) which
herpesviruses need to reproduce. Viral enzymes mistakenly add this drug
into a growing strand of DNA and stop its production. Acyclovir is given
to patients by injection, as a pill, or in ointment applied to the skin.
Other drugs, such as zanamivir, used to treat influenza, can be inhaled.
Many antiviral drugs available today have been developed to treat HIV,
the virus that causes AIDS.

Scientists are developing new antiviral drugs as they
learn more about the structure and life cycle of viruses. Researchers
analyze the chemical structure of viral enzymes and proteins. They use
computers to design chemical compounds that will bind to the viruses
without causing side effects in the host.

References:

MS Encarta Encyclopædia.

World Book 2003

Copyright © Dr. Sayeed Ahmad
2004

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