Let us make an in-depth study of virology-the branch of medical science that studies viruses and viral diseases. The below given essay will help you to understand the following things:- 1. History of Viruses 2. Definition of Virus 3. Classification of Viruses 4. Chemical Composition 5. Effects of Physical and Chemical Agents and 6. Design & Construction.
Essay # General Introduction to Virology:
History of Viruses:
Many common contagious diseases of animals and man were clinically known since ancient time for which no bacterial aetiology has been assigned. From pre-Christian times; Small pox was considered as a deadly infection and Hippocrates (460 -377 B.C.) described swollen neck in mumps, about 2,400 years ago. In 1884, Pasteur believed that Rabies of dogs is caused by a “microorganism infinitesimally small” as he could not detect bacteria in the infective material of rabid dogs.
In 1892, Iwanowsky showed that the mosaic disease of tobacco plants was caused by a minute agent which was so small, ultramicroscopic and could pass through the pores of bacteria stopping filters. In 1898, Loeffler and Frosch have shown that foot and mouth disease of cattle was caused by a filterable infectious agent. In 1901, Walter Reed et al proved that yellow fever was caused by a mosquito-borne virus.
Viruses were originally described as ultramicroscopic and filter passing. Later, it was shown that the viruses cause serious diseases in animals such as foot and mouth diseases and rinderpest in cattle, distemper and rabies in dogs and fowl pest in poultry. Rous, in 1911, demonstrated that certain sarcomata of fowls could be transmitted with cell free filtrates of the tumours.
Since that time, it was established that virus can cause tumour in animal and they are called “Oncogenic viruses”. ‘Fibromas, papilloma’s, and related tumours in rabbit and other mammals; mammary carcinomas, leukemia’s and parotid tumours (polyoma virus) in mice and leucosis in poultry were caused by Oncogenic viruses. Insects are vectors of viruses which attack man, animals and plants. Nematodes, parasitic to pigs, spread swine influenza virus as in case of plant nematodes.
Twort in 1915 and d’Herelle in 1917 observed independently that viruses can infect and cause lysis of bacteria (clear zone of lysis — plaque), these viruses are called Bacteriophage — a name later abbreviated as “phage” This phenomenon of lysis was first noted in Shigella shigae—causative agent of bacillary dysentery. Because of this, knowledge of mechanism of phage infection and reproduction is much advanced than knowledge of the corresponding mechanism of animal viruses.
Definition of Virus:
Viruses (Vira—poison) were earlier defined as an ultramicroscopic agent which can be seen only under electron microscope, can pass through the bacteria stopping filters, reproduce by replication, can grow only in living cells, resist the action of antibiotics and contain Ribonucleic acid (RNA) or Deoxyribonucleic acid (DNA) covered by a protein coat.
Luria, in 1967, produced a composite definition incorporating all the accepted essential features of viruses:
“Viruses are entities whose genome is an element of nucleic acid, either DNA or RNA, which reproduces inside living cells and uses the synthetic machinery to direct the synthesis of specialised particles, the virions, which contain the viral genome and transfer it to other cells.”
The recent definition is:
A virus is one of a group of minute infectious agents characterised by a lack of independent metabolism and by the ability to replicate only within living host-cells. They range from 15 – 300 nm in size and are morphologically heterogeneous (DORLAND’S MEDICAL DICTIONARY).
Following are two cardinal features of viruses:
1. They possess the genetic material which, when it is within the host cell, behaves as a part of the cell.
2. They also exist in an intracellular form, which is the product of the host cell under the control of the virus itself. The extracellular form — the virion — serves as a vehicle to carry the viral genome to other cells.
Viruses can reproduce only when they are within their host cells and are of necessity intracellular parasites. They have no ribosomes, no enzymes to generate high energy bonds, no mitochondria. They lack a rigid cell wall. Thus, there is no muramic acid in their outer coatings, hence, they are unaffected by antimicrobial agents. Indeed, viruses have no real cell structures and it is difficult to regard them as microorganisms. True microorganisms multiply by binary fission.
Structure:
A complete infective virus particle is called as virion and the genome is in core and consists of either deoxyribonucleic acid (DNA) or of ribonucleic acid (RNA) but not both. The outer shell of protein covering the nucleic acid is known as capsid.
The protein and nucleic acids are closely integrated to form a nucleoprotein of distinct symmetry, when a virion enters the host cell, its protein capsid is stripped off and its nucleic acid is liberated within the host cell. At this stage, it ceases to exist as a particle although its nucleic acid is still present and intact. Component parts are still detectable. This stage in viral multiplication is known as the eclipse phase. All true viruses have an eclipse phase in their reproductive cycle (Fig. 3.1).
Essay # Classification of Viruses:
Animal viruses may be arranged into groups. The names of these groups end in the suffix-Virus e.g., adenovirus, herpes virus and poxvirus. Members of these groups share similar features and constitute families ending with the suffix — idae, i.e., the family Herpesviridae would have as its type genus Herpes virus which is a group that contains the Herpes simplex, varicella-zooster.
Following are the important distinguishing features of animal viruses:
(1) Type of nucleic acid;
(2) Chemical composition;
(3) Susceptible to chemical and physical agents;
(4) Size and measurement;
(5) Design and construction of virus;
(6) Antigenic characters.
Deoxyriboviruses, at present, are placed in five groups (Poxvirus, Herpes virus, Adenovirus, Papavovirus, Parvovirus) and Riboviruses in nine groups (Orthomyxovirus, Paramyxovirus, Rhabdovirus, Togavirus, Arenavirus, Reovirus, Picornavirus, Leukovirus, Coronavirus).
There are many viruses that do not fit neatly into these groups. They are known as Arboviruses, because they are transmitted by the bite of arthropods.
Essay # Types of Nucleic Acid:
DNA or RNA carries in their nucleic acids the genome and infectivity of viruses. Most de-oxyribo viruses have linear double stranded DNA; whereas, in the parvoviruses, the molecule is single stranded RNA. When the protective capsids of the viruses are removed, some fragile viral nucleic acids retain their infectivity for some time, they can be seen and measured under electron microscope. Most of them are linear but some are circular.
Essay # Chemical Composition of Viruses:
Proteins are the main components of all viruses. The most important function of these proteins is to provide the shield for the underlying delicate nucleic acid molecules. The surface proteins of the virion have special affinities for specific receptor sites on the host cell and provide a means of attachment of the particle to a position where it can enter the host cell and initiate the process of infection.
Viral antigens, which are protein in nature, stimulate the host immune response during infection. They are responsible for the distinctive serological characteristics of the viruses. The capsomeres of most pathogenic viruses contain several different polypeptides. These polypeptides make up the surface configuration. They constitute the hem agglutinin prickles of ortho and paramyxoviruses and form a part of the mushroom- shaped projections of neuraminidase subunits.
Viral Enzymes:
There are many enzymes coded in the nucleic acids.
Lipids:
When some mature viruses are liberated from nuclear and cytoplasmic membrane, they acquire a surface layer of lipoprotein derived in part from the host cell. The lipids may cement the proteins of the envelopes of the viruses. These viruses are inactivated by lipid solvents (ether, chloroform, bile salt).
Carbohydrates:
Some viruses contain minimal amounts of carbohydrates.
Essay # Effects of Physical and Chemical Agents:
Heat and Cold:
The viruses causing the diseases in man and animals are readily inactivated by moderate heat (56° to 60°C for 30 minutes) except serum hepatitis and poliomyelitis viruses. Viruses are resistant to extremes of cold; freezing at -35°C or -70°C is a satisfactory common method for their preservation in the laboratory. The majority of viruses can be well preserved by freeze drying method. pH variation. Viruses remain viable within a range of pH 5 to 9, but they are destroyed by extreme acidity or alkalinity.
Glycerol:
Many viruses remain viable for many months or years in a 50 percent solution of glycerol. Poliomyelitis, rabies and herpes simplex viruses can be kept for long period in glycerol at 4°C. Rinderpest virus and certain bacteria do not tolerate glycerol.
Virucidal Agents:
The most efficient disinfectants for use against viruses are oxidising agents (hydrogen peroxide, potassium permanganate and organic iodine derivatives). Formaldehyde (a reducing agent) is lower in action but is valuable in defined concentration in the preparation of inactivated poliomyelitis vaccines. Glutaraldehyde has an important role in the disinfection of apparatus used in renal dialysis units.
Phenol and certain cresol disinfectants (i.e., lysol) are active against only a few viruses and are not to be recommended for material contaminated with the poliomyelitis or small pox viruses. Antiviral agents acting on Human Immunodeficiency virus (HIV) are: U-90.152 (drug); Zidovudine; Didanosine and Zalcitabine are currently licensed, Acyclovir acts on Herpes simplex and Varicella Zoster virus.
Ether:
Viruses containing lipids in their envelopes are sensitive to ether. Naked viruses (polio, echo, coxsachie, adeno, papova and parvoviruses and some pox viruses) are resistant to ether, whereas herpes, orthomyxo and paramyxo, rhabdo, corona and leuko viruses are inactivated by ether.
Vital Dyes:
Acridine dyes, neutral red and toluidine blue permeate the nucleic acid and render virions vulnerable to ultraviolet rays. Herpes simplex and vaccinia viruses are more severely damaged than polioviruses. Double stranded DNA viruses stained with acridine dyes fluoresce with a yellow colour when seen under electron microscope, while single stranded DNA viruses fluoresce red.
Antibiotics and chemotherapeutic agents have no effect on viruses, but the agents of psittacosis lymphogranuloma group are susceptible to these drugs. The unit measurement of whole virus particle is the nanometer (nm) or millimicron (mµ) or one- thousandth part of a micron (n) or micrometer (µ) or 0.000001 nm or it is 10-9 meter; whereas the component parts or the internal structure (capsomere) of the virus is expressed as Angstrom (A0) which is one-tenth of a millimicron (0.0000001 mm).The Angstrom is the unit used to measure the wave length of light in microscope.
Virions vary in diameter from 300 to 18 nm. The largest ones are nearly half the size of small bacteria and can just be seen under an optical microscope. Most viruses can be seen only under electron microscope. Largest virus is vaccinia virus (330 x 100 nm), smallest one is picorna virus and parvovirus (40- 20nm).
It is customary to include in a suspension of purified viruses some latex particles of known size (e.g., 88 nm) in electron microscope, so in electron micrographs, the size and shape can be determined accurately: Vaccine virus is brick shaped; influenza virus is round or filamentous; bacteriophage is sperm like with head and tail. Formerly, the size of viruses was measured by their capacity to pass through the filters or by their rate of sedimentation in a suspended fluid.
Essay # Design and Construction of Virus:
A virion (the old term which is still in Use) in its simplest form is a single molecule of nucleic acid enclosed within a protein capsid. It was known as “elementary body. The shape taken by the capsid is that of rigid spherical cage or of a tube; mainly the capsid protects the nucleic acid. Viral capsids are made up of large number of “morphological units” or “capsomeres” that are attached to each other by their bonds.
Cubical Capsids:
Viruses with cubical symmetry take the form of an icosahedron. An icosahedron has 20 facets, each an equilateral triangle and 12 vertices or corners. Antigenic classification of animal viruses depends upon the antigenic patterns.
Virus hem-agglutinin:
Many different viruses agglutinate red blood cells when virions and erythrocytes are in a suspension, they collide and adhere to each other; the cells become speckled with attached virions and they are bound to each other by viral bridges.
Inclusion Bodies:
During the course of multiplication, the viruses are associated with the appearance of large distinctive structures known as “inclusion bodies” They may be situated either in the cytoplasm (intra-cytoplasmic inclusion bodies) or in the nucleus (intra-nuclear inclusion bodies) or as in the case of measles, in both.
The inclusion bodies are usually acidophilic and appear as pink masses in smears stained with Giemsa stain or Mann’s eosin methylene blue stain; basophilic inclusion bodies are characteristic of psittacosis-lymph granuloma group. They vary in size from 1 -30 n in diameter. The presence of large intra-cytoplasmic acidophilic inclusion bodies in the nerve cells of hippocampus of the brain is the presumptive diagnosis of Rabies.
Acidophilic Intra-nuclear Inclusion Bodies are of two types:
Cowdry Type A is granular in appearance and of variable size (herpes simplex, zoster varicella, yellow fever); Type B is more round and multiple (Poliomyelitis, adenovirus).
Virus tropism:
Viruses are classified according to their particular tissue predilection as dermotropic, neurotropic, pneumotropic, enterotropic and viscerotropic.
Viral pathogenicity:
When the virus multiplies in the cell, it may spread from cell to cell producing a small focal lesion (Wart or molluscum contagiosum). When the viruses are deposited on the mucous membrane of respiratory tract, they infect, multiply inside the mucosa and spread over a large area producing common cold; when trachea and bronchi are infected, influenza ensues.
Transmission of virus infection:
Inclusion conjunctivitis is transmitted by the direct contact between persons during intercourse from one concert to another. Later it reaches the eyes of the new born as it passes through the infected birth canal. Polioviruses and other entero viruses may be transmitted after the ingestion of food or drink.
Interferon:
Among animal viruses, interference of one virus with the reproduction of another may be mediated by a protein called “interferon” which is produced by the cell after infection with the first virus. Interferon is a protein with a molecular weight of 30,000 Daltons. Because of its size, the interferon is released from cells and can diffuse in the body throughout the extracellular fluids.
Cultivation of Virus:
Viruses can be grown in:
(1) The animal;
(2) Chick embryo;
(3) Tissue culture
Bacteriophage:
The structure of bacteriophage is complex and sophisticated than that of virus. The basic design is sperm shaped with a polyhedral head and a cylindrical tail. The head of T2 phage of E coli is bi-pyramidal hexagonal prism and consists of an outer shell of about 1,000 subunits of a protein with a molecular weight of 80,000 and an inner central mass of DNA. The tail is about 100 mµ long and 25 mµ wide and is composed of contractile sheath surrounding a central hollow core.
At the distal extremity of the tail, there is a hexagonal plate to which six fibers are attached. The sheath of the tail contracts less than half of the original length. The central core extrudes through the bacterial wall to enable the transference of DNA to the host cell (Fig. 3.2). The epidemiology of the disease can be studied by bacteriophage typing. The cholera phage typing method has obvious epidemiological uses in tracing the epidemic spread of cholera (Fig.3.2).
Essay # Arthropod Borne Virus (ARBOR) Viruses:
The term arbor virus is used to denote a large number of viruses transmitted by arthropod. These viruses are taken in by the arthropods, while sucking blood of man or animal. These viruses — multiplying within the body of the arthropods without causing damage to the tissues of the arthropod — produce the disease in man.
Slow and Oncogenic Viruses:
They have in common the long continued association with the host. In general, the associated diseases occur sporadically and they exceptionally assume epidemic features (slow virus infections scrapie, Kuru, Animal virus tumours).
Application to Nursing:
Professional nurses – working particularly in communicable disease ward and public health should have a sound knowledge of virology and bacteriophage which is used to study the epidemiology of communicable diseases.