Hello steemians welcome to my blog.
Today's topic, is about viruses.
What is Virology?
Virology in a simple language, is the branch of science that deals with the study of viruses.
Medical Virology, also known as clinical virology is a branch of medicine (that deals with more of clinical pathology) which consists of isolating and characterising one or several viruses responsible for some human pathologies by various direct or indirect techniques. source
Viruses
Virus-pixabay
Viruses are the smallest infectious agents measuring 20 to 300 mm in diameter. They are obligate intracellular parasites and contain no enzymes necessary for energy metabolism, and are entirely dependent on the host cell for biosynthesis of macromolecules. The discovery of viruses was in 1886-1903. This period was the discovery period when viruses were actually found. A Russian botanist
Dmitri Iosifovich Ivanovski whom was one of the discoverer of viruses in 1898, observed bacteria like substance. Also a Dutch microbiologist and botanist, whom was considered as one of the founders of virology and environmental microbiology named Martinus Willem Beijerinck, demonstrated filterable characteristic of the virus and found that the virus is an obligate parasite, meaning that the virus is unable to live on its own.
A virus contains only one of the two nucleic acids, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
The nucleic acids are encased in a protein shell, which may again be surrounded by a lipid containing membrane.
Viruses might have evovled from free living cells, or they might have been components of host cells which separated and started independent existence. They behave like "living chemicals" and are the best tools to understand the chemistry of life.
Viruses infect unicellular organisms such as bacteria, algae, and all he higher plants and animals.
The medical importance viruses lies in their capacity to cause a wide variety of human diseases, ranging from minor ailments such as the common cold to highly fatal diseases such as yellow fever.
Morphology Of Viruses.
Viruses can be studied in,
- Sizes
- Structure
- Complex structure
- Envelope
Details
1, Size: Viruses different from bacteria by their ability to pass through filters that hold back bacteria. With the exception of some larger viruses, they are usually too small to be seen with the light microscope, and hence are called 'ultramicroscopic'.
The size of virus can be measured in a number of ways.
i) Filtration through collodion membranes of graded porosity.
ii) Direct observation and measurement in the electron microscope.
iii) Determination of the sedimentation rate in the ultracentrifuge.
iv) Comparative measurements with other microorganisms of known sizes.
The size of viruses ranges from 20 nm to 300 nm.
The pox viruses are the largest of the viruses, which are about the same size as the smallest bacteria.
They are brick-shaped (240 nm by 300 nm) and have a complex internal structure including a double-stranded DNA genome (130–260 kb) and associated enzymes.
2, Structure: The complete Virus particle, virion, consists of a nucleic acid core surrounded by an impenetrable protein coat, the capsid. The capsid is composed of a large number of morphological units called capsomers. The capsid and the nucleic acid core together are known as the nucleocapsid. The capsomers are symmetrically arranged around the core, which is of two types
i. Icosahedral symmetry.
ii. Helical symmetry.
Now what are icosahedral and helical symmetries.
- Icosahedral symmetry: An Icosahedral is a polygon with 12 vertices (corners) and 20 facets (sides).
Icosahedron structure-wikimedia commons By en:User:Cyp under CC-BY-SA-3.0
Each facer being an equilateral triangle e.g. adenovirus. Icosahedral capsids are usually formed independant of nucleic acid.
- Helical symmetry:
Helical structure-wikimedia commons By RobHar under CC BY-SA 3.0
In helical symmetry, the capsomers are bound to the nucleic acid in such a way that the nucleic acid winds spirally to form a helix. Thus, unlike the icosahedral symmetry, the helical symmetry involves interaction between the nucleic acid core and the capsid, e.g. tobacco-mosaic virus and orthomyxoviruses.
3, Complex structure: Some viruses do not exhibit any symmetry, but are complex in structure. For example, poxviruses are brick shaped with ridges on the external surface.
4, Envelope: Some viruses have a covering or an envelope outside the capsid, and are said to be envelopes. The envelope is derived from the host cell membrane when the virus is related from the cell. The envelope is made up of lipids and proteins. The protein subunits may project from the envelope as spikes.
Cytomegalovirus-wikimedia commons By Emmanuel Boutet under CC BY-SA 2.5
A virus may have different types of spikes e.g. influenza virus which has two types of spikes namely, haemagglutinin and neuraminidase spikes.
CHEMICAL CONSTITUTES OF VIRUSES.
Viruses chemically consists of
- Protein
- Nucleic acid
- Lipids
- Carbohydrates.
Details.
Proteins: The protective covering, capsid, consist of proteins. These proteins determine the antigenic characteristics of the virus. Some proteins are in the form of enzymes. And these enzymes though present in very small amounts, are essential for the initiation of the replication cycle of the virus within the host cell. Fir example, some viruses contain RNA polymerase (e.g. orthomyxoviruses), while retroviruses contain reverse transcriptase, the enzyme which makes complementary DNA copy of the viral RNA.
Nucleic acid: The nucleic acid of a virus, which is either DNA or RNA, may be single or double stranded, circular or linear in shape, and may or may not be segmented. The nucleic acid contains the genetic information necessary for the replication of the virus. The sequences and composition of nucleotides of viral nucleic acid are specific for each virus, and the characteristics are useful in classifying the viruses into families.
Lipids: Lipids are present in the envelopes of viruses. As the envelope is formed by budding from the cell membrane of the host cell, it shows a lipid composition. Some viruses, such as herpesviruses, bud through the nucleic membrane of the host cell, and shows the lipid composition of the nuclear membrane instead of that of the cell membrane.
Carbohydrates: Viral envelopes also contain glycoproteins coded by the virus itself. Unlike the lipids which are derived from the host cell. These glycoproteins help the virus in attachment to the host cell. Because of their antigenic nature, the glycoproteins are also involved in the interactions.
CLASSIFICATION OF MEDICALLY IMPORTANT VIRUSES.
The scheme for the classification of viruses depends upon their structure, antigenic composition and other properties. Viral families and genera have been designated, though differentiation into species is still incomplete.
Viruses are classified into two major divisions depending on the type of nucleic acid.
Deoxyriboviruses, which contain DNA.
Riboviruses, which contain RNA.
Both of these are further subdivided mainly on the basis of size and shape of the virions, symmetry of the nucleocapsid and strandedness of the nucleic acid.
Other classifications are Arboviruses and the unclassified viruses.
Now let's know about the families of the viruses.
Details.
- Deoxyriboviruses.
DNA-wikimedia commons by Madprime under CC-BY-SA-3.0
It consist of DNA which is also known as deoxyribonucleic acid. The DNA is a chemical substance present in the nucleus of every cell, which is made up of two chains of small chemical units called nucleotides. Each nucleotide consists of a phosphate group, a sugar called deoxyribose and a nitrogenous base.
The nitro-genous bases are purines and pyrimidines.
The purines are adenine (A) and guanine (G), while the pyrimidines are thymine (T) and cytosine (C).
The nucleotides are twisted into each other to form a double helix, something like a spiral staircase. The chemical structure of these bases are bond in certain pattern as adenine always joins thymine and guanine always joins cytosine. In other words the nitrogenous base from one chain (purine) links up with a compatible base (pyrimidine) of the opposite chain like rungs (steps) of a ladder. The four bases of the DNA chains are held together by hydrogen bonds.
The DNA viruses are grouped into families which I would briefly discuss below.
a) The parvoviridae family
The parvoviridae family has the size of 18-26 nm. They are icosahedral symmetry with absence of envelope. And their DNA are usually single stranded. Example, parvovirus.
Some members of the family cause gastroenteritis while others are associated with haemolytic disease.
b) The papovaviridae family
The papovaviridae family has the size of 40-55 nm. They are icosahedral symmetry with absence of envelope. And their DNA are usually double stranded. Example,
i) Papilloma virus which causes cutaneous, genital and laryngeal warts.
ii) Polymavirus which Produces neurological diseases.
c) The Adenoviridae family
The adenoviridae family has the size of 70-90 nm. They are icosahedral symmetry with absence of envelope. And their DNA are usually double stranded. Example, Adenovirus: There are at least 41 types of adenoviruses that can infect humans. They attack the lymphoid tissue and the mucous membranes. Also some adenoviruses can cause acute respiratory disease and conjunctivitis.
d) The herpesviridae family
The herpesviridae family has the size of 100-200 nm. They are icosahedral symmetry with presence of envelope. And their DNA are usually double stranded.
Examples:
i) Herpes simplex virus: Herpes simplex virus type 1 may cause oral infection (gingivostomatitis) or keratitis, while herpes simplex virus type 2 causes genital lesions.
ii) Varicella/Zoster: Varicella (chicken pox) is the primary infection. The virus may remain latent in the nerve ganglia, and may be reactivated when the immunity is lowered, resulting in an attack of zoster (shingles)
iii) Cytomegalovirus (CMV): Usually CMV infections are symptomless. It may cause severe generalized neonatal infection or an infection similar to glandular fever in adults. Generalized infection may occur in immunosuppressed patients, especially in AIDS.
iv) Epstein-Barr virus: This virus causes infectious mononucleosis (glandular fever). It is also associated with Burkitt's lymphoma although it's exact role in the development of this malignancy is not certain.
This viruses often cause asymptomatic and latent infections.
e) The poxviridae family
The poxviridae family has the size of 300-450 nm × 170-260 nm.Their symmetry are unknown and has the presence of envelope. And their DNA are double stranded.
Example; All poxviruses tend to produce skin lesions. The human pathogens include variola (small pox), vaccinia (used for small-pox vaccine) and molluscum contagiosum virus. Some animal pathogens can also infect humans e.g. cowpox and monkey pox.
Variola (smallpox) is a severe disease with a high mortality rate in non-immunized persons. The virus causes skin lesions through the blood stream which later pustulate. The pustulates breaks down and discharges the virus into the environment. And the main route of transmission is respiratory. Smallpox vaccine is prepared from the closely related but much less virulent, vaccinia virus.
f) The hepadnaviridae family
The hepadnaviridae family has the size of 42 nm. Their symmetry is unknown and has the presence of envelope. And their DNA are partially double stranded.
The virion also contains DNA-polymerase that repairs the single stranded region to make fully double stranded molecules.
Example; Hepatitis-B virus may be transmitted from person to person by inoculation with infected blood, e.g. from a contaminated hypodermic syringe and needle. Procedures like tattooing and acupuncture can also be source of infection.
The virus can cause hepatitis, liver cirrhosis and hepatocellular carcinoma.
However, the number of silent carriers is very high. Most carriers of the virus remains healthy but a few develop chronic active hepatitis and cirrhosis, which may lead to hepatoma.
2.Riboviruses
RNA-wikimedia commons by vossman under CC BY-S.A 3.0
It consists of RNA also known as ribonucleic acid. The RNA and DNA are both referred to as nucleic acud because they are acidic and are found in the nuclear even though RNA is widely distributed in the cytoplasm of the cell.
The RNA is different from the DNA in having a sugar which contains oxygen while the sugar of DNA has no oxygen hence the name deoxyribonucleic acid.
The nitrogenous bases purines are the same in both DNA and RNA, while they differ in the pyramidines.
The pyramidines of DNA consist of thymine and cytosine while those of RNA are made up of uracil and cytosine. Both DNA and RNA Carry's genetic code. This is why some viruses that lack DNA, have RNA to carry out their genetic functions.
RNA occurs as half of a ladder. It consists of a single upright strand of suger and phosphate units. The nitrogenous bases of RNA are not in pairs, they occur singly. Three of the four nitrogenous bases are the same as those of DNA, adenine (A), cytosine (C), guanine (G). The fourth base thymine (T) is replaced with uracil (U) in RNA.
Now lets know about the families of the viruses.
a) The picornaviridae family
The picornaviridae family do have the size between 22-30 nm. They are icosahedral symmetry with the absence of envelope. Their RNA are usually single stranded.
Examples;
Rhinovirus: There are more than 100 serotypes which cause the common cold.
Enteroviruses: The Poliovirus, Echovirus and Coxsackievirus belongs to this geneus. These viruses are ingested and multiplied in the alimentary tract. They spread to the tissues and organs to which they have affinity, via the blood stream. The viruses are excreted in the faeces.
- Poliovirus causes poliomyelitis. It is an acute infectious disease and in it's serious form affects the central nervous system. It destroys the motor neurons in the spinal cord resulting in paralysis.
Though most of the Poliovirus infections are subclinical.
Coxsackievirus is a large subgroup of the enteroviruses, and is subdivided into two groups, A and B. They Produces a variety of illnesses in humans.
Herpangina, hand, foot, and mouth diseases, and acute haemorrhagic conjunctivitis are caused by some group A Coxsackieviruses.
Group B serotypes are associated with myocarditis, pericarditis and meningoencephalitis. In addition, viruses in both groups can be responsible for aseptic meningitis, respiratory and undifferentiated febrile illnesses, hepatitis and paralysis. The paralysis is usually incomplete and reversible.Echoviruses (enteric cytopathogenic human orphan viruses) have about 30 serotypes. Some of them causes aseptic meningitis, febrile illnesses with or without rash, and common cold.
New enteroviruses are newly recognized enteroviruses which are given numbers instead of names. Enterovirus type 70 is known to cause acute haemorrhagic conjunctivitis. Enterovirus type 71 can cause meningitis, encephalitis and a respiratory infection.
b) The Reoviridae family
The Reoviridae family are known to have the size of 60-80 nm. They are icosahedral symmetry with absence of envelope. Their RNA are double stranded.
Example: The family Reoviridae includes the original Reovirus genus, rotaviruses and other insect and plant viruses.
Reoviruses are known for causing minor febrile illnesses, diarrhea or enteritis and are not known in causing any severe illness.
Rotaviruses are a major cause diarrhoea in infants and children. Typical Symptoms include diarrhoea, fever, abdominal pain and vomiting, leading to dehydration. The name rotavirus is based on the appearance of the virus in electron microscopy.
c) The Orthomyxoviridae family
The Orthomyxoviridae family are known to have the size of 100nm. They are either spherical or pleomorphic and helical symmetry with the presence of envelope. Their RNA are single stranded and segmented in 8 pieces.
Examples: All orthomyxoviruses are influenza viruses (type A, B and C). They have, as a part of their surface, projections that exhibit hemagglutinin and neuraminidase activity. Influenza is an acute respiratory tract infection which usually occurs in epidemics. Antigenic variation is observed very frequently in group A serotypes and less frequently in group B serotypes, while the type C is antigenically stable.
d) The Paramyxoviridae family
The Paramyxoviridae family are known to have the size of 150-300 nm. They are either spherical or plemorphic and helical symmetry with the presence of envelope. Their RNA are single stranded and non-segmented.
Examples:
Mump's virus: It causes an acute contagious disease characterized by a non-suppurative enlargement of one or both of the parotid glands, and other organs may be involved too.
Measles (Rubeola) virus: It causes an acute, highly infectious disease characterised by a maculopapular rash and fever. It is associated with respiratory involvement.
Parainfluenza viruses: parainfluenza group of viruses can cause serious illness such as laryngotracheitis and croup, bronchitis and pneumonitis and croup, bronchitis and pneumonitis, especially in the first year of life.
Respiratory Syncytial virus (RSV): This paramyxovirus causes the most serious bronchiolitis, RSV produces a characteristic fusion of cells, Syncytia, in human cell cultures.
e) The Togaviridae family
This family is known to have sizes between 40-70 nm. They are icosahedral symmetry with presence of envelope. Their RNA are single stranded.
Examples:
- Rubella virus: It causes an acute febrile illness with rash and lymphadenopathy. It affects children and young adults. Infection in the early pregnancy may result in abnormalities of the foetus.
- Arboviruses (Arthropode-borne viruses): Many Arboviruses are from the family Togaviridae. Fir example, Yellow fever virus which causes an acute, febrile mosquito borne illness. Severe cases may show jaundice, proteinuria and haemorrhage. Other examples of arboviruses in this family incude, Chikungunya, Semliki Forest and Sindbis viruses.
Note: The family Togaviridae is further divided into two families, Togaviridae and Flaviviridae. Hepatitis C virus is proposed to be included in the family Flaviviridae.
f) The Retroviridae family
This family are known to have their sizes between 80-100 nm. Their symmetry are unknown with the presence of envelope. While their RNA are single stranded.
Example: The members of this family contain a unique enzyme, reverse transcriptase, which is an RNA directed DNA polymerase.
The family has many tumour producing viruses such as the Sarcoma viruses of birds and mice, the leukaemia viruses of mice, cats birds and humans. And most importantly The Human Immunodeficiency syndrome (HIV), the causative agent of Acquired Immunodeficiency Syndrome (AIDS).
HIV/AIDS
Human Immunodeficiency syndrome virus (HIV) is the causative agent of Acquired Immunodeficiency syndrome (AIDS). HIV has a cylindrical core with nucleic acid cloned and sequenced.
It is a retrovirus, but differs from other retroviruses such as human To lymphotrophic viruses (HTLV) 1 and 2. Cells bearing CD4 antigen which is a component of the viral receptor required for cell entry are susceptible to HIV infection, but HIV has a particular affinity for the T4 helper lymphocytes.
An envelope glycoprotein of the virus, specifically binds to the CD4 molecule receptor site on the surface of the T4 lymphocytes. Once the virus is bound to the cell receptor, it can enter the cell. The viral reverse transcriptase enzyme, on entering into the infected cell, makes A DNA copy of the RNA genome (pro-viral DNA). The pro-viral DNA can thereby bring about the destruction of the cell. The T4 lymphocyte has a central role in many immunological functions, and an infected cell can cause severe damage to the cellular immune system, reduce immunity results in infections with many opportunistic pathogens and development of certain tumours such as Kaposi's sarcoma.
Mode of infection.
- By sexual intercourse.
- By transfusion of bloodbof an HIV infected person to an uninfected person.
- By oral sex.
- By using unsterilized infected needless.
- From infected mother to her baby in the womb or during birth.
AIDS is an illness characterized by one or more indicator do. Certain diseases, when definitely diagnosed in immunocompromised individuals without the Symptoms of any other cause of immune deficiency. If there is a laboratory evidence of HIV infection, certain indicator diseases that requires presumptive and definitive diagnosis that requires presumptive and definitive diagnosis are diagnostic of AIDS.
Acute Hiv infection is usually accompanied by transient non-specific illness characterized by fever, malaise, myalgia, lympadenopathy, pharyngitis and rash. Most of these conditions are, however, subclinical. In two to six weeks, antibodies to the core and surface proteins are usually detected by enzyme immunoassay (EIA) and confirmed by immunofluorescence.
A chronic infection of AIDS that follows is asymptomatic in early stage. In some patients, a persistent generalised lymphadenopathy in the form of nodes of 1 cm or more in diameter in two or more non-contiguous extra-inguinal sites, commonly in the cervical and the axillary lymph nodes, usually develops. In the later stages of the illness, there may develop symptoms such as fever, night sweats, diarrhoea and wieght loss. Patients may also suffer from minor opportunistic infections such as oral candidiasis, oral hairy leucoplakia, herpes simplex, folliculitis, seborrheic dermatitis, impetigo and tinea infections.
The incubation period of HIV infection is long, lasting up to 10years before the Symptoms develop.
Facts About Viruses.
pixabay
Viruses don't have life in them.
Viruses like bacteriophages, meaning ‘bacteria eater’ arw useful. They kill bacteria and are used to protect people against harmful bacteria in food.
cells are absence in viruses.
Viruses are 20 to 250 nanometres in size.
Viruses don't change food into energy.
Viruses don't really die. With their genes, the viruses reproduces and evolve through what is known as natural selection.
Viruses don't usually survive without a host, because without it they are just inert packets of chemicals.
Antibiotics cannot be used in curing viral infections. Some infections like flu and cold can get better on it's own.
It is believed that, half of all human DNA was originated from viruses, which infect and embedd themselves in our ancestor's egg and sperm cells.
The first human virus, yellow virus was found in 1901 by Walter Reed.
Thanks For Reading.
References And Further Reading.
√Medical Laboratory Science Theory And Practice By J OCHEI & A KOLHATKAR.( section V medical microbiology chapter1)
√https://en.m.wikipedia.org/wiki/Clinical_virology
√Idodo Umeh College Biology, by G. IDODO. UMEH, B.Sc (Hons.) M.Sc Phd and PhD. (page 410,411,412)
√https://en.m.wikipedia.org/wiki/Dmitri_Ivanovsky
√https://en.m.wikipedia.org/wiki/Martinus_Beijerinck
√http://discovermagazine.com/2010/apr/20-things-you-didnt-know-about-viruses
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