VIROLOGY: HISTORY, WHAT IT STUDIES, TYPES OF VIRUSES, EXAMPLES - BIOLOGY - 2024

The virology is the branch of biology that studies the origin, evolution, classification, pathology and biomedical and biotechnological applications of viruses. Viruses are small particles, 0.01–1 µm, whose genetic information is solely for their own replication.

The genes of viruses are decoded by the molecular machinery of the infected cell for multiplication. Therefore, viruses are obligate intracellular parasites dependent on the metabolic functions of living cells.

Source: Photo Credit: Cynthia Goldsmith Content Providers (s): CDC / Dr. Erskine. L. Palmer; Dr. ML Martin

The most abundant genetic material on the planet corresponds to that of viruses. They infect other viruses and all living things. Immune systems do not always defend successfully against viruses: some of the most devastating diseases of humans and animals are caused by viruses.

Human viral diseases include yellow fever, polio, influenza, AIDS, smallpox, and measles. Viruses are involved in about 20% of human cancers. Every year, viral respiratory and intestinal infections kill millions of children in developing countries.

Some viruses are useful for typing bacteria, as sources of enzymes, for pest control, as antibacterial agents, for fighting cancer, and as gene vectors.

History

In the late 19th century, Martinus Beijerinck and Dmitri Ivanovski independently determined that bacteria-free filtrates from diseased tobacco plants contained an agent capable of infecting healthy plants. Beijerinck called this agent contagium vivum fluidum.

We now know that the Beijerinck and Ivanovski filtrates contained the tobacco mosaic virus. Also in the 19th century, Friedrich Loeffler and Paul Frosch concluded that FMD in cattle is caused by a non-bacterial agent.

In the first decade of the 20th century, Vilhelm Ellerman and Olaf Bang demonstrated the transmission of leukemia in chickens, using cell-free filtrates. These experiments allowed to conclude that there are animal viruses that can cause cancer.

In the second decade of the 20th century, Frederick Twort observed the lysis of micrococci on agar plates in which he was trying to grow the smallpox virus, assuming that this lysis had been caused by a virus or by enzymes of bacteria. For his part, Felix d'Hérelle discovered that the bacilli that cause dysentery were lysed by viruses that he called bacteriophages.

In 1960 Peter Medawar received the Nobel Prize for discovering that viruses contained genetic material (DNA or RNA).

Virus types

Viruses are classified according to the characteristics they possess. These are the morphology, the genome and the interaction with the host.

The classification based on the interaction of the virus with the host is based on four criteria: 1) production of an infectious progeny; 2) whether or not the virus kills the host; 3) if there are clinical symptoms; 4) duration of infection.

The immune system plays an important role in the interaction between virus and host because it determines the development of the infection. Thus the infection can be acute and subclinical (the virus is eliminated from the body), or persistent and chronic (the virus is not eliminated from the body).

Classification based on genome differences (Baltimore System) and taxonomic classification, which takes into account all the characteristics of viruses, are the systems most used today to catalog viruses.

Classification based on morphology

To understand this classification it is necessary to know the parts that make up a virus. Viruses consist of a genome and capsid, and may or may not have an envelope. The genome can be DNA or RNA, single or double stranded, linear or circular.

The capsid is a complex structure made up of many identical viral protein subunits, called capsomeres. Its main function is to protect the genome. It also serves to recognize and bind to the host cell, and to ensure transport of the genome into the cell.

The envelope is the membrane composed of lipids and glycoproteins that surrounds the capsid. It is derived from the host cell. It varies considerably in size, morphology, and complexity. The presence or absence of envelopes serves as a criterion for virus classification.

Three categories of non-enveloped viruses are recognized: 1) isometric, approximately spherical in shape (icosahedrons or icosadeltahedrons); 2) filamentous, with a simple helix shape; 3) complex, without the previous forms. Some viruses, such as bacteriophage T2, combine the isometric and filamentous forms.

If the virus is enveloped, they can also be assigned to morphological categories based on the characteristics of the nucleocapsid within the membrane.

Genome-based classification: Baltimore system

This classification, proposed by David Baltimore, considers the nature of the virus genome in terms of the mechanism it uses to replicate nucleic acid and transcribe messenger RNA (mRNA) for protein biosynthesis.

In the Baltimore system, viruses whose RNA genome has the same sense as the mRNA are called viruses with positive sense RNA (+), while viruses whose genome has the opposite sense (complementary) to the mRNA are called viruses with Negative sense RNA (-). Double-stranded genome viruses go both ways.

A disadvantage of this classification is that viruses that have similar replication mechanisms do not necessarily share other characteristics.

Baltimore System Classes

Class I. Virus with a double-stranded DNA genome. Transcription similar to that of the host cell.

Class II. Viruses with a single-stranded DNA genome. DNA can be of (+) and (-) polarity. Converted to double stranded prior to mRNA synthesis.

Class III. Viruses with a double-stranded RNA genome (dsRNA). With segmented genome and mRNA synthesized from each segment of the DNA template. Enzymes that participate in transcription encoded by the virus genome.

Class IV. Viruses with single stranded RNA genome (ssRNA), polarity (+). Synthesis of mRNA preceded by synthesis of the complementary strand. Transcription is similar to that of class 3.

Class V. Virus with ssRNA genome of the opposite sense to that of the sense mRNA (-). Synthesis of mRNA that requires virus-encoded enzymes. The production of new generations of the virus requires the synthesis of intermediate dsRNA.

Class VI. Virus with ssRNA genome that produces intermediate dsDNA before replication. It uses enzymes that the virus carries.

Class VII. Viruses that replicate their dsDNA via an intermediate ssRNA.

Taxonomic classification

The International Committee on Taxonomy of Viruses established a taxonomic scheme to classify viruses. This system uses the divisions order, family, subfamily and gender. There is still a debate about the application of the species concept to viruses.

The criteria used for taxonomic classification are host range, morphological characteristics, and the nature of the genome. In addition, other criteria are considered, such as the length of the phage tail (virus that infects bacteria), the presence or absence of certain genes in the genomes, and the phylogenetic relationships between viruses.

An example of this classification is: order Mononegavirales; family Paramyxoviridae; Paramyxovirinae subfamily, genus Morbillivirus; species, measles virus.

The names of families, subfamilies, and genera are inspired by the place of origin, the host, or the symptoms of the disease that the virus produces. For example, the Ebola River in Zaire gives the name to the genus Ebola; the tobacco mosaic gives its name to the genus Tomabovirus.

Many virus group names are words of Latin or Greek origin. For example, Podoviridae, is derived from the Greek podos, which means foot. This name refers to short-tailed phages.

Examples of viruses

Influenza virus

They infect birds and mammals. They have diverse morphology, with envelope. Single stranded RNA genome. They belong to Class V of Baltimore and to the family Orthomyxoviridae.

Influenza viruses belong to this family. Most cases of influenza are caused by influenza A viruses. Outbreaks caused by influenza B viruses occur every 2–3 years. Those produced by influenza C viruses are less frequent.

The influenza A virus has caused four pandemics: 1) the Spanish flu (1918–1919), a subtype of H1N1 virus of unknown origin; 2) Asian flu (1957–1958), H2N2 subtype, of avian origin; 3) Hong Kong flu (1968–1969), subtype H3N3, of avian origin; 4) swine flu (2009–2010), subtype H1N1, of swine origin.

The most devastating pandemic known was caused by the Spanish flu. It killed more people than the First World War.

The letters H and N come from the membrane glycoproteins hemagglutinin and neuraminidase, respectively. These glycoproteins are present in a great variety of antigenic forms and are involved in new variants.

Retrovirus

They infect mammals, birds, and other vertebrates. Spherical morphology, with envelope. Single stranded RNA genome. They belong to the Baltimore class VI and the Retroviridae family.

The human immunodeficiency virus (HIV), genus Lentivirus, belongs to this family. This virus causes damage to the immune system of the infected person, making it susceptible to infection by bacteria, viruses, fungi and protozoa. The disease caused by HIV is known as acquired immunodeficiency syndrome (AIDS).

Other genera belonging to Retroviridae also cause serious diseases. For example: Spumavirus (simian fluffy virus); Epsilonretrovirus (Walleye dermal sarcoma virus); Gammaretrovirus (murine leukemia virus, feline leukemia virus); Betaretrovirus (murine mammary tumor virus); and Alpharetrovirus (Rous sarcoma virus).

Herpes virus

It infects cold-blooded mammals, birds, and vertebrates. Morphology of the virus: icosahedral capsule, with envelope. Double-stranded DNA genome. They belong to class I of Baltimore and to the Herpesviral order.

Some members are: Herpes simplex virus 2 (causes genital herpes); human cytomegalovirus (causes birth defects); Herpesvirus KaposiB ^™ s sarcoma (causes Kaposi's sarcoma); EpsteinBƂBarr virus or EBV (causes glandular fever and tumors).

Viruses that cause polio and other related viruses

It infects mammals and birds. Morphology of the virus: isometric or icosahedral. Single stranded RNA genome. They belong to the Baltimore class IV and the Picornaviridae family.

Some genera of this family are: Hepatovirus (causes hepatitis A); Enterovirus (causes poliomyelitis); Aphthovirus (causes foot and mouth disease).

Viruses that cause rabies and related viruses

They infect mammals, fish, insects, and plants. Helical morphology, with envelope. Single stranded RNA genome. They belong to class V of Baltimore and to the family Rhabdoviridae.

The viruses that cause diseases such as rabies, caused by the genus Lyssavirus, belong to this family; vesicular stomatitis, caused by the genus Vesiculovirus; and the yellow dwarf potato, caused by the genus Novirirhabdovirus.

Virus that causes infectious erythrema

It infects mammals, birds, and insects. Symmetric icosahedral morphology. Single-stranded DNA genome. They belong to Class II of Baltimore and to the family Parvoviridae.

One member of this family is the B19 virus, belonging to the genus Erithrovirus, which causes infectious erythrema in humans, which usually does not produce symptoms. The B19 virus infects the precursor cells of red blood cells.

Some members of Parvoviridae are used as gene vectors.

Virus Applications

Viruses can be used for the benefit of man by constructing recombinant viruses. They have a genome modified by molecular biology techniques.

Recombinant viruses are potentially useful for gene therapy, the purpose of which is to cure specific diseases, or the production of vaccines.

HIV has been used to construct gene vectors (lentiviral vectors) for gene therapy. These vectors have been shown to be efficient in animal models of retinal pigment epithelial disease, such as retinitis pigmentosa caused by autosomal recessive inheritance or mutations.

Viruses used as vaccine vectors should have low pathogenic potential. This is verified using animal models. This is the case of vaccines developed or in development against smallpox viruses, vesicular stomatitis and Ebola.

References

1. Carter, JB, Saunders, VA 2013. Virology: principles and applications. Wiley, Chichester.
2. Dimmock, NJ, Easton, AJ, Leppard, KN 2007. Introduction to modern virology. Blackwell Malden.
3. Flint, J., Racaniello, VR, Rall, GF, Skalka, AM, Enquist, LW 2015. Principles of virology. American Society for Microbiology, Washington.
4. Hull, R. 2009. Comparative plant virology. Elsevier, Amsterdam.
5. Louten, J. 2016. Essential human virology. Elsevier, Amsterdam.
6. Richman, DD, Whitley, RJ, Hayden, FG 2017. Clinical virology. American Society for Microbiology, Washington.
7. Voevodin, AF, Marx, PA, Jr. 2009. Simian virology. Wiley-Blackwell, Ames.
8. Wagner, EK, Hewlett, MJ, Bloom, DC, Camerini, D. 2008. Basic virology. Blackwell Malden.