Virus Classification PDF

CLASSIFICATION OF VIRUSES  Virus classification is the process of categorizing viruses into a structured system based on shared characteristics. This system uses a hierarchy that groups viruses into orders, families, g...

CLASSIFICATION OF VIRUSES  Virus classification is the process of categorizing viruses into a structured system based on shared characteristics. This system uses a hierarchy that groups viruses into orders, families, genera, and species based on factors like: Genome type (DNA or RNA, single-stranded or double- stranded) Replication method Morphology (shape, size, presence of an envelope) Host range (types of organisms they infect) The most widely recognized classification system is the Baltimore classification, which groups viruses based on their replication strategy and genome type. Purpose of Virus Classification 1.Understanding Relationships: Helps in understanding the evolutionary relationships between different viruses. 2.Facilitating Research: Provides a systematic way to study viruses, allowing researchers to predict characteristics and behavior based on classification. 3.Developing Treatments: Aids in the development of antiviral drugs and vaccines by identifying common targets among related viruses. 4.Diagnosing Diseases: Assists in identifying the cause of viral infections, which is crucial for accurate diagnosis and treatment. 5.Public Health: Supports surveillance and control of viral outbreaks by enabling quick identification and categorization of new viruses. History of Virus Nomenclature The classification of viruses is a relatively new exercise because it was not till the 1890s that the viruses were discovered. It was in 1927 that Jonhson, a plant virologist first drew attention to the need for the classification of viruses. The classification of viruses then began by considering their biological, ecological and pathogenic characteristics. In 1939, Holmes divided the plant viruses based on their host reactions and host species, but could only describe 89 viruses. The electron microscopic studies were advanced in the 1950s and thus came the classification of viruses based on their virion properties, such as Herpesvirus, Myxovirus and Poxvirus. At that time new viruses were being discovered every other day and many scientists tried to give a way taxonomy is the scientific of classification, but none of them could succeed. study of naming, defining, classifying groups of biological organisms based on the shared In 1966, at the International Congress for Microbiology, held in Moscow, 43 virologists came characteristics forward to form a committee and named it the International Committee on Nomenclature of Viruses (ICNV). The main objective of creating this committee was to create a taxonomic and nomenclature system of viruses that was accepted worldwide. The name of this committee was changed to the International Committee on Taxonomy of Viruses (ICTV) in 1974. DIFFERENT CRITERIA FOR CLASSIFICATION OF VIRUSES (1) Nucleic acid type: RNA or DNA; single-stranded or double- stranded; strategy of replication. (2) Size and morphology, including type of symmetry, number of capsomeres, and presence of membranes. (3) Presence of specific enzymes, particularly RNA and DNA polymerases, and neuraminidase (4) Susceptibility to physical and chemical agents, especially ether. (5) Immunologic properties. (6) Natural methods of transmission. (7) Host, tissue, and cell tropisms. (8) Pathology; inclusion body formation. (9) Symptomatology Classification by Biological, Chemical, and Physical Properties RNA-Containing Viruses Picornaviridae Reoviridae Togaviruses Arenaviridae Coronaviridae Retroviridae  The International Committee on the Taxonomy of Viruses (ICTV) The International Committee on the Taxonomy of Viruses (ICTV), which is a committee of the Section on Virology of the International Association of Microbiological Societies (IAMS), completed a round of meetings during the Third International Congress for Virology that was held in Madrid from IO to 17 September 1975. Since I C T V only meets during these conferences, which are held every four years, the meetings are important occasions for reviewing the classification and nomenclature of viruses. Decisions on new names, which encapsulate the recognition of natural ' g r o u p s ' of viruses, evolve slowly. The ICTV Nomenclature According to the latest meeting of ICTV in March 2021, there are 10434 species, 2606 genera, 168 subfamilies, 233 families, and 65 orders of viruses. The ICTV began the process of naming and classification of viruses in the 1970s and the effort continues till today. The objectives of ICTV are: 1. To develop an internationally agreed taxonomy for viruses and other mobile genetic elements (MGEs) that are part of the virosphere; 2. To develop internationally agreed names for virus taxa; 3. To communicate taxonomic decisions to the international community of virologists; 4. To maintain an Index of agreed names of virus taxa. The International Committee on Taxonomy of Viruses (ICTV) is responsible for classifying and naming viruses. The ICTV classification system organizes viruses into a hierarchical structure based on their genetic material, replication strategy, morphology, and other characteristics. Here is an overview of the main taxonomic levels in the ICTV classification system: 1.Order: The highest taxonomic rank in the classification system, orders group together families of viruses with similar characteristics. For example, the order Picornavirales includes families such as Picornaviridae (which includes viruses like poliovirus and rhinovirus). 2.Family: Families consist of viruses that share common characteristics, such as genome structure, replication strategy, and morphology. Examples of virus families include Flaviviridae (e.g., hepatitis C virus) and Herpesviridae (e.g., herpes simplex virus). 3.Subfamily: Some families are further subdivided into subfamilies, which contain viruses with more specific shared characteristics. Subfamilies are not present for all families and are used when necessary to organize viruses more precisely. 4.Genus: Within a family or subfamily, viruses are grouped into genera based on similarities in genetic makeup, replication strategy, and other characteristics. For example, the genus Flavivirus includes viruses such as Zika virus and dengue virus. 5. Species: The species is the fundamental unit of virus classification. Each species represents a group of viruses with similar characteristics, often including viruses that share a common ancestor and are genetically related. Species names are italicized and include the virus name followed by the word "virus." For example, Human immunodeficiency virus is a species within the genus Lentivirus. 6. Strain: Strains refer to specific variants or isolates of a virus species. Strains may differ in characteristics such as virulence, host range, or antigenicity. Strain names are often designated using letters, numbers, or other identifiers, such as H1N1 influenza virus or HIV-1 strain. The ICTV classification system provides a standardized framework for organizing and naming viruses, facilitating communication among researchers and public health professionals and enabling the study of viral evolution, epidemiology, and pathogenesis. Official approval for new names depends upon a series of sequential steps; recommendations by one or more of the subcommittees of the Executive Committee of ICTV (subcommittees on Bacterial, Invertebrate, Plant and Vertebrate Viruses respectively, and for some of the larger viral groups which span several kinds of h o s t s - the Coordination Subcommittee), which are considered by the Executive Committee of ICTV and The results of the last five years of work by ICTV and its may finally be submitted for approval committees will be published during 1976 as a separate volume of by ICTV itself. 'Intervirology', the official journal of the Section on Virology of IAMS, and additional copies will be produced for sale as separates. Only after this final approval does a name become This ' Second R e p o r t ' will include, b e s i d e s ' a p p r o v e 'official'. d ' and common names of virus groups, a brief description of the properties of each group and a listing of probable and possible members of the group. To ensure that information upon the present state of viral taxonomy is available to as many virologists as possible, as soon as possible, the following brief paper has been prepared. It includes an up-to-date version of the Rules of Nomenclature, the names of all members of the new Executive Committee of ICTV, and a summary of the present position regarding viral families, genera and groups and their names. For the convenience of working virologists, the last part has been set out with some relation to the 'host-directed' interests of most virologists, although it will be seen that in several cases viral groupings transgress such boundaries. The Rules of Viral Nomenclatures The official approval of names for viral groups is subject to their legitimacy in terms o f several 'Rules', which are set out in the First Report on the Classification and Nomenclature of Viruses (Wildy, 1971). At the meetings in Madrid several minor and two major changes in the Rules were approved by ICTV. The latter relate to Rule 4 (previously ' An effort will be made towards a latinized binomial nomenclature’) and Rule 7 [New sigla (i.e. names made up from a few or initial letters) shall not be introduced’]. Over the nine years that have elapsed since the formation of ICTV (initially known as the International Committee on the Nomenclature of Viruses), it has become clear that virologists and teachers of virology greatly value the grouping of viruses into genera and families, based primarily on their chemistry, morphology and strategy of multiplication. Latinized names, ending in '... virus' for a genus and '... viridae' for a family name, have been widely accepted. The ICTV does not classify ranks below species such as variants, subspecies or serotypes. The International Committee on Taxonomy of Viruses (ICTV) classification system offers several advantages: 1.Standardization: Provides a uniform and globally accepted framework for naming and classifying viruses, facilitating clear communication among researchers and healthcare professionals. 2.Comprehensive Classification: Organizes viruses based on a range of criteria, including genetic material, replication strategy, and morphology, offering a detailed and structured approach to virus taxonomy. 3.Consistency: Ensures consistency in virus names and classifications, which is crucial for accurate identification, research, and comparison of viral species. 4.Adaptability: The system is regularly updated to incorporate new discoveries and advancements in virology, allowing for the inclusion of newly identified viruses and revised taxonomic relationships. 5.Integration with Molecular Data: Utilizes molecular and genetic data to classify viruses, which provides a more precise and scientifically grounded taxonomy compared to traditional methods. 6.Facilitates Research and Communication: Aids in research by providing a clear taxonomy that helps in studying viral epidemiology, pathogenesis, and vaccine development. It also enhances communication and collaboration across different fields of virology. SHORTCOMINGS ASSOCIATED WITH ICTV FRAMEWORK The International Committee on Taxonomy of Viruses (ICTV) classification system is widely used for organizing and categorizing viruses based on their genetic material, replication strategy, morphology, and other characteristics. However, like any classification system, it has its shortcomings: 1.Complexity: The ICTV classification system can be complex and difficult to navigate, especially for non-experts. It relies heavily on molecular data and may require specialized knowledge to understand fully. 2.Lack of Universal Application: While the ICTV classification system is widely accepted in the scientific community, it may not be universally applied across all fields or regions. Different researchers or institutions may use alternative classification systems, leading to inconsistencies in virus naming and classification. 3.Rapid Evolution: Viruses are known for their rapid evolution, which can lead to changes in their genetic makeup and characteristics over time. This rapid evolution can make it challenging to maintain an up-to-date and accurate classification system. 4. Uncertainty in Taxonomic Assignments: Due to the complexity of viral genomes and the diversity of viruses, taxonomic assignments within the ICTV classification system may sometimes be uncertain or subject to revision as new information becomes available. 5. Bias Towards Culturable Viruses: The ICTV classification system may have a bias towards viruses that are easily culturable or well-studied in laboratory settings. Viruses that are difficult to culture or study may be underrepresented or poorly characterized within the classification system. 6. Inadequate Representation of Viral Diversity: Despite efforts to categorize viruses based on their genetic characteristics, the ICTV classification system may not fully capture the diversity of viruses present in nature. This could result in certain viral groups being underrepresented or overlooked. 7. Limited Predictive Power: While the ICTV classification system provides a framework for organizing viruses based on their known characteristics, it may have limited predictive power for identifying novel or emerging viruses with unique properties. Overall, while the ICTV classification system serves as a valuable tool for organizing and categorizing viruses, it is not without its limitations. Continued research and refinement of classification methods are essential to address these shortcomings and improve our understanding of viral diversity and evolution.