Classification of Viruses

CLASSIFICATION OF VIRUSES Criteria for classification of viruses • Originally the criterion was based on the host affected – Animal virus – Plant virus – Bacteriophages • Some virus groups affect more than one kingdom of organisms or hosts – Bunyaviridae Plant and animals – Partiviridae Plant and fungi – Reoviridae Plant and animals – Rhabdoviridae Plant and animals – Phycodnaviridae Plant and protozoa – Picornaviridae Plant and animals – Totiviridae Protozoa, fungi and insects • Viruses infecting across different phyla (insects and vertebrates) – Flaviviridae – Iridoviridae – Asfarviridae – Parvoviridae – Poxviridae – Togaviridae Taxonomic criteria: Host organism(s) • Procaryotes • Eucaryotes – Archbacteria – Algae – Eubacteria – Protozoa – Mycoplasma – Fungi – Spiroplasma – Plants – Invertebrates – Vertebrates Criteria for Classification of viruses • Classical virus classification schemes have been based on the consideration of five major properties of viruses: – The type of nucleic acid which is found in the virion (RNA or DNA) – The symmetry and shape of the capsid – The presence or absence of an envelope (ether sensitivity) – The size of the virus particle – The host species • More recently the strategy of NA replication (Baltimore’s classification scheme) Criteria for classification • International Committee on Viral Taxonomy (ICTV) have really emphasized the viral genome as the primary determinant for viral taxonomy. • there is a drift towards the use of genomics for virus classification • The naming system for viruses that has been adopted by the ICTV (http://www.ncbi.nlm.nih.gov/ICTV) is very useful for animal viruses, and is widely used. • Latinized virus family names start with capital letters and end with the suffix –viridae (e.g., Herpesviridae). These formal names are often used interchangeably with the common names for viruses (e.g., herpesviruses). Levels of taxonomy Suffix Taxonomic level Example (comment) -virales (a group Order of related Mononegavirales families) Family -viridae Paramyxoviridae Subfamily -virinae Paramyxovirinae Genus -virus Morbillivirus (an individual Species Measles virus virus) • Viruses are broadly classified primarily upon the type of genomic nucleic acid, e.g. DNA or RNA, and • then further by the number of strands of nucleic acid (e.g. double-stranded DNA, double-stranded RNA or single-stranded RNA, with a positive or negative "sense" of that single strand). • Retroviruses are a special category of RNA viruses that require reverse transcription of their RNA to DNA and then integration of that DNA into the host cell genome before replication can take place. They carry a reverse transcriptase enzyme as part of the virion. Genome type/diversity • DNA – Double-stranded – Single stranded – Linear or circular – Single or multiple molecule • RNA – Double stranded – Single stranded • Positive sense • Negative sense – Linear or circular – Single or multiple molecule The Most Common DNA Viruses • Adenoviruses • Cytomegalovirus • Epstein-Barr virus • Hepatitis B virus • Herpes simplex Types 1 &2 • Papovavirus • Varicella-Zoster virus RNA viruses • Dengue viruses • Ebola virus • Picorna viruses • Hanta virus • Hepatitis A and C • HIV • Influenza virus • Mumps virus • Norwalk virus • Corona virus • Rotavirus • Rubeola Virus Others criteria • Disease symptoms • Antigenic diversity • Protein profiles • Host range Genome Diversity DNA RNA Double stranded Single stranded Double stranded Single stranded Linear Circular Linear Linear/*Circular Single Single molecule Multiple molecules Multiple Positive sense Negative sense Single molecule Multiple molecules Baltimore classification scheme • Viruses exhibit great diversity in terms of morphology, genome structure, mode of infection, host range, tissue tropism, disease or pathogenesis • Each of these criteria can be used to place a virus into a group, but many share these properties • David Baltimore proposed a scheme that encompassed all viruses based on the nature of their genomes and their modes of replication and gene expression • The ICTV uses these together with other parameters to place viruses into families and genera • Viruses are grouped according to their mechanism of mRNA synthesis and their replication strategy, • The scheme uses the criteria that are the unchanging characteristics of the virus, but takes no account of the biological properties of the viruses • Class I: – Have ds DNA genomes – mRNA may come from either strand – Transcription can occur using a process similar to that found in the host cells • Class II: – Have ss DNA genomes – DNA can be + or – sense – DNA must be converted to a ds form before mRNA synthesis can proceed • Class III: – Have dsRNA genomes – Most have segmented genomes – mRNA is synthesized from only one template strand of each segment – Enzymes are carried into the cell by the virus to initiate the infectious process • Class IV: – Have ssRNA genomes of positive sense – Complementary strand is synthesized to form a dsRNA intermediate and precedes synthesis of mRNA – Enzymes required for RNA synthesis are produced after infection is initiated • Class V: – Have ssRNA genome of negative sense – mRNA is synthesized by transcription from the genome strand and requires novel virus-encoded enzymes – The positive sense antigenome strand is used as a template for replication – Some viruses use the newly synthesized strand as a template for production of mRNA (referred to as ‘ambisense’ viruses) – Enzymes are virus encoded, they are packaged and carried into the cell by the virion • Class VI: (Retroviruses) – Have positive-sense ssRNA genomes – Generate a dsDNA intermediate as a prelude to replication – Virus carry the virus-coded enzyme (reverse transcriptase) not found in the host cells • Class VII: (‘Reversiviruses’) – Have a dsDNA genome – Replicate via a positive-sense ssRNA intermediate – Possess a reverse transcriptase – Example: hepadnaviridae .

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