Classification of Medically Important Viruses Prepared by : Nour I. El-Haddad

Introduction The classification of viruses is based on chemical and morphologic criteria. The two major components of the virus used in classification are 1- the nucleic acid (its molecular weight and structure) 2- the capsid (its size and symmetry and whether it is enveloped)

DNA Viruses The three naked (nonenveloped) icosahedral virus families: 1- the parvoviruses 2- papovaviruses 3- adenoviruses

The three enveloped families: 1- The hepadnavirus family  hepatitis B virus 2- the herpesviruses 1,2 are enveloped icosahedral viruses 3- The largest viruses, the poxviruses, have a complex internal symmetry.

Parvoviruses

* These are very small (22 nm in diameter), naked icosahedral viruses. * single-stranded linear DNA * There are two types of parvoviruses:defective and nondefective 1- The defective parvoviruses, e.g., adeno-associated virus, require a helper virus for replication. The DNA of defective parvoviruses is unusual because plus-strand DNA, and minus-strand DNA are carried in separate particles 2-The nondefective parvoviruses are best illustrated by B19 virus is associated with aplastic crises in sickle cell anemia patients and with erythema infectiosum, an innocuous childhood disease characterized by a "slapped-cheeks" rash

Papovaviruses

*These are naked icosahedral viruses (55 nm in diameter). *with double-stranded circular supercoiled DNA. *The name "papova" is an acronym of papilloma, polyoma, and simian vacuolating viruses. *Polyomavirus and simian vacuolating virusare papovaviruses of mice and monkeys, respectively.

Three human papovaviruses: 1-JC virus, isolated from patients with progressive multifocal leukoencephalopathy. 2-BK virus, isolated from the urine of immunosuppressed kidney transplant patients. 3- human papillomavirus.

papillomavirus

Adenoviruses

- These are naked icosahedral viruses (75 nm in diameter). -with double-stranded linear DNA. -They cause pharyngitis, upper and lower respiratory tract disease. -at least 40 antigenic types, some of which cause sarcomas in animals but no tumors in humans.

Hepadnaviruses

- These are double-shelled viruses (42 nm in diameter) with an icosahedral capsid covered by an envelope. -The DNA is a double-stranded circle that is unusual because the complete strand is not a covalently closed circle and the other strand is missing approximately 25% of its length. - Hepatitis B virus is the human pathogen in this family.

Herpesviruses

-These are enveloped viruses (100 nm in diameter) with an icosahedral nucleocapsid. -double-stranded linear DNA. -They are noted for causing latent infections. - The five important human pathogens are herpes simplex virus types 1 and 2, varicella-zoster virus, cytomegalovirus, and Epstein-Barr virus (the cause of infectious mononucleosis).

Poxviruses

-These are the largest viruses, with a bricklike shape, an envelope with an unusual appearance, and a complex capsid symmetry. -They are named for the skin lesions, or "pocks," that they cause. - Smallpox virus and vaccinia virus are the two important members. The latter virus is used in the smallpox vaccine.

RNA Viruses The 14 families of RNA viruses: -Three naked icosahedral virus families. -Three enveloped icosahedral. -The remaining eight families are enveloped helical viruses. ( the first five have single-stranded linear RNA as their genome) (whereas the last three have single-stranded circular RNA.)

Picornaviruses

-These are the smallest (28 nm in diameter) RNA viruses. -They have single-stranded, linear, nonsegmented, positive-polarity RNA within a naked icosahedral capsid. -The name "picorna" is derived from pico (small), RNA-containing.

-There are two groups of human pathogens: 1- enteroviruses: poliovirus, coxsackievirus, echovirus, and hepatitis A virus. 2- rhinoviruses.

Rhinoviruses

Caliciviruses

-These are naked viruses (38 nm in diameter) with an icosahedral capsid. -They have single-stranded, linear, nonsegmented, positive-polarity RNA. -There are two human pathogens: Norwalk virus and hepatitis E virus.

Caliciviruses

Reoviruses

-These are naked viruses (75 nm in diameter) with two icosahedral capsid coats. - They have 10 segments of double- stranded linear RNA. -The name is an acronym of respiratory enteric orphan, because they were originally found in the respiratory and enteric tracts and were not associated with any human disease.

-The main human pathogen is rotavirus, which causes diarrhea mainly in infants

Flaviviruses

-These are enveloped viruses with an icosahedral capsid. -Single-stranded, linear, nonsegmented, positive- polarity RNA. - The flaviviruses include hepatitis C virus, yellow fever virus, dengue virus, West Nile virus, and St. Louis and Japanese encephalitis viruses.

Togaviruses

*These are enveloped viruses with an icosahedral capsid. *Single-stranded, linear, nonsegmented, positive- polarity RNA. *There are two major groups of human pathogens: the alphaviruses and rubiviruses. The alphavirus group includes eastern and western encephalitis viruses. The rubivirus group consists only of rubella virus.

Retroviruses

*These are enveloped viruses with an icosahedral capsid. *Two identical strands (said to be "diploid") of single- stranded, linear, positive-polarity RNA. * The term "retro" pertains to the reverse transcription of the RNA genome into DNA. * There are two medically important groups: (1) the oncovirus group, which contains the sarcoma and leukemia viruses, e.g., human T-cell leukemia virus (HTLV) (2) the lentivirus ("slow virus") group, which includes human immunodeficiency virus (HIV) and certain animal pathogens, e.g., visna virus.

Orthomyxoviruses

* Enveloped, with a helical nucleocapsid. Eight segments of linear, single-stranded, negative- polarity RNA. The term "myxo" refers to the affinity of these viruses for mucins, and "ortho" is added to distinguish them from the paramyxoviruses. Influenza virus is the main human pathogen.

Paramyxoviruses

* These are enveloped viruses with a helical nucleocapsid. * Single-stranded, linear, nonsegmented, negative-polarity RNA. * The important human pathogens are measles, mumps, parainfluenza, and respiratory syncytial viruses.

Rhabdoviruses

*These are bullet-shaped enveloped viruses with a helical nucleocapsid. *Single-stranded, linear, nonsegmented, negative-polarity RNA. *The term "rhabdo" refers to the bullet shape. * Rabies virus is the only important human pathogen.

Filoviruses Marburg virus

* These are enveloped viruses with a helical nucleocapsid. * Single-stranded, linear, nonsegmented, negative- polarity RNA. * They are highly pleomorphic, long filaments that are 80 nm in diameter but can be thousands of nanometers long. * The term "filo" means "thread" and refers to the long filaments. * The two human pathogens are Ebola virus and Marburg virus.

Coronaviruses

-These are enveloped viruses with a helical nucleocapsid -Single-stranded, linear, nonsegmented, positive-polarity RNA. - The term "corona" refers to the prominent halo of spikes protruding from the envelope. -Coronaviruses cause respiratory tract infections, such as the common cold and SARS (severe acute respiratory syndrome), in humans.

Arenaviruses

*These are enveloped viruses with a helical nucleocapsid. *Single-stranded, circular, negative-polarity RNA in two segments. *(A part of both segments is positive-polarity RNA, and the term "ambisense RNA" is used to describe this unusual genome.) * The term "arena" means "sand" and refers to granules on the virion surface that are nonfunctional ribosomes. * Two human pathogens are lymphocytic choriomeningitis virus and Lassa fever virus.

Bunyaviruses Hantaviruse

-These are enveloped viruses with a helical nucleocapsid. - Single-stranded, circular, negative-polarity RNA in three segments. -Some bunyaviruses contain ambisense RNA in their genome. -The term "bunya" refers to the prototype, Bunyamwera virus, which is named for the place in Africa where it was isolated. -These viruses cause encephalitis and various fevers such as Korean hemorrhagic fever. - Hantaviruses, such as Sin Nombre virus, are members of this family.

Deltavirus

Hepatitis delta virus (HDV) is the only member of this genus.- It is an enveloped virus with an RNA genome that is a single- stranded, negative-polarity, covalently closed circle. - The symmetry of the nucleocapsid is uncertain. - It is a defective virus because it cannot replicate unless hepatitis B virus (HBV) is present within the same cell. -- HBV is required because it encodes hepatitis B surface antigen (HBsAg), which serves as the outer protein coat of HDV. - The RNA genome of HDV encodes only one protein, the internal core protein called delta antigen.