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CLASSIFICATION OF VIRUSES Assigned by: Dr. Ummarah Kanwal

Introduction of virus:

Introduction of virus Edward Jenner first introduced the term “Virus” to microbiology. Later it was first discovered by the Russian microbiologist “ Dmitri Ivanosky” in 1890. Virus is Latin word derived from Venom which means “poison”.

VIRUSES ( Latin, poison):

VIRUSES ( Latin, poison) Are non-cellular Size 20-200nm Always have at least two parts _an outer capsid composed of protein subunits _an inner core of nucleic acid (RNA or DNA) Outside of a living cell, a virus is dormant, but once inside, it takes over the resources of the host cell and begins the production of more virus particles.

Mode of transmission of virus:

Mode of transmission of virus Shedding Droplet contact or respiratory secretions Sexual Transmission Fecal-oral Transmission Vertical Transmission Iatrogenic Transmission Vector borne Transmission


CLASSIFICATION The ICTV (International committee on Taxonomy) of viruses began to devise and implement rules for the naming and classification of viruses in the early 1990’s. Later on viruses were classified by the LHT system approved by PNVC ( Provisional committee on nomenclature of virus). The Linnaean system Baltimore classification of viruses

Basis of classification:

Basis of classification Viruses are classified on the basis of : Morphology Nucleic acid type Taxonomy The type of disease they cause.

Classification based on morphology:

Classification based on morphology Viruses may be classified into several morphological types on the basis of their capsid. The four classes of coat shape include: Helical viruses Polyhedral viruses Enveloped viruses Complex viruses

1- Helical viruses:

1- Helical viruses A virus in which the protein capsid appears in a coiled pattern. They are non-enveloped which are arranged helically around the virus genome. Examples: Tobacco mosaic virus Rabies virus

2-Polyhedral viruses:

2-Polyhedral viruses A virus consisting of nucleic acid surrounded by a hollow protein,multi-sided capsid. The capsid of most polyhedral viruses is in the shape of a regular polyhedron with 20 triangular faces and 12 corners. Examples: Adenoviruses Polio viruses

3-Enveloped virus:

3-Enveloped virus Enveloped viruses are roughly spherical but variable in shape. When helical or polyhedral viruses are enclosed by envelopes, they are called enveloped helical and enveloped polyhedral viruses . Examples: Influenza virus Herpes simplex virus

4-Complex Virus:

4-Complex Virus A virus that has neither helical nor polyhedral forms and is combination of both these forms is formely called complex. Examples: Bacteriophage Poxvirus


CLASSIFICATION ON THE BASIS OF OUTER COVERING On the basis of outer covering viruses are classified as: Enveloped viruses Non-enveloped viruses


ENVELOPED VIRUSES A virus that has an outer wrapping or envelope. This envelope comes from the infected cell, or host, in a process called "budding off." During the budding process, newly formed virus particles become "enveloped" or wrapped in an outer coat that is made from a small piece of the cell's plasma membrane, lipids and carbohydrates. The envelope may play a role in helping a virus survive and infect other cells .

Formation of envelope::

Formation of envelope: Some viruses are released from the host cells by an extrusion process that coats the virus with a layer of host’s cells plasma membrane, that layer becomes the viral envelope. Depending on the virus, envelope may or may not be covered by spikes, which are carbohydrate-protein complexes that projects from the surface of envelope.

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Examples of enveloped viruses: Influenza Respiratory syncytial virus (RSV) Herpes virus HIV

Non enveloped viruses:

Non enveloped viruses Viruses that lack envelopes are termed as non-envelope viruses. These viruses consist only a nucleo capsid. In non-enveloped viruses the capsids are solely responsible for attachment of the virus to cell and to infect cell. The capsid of the non-enveloped virus protects the nucleic acid from nuclease enzymes in biological fluids and promotes the virus attachment to susceptible host cells.

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Examples: Norovirus Rotavirus Human pappillomavirus (HPV)

DNA VIRUS: Dna virus :

DNA VIRUS: Dna virus . A virus that has genetic material and replicates using a DNA-dependent DNA polymerase and the nucleic acid is usually double-stranded.


CLASSIFICATION OF VIRUS ON THE BASIS OF DNA Viruses are classified on the basis of DNA as: 1-Parvoviruses: These are very small(22 nm in diameter),naked icosahedral virus with single stranded linear DNA. Replication occur only in actively dividing cells. Examples: Adeno-associated viruses B19 viruses


2-Polyomaviruses: They are small viruses of about 40nm Non-enveloped and heat stable. Cubic symmetry with 72 capsomeres Have double stranded DNA Produce latent and chronic infections in host Examples: IC viruses BK viruses


3-Papillomaviruses: These are similar to polyomaviruses in many respects, but they have larger genome. These have particle size of about 55nm. These are very host and tissue specific.


4-Adenoviruses These are naked icosahedral viruses (75 nm in diameter) with double stranded linear DNA. They cause phryngitis, upper and lower respiratory tract infections. They have double stranded genome.


5-Hepadnaviruses These are double-shelled viruses (45 nm in diameter) with an icosahedral capsid covered by an envelope. The DNA is circular double stranded They cause acute and chronic hepatitis.


6-Herpesviruses These are enveloped viruses (100 nm in diameter) Have an icosahedral nucleocapsid and double stranded linear DNA. They are noted for causing latent infections.


7-Poxviruses These are the largest viruses, with bricklike shape, an envelope with an unusual appearance and a complex capsid symmetry. they are named for skin lesion. Small pox virus and vaccinia virus are two important members.

RNA virus:

RNA virus An RNA virus is a virus that has RNA (ribonucleic acid) as its genetic material. This nucleic acid is usually single-stranded RNA, but may be double-stranded RNA . Human diseases caused by RNA viruses include SARS, influenza,hepatitis C, West Nile fever and polio.


CLASSIFICATION OF VIRUSES ON THE BASIS OF RNA On the basis of RNA viruses are classified as: 1-PICORNAVIRUSES: These are the smallest (28 nm in diameter) RNA virus. They have single stranded, linear, no segmented positive-polarity RNA within a naked icoshedral capsid. There are two groups of human pathogen (1) enterovirus (2) rhinovirus.


2-Calciviruses These are naked viruses (38 nm in diameter)with an icosahedral capsid have single stranded, linear, non segmented positive-polarity RNA.the two human pathogens are Norwalk virus and hepatitis E virus.


3-Astroviruses Similar to size of picno viruses (28-30 nm) Particles display a distinctive star-shaped outline on their surface. The genome is linear, positive sense, single stranded RNA.


4-Reoviruses These are naked viruses (75 nm in diameter) with icosahedral capsid coats. They have 10 segments of double-stranded linear RNA. The main human pathogens is rotavirus which cause diarrhoea mainly in infants.


5-Togaviruses These are enveloped viruses with an icosahedral capsid and single stranded linear, non segemented,positive polarity RNA. These are enveloped viruses with an icosahedral capsid and single stranded linear, non segemented,positive polarity RNA. There are two major groups of human pathogens: _The alpha viruses _The rubiviruses.

6-Flavi viruses:

6-Flavi viruses These are enveloped viruses (40-60nm) with an icosahedral capsid and single stranded linear, non segemented,positive polarity RNA. The flavivirus include hepatitis C virus, yellow fever virus, dengue virus, west Nile virus and St Louis and Japanese virus. Their genome size vary from 9.5-12.5 kb.


7-Arenaviruses Polymorphic, enveloped viruses ranging in size from 300-500nm. The genome is segmented, circular, single stranded RNA, negative-sense and 10-14 kb total size.


8-Retroviruses These are enveloped viruses with an icosahedral capsid and two identical strands of single stranded linear,non segemented,positive polarity RNA.


9-Orthomyxovirus These virus are enveloped with a helical nucleocpsid and eight segments of linear single stranded negative Polarity RNA. Influenza is the main human pathogen.


10-Paramyxovirus These are enveloped viruses with a helical nucleocapsid and single stranded linear,non segmented, negative-polarity RNA. The main important human pathogens are measles mumps and Para influenza.


11-Rhabdoviruses These are bullet-shaped enveloped viruses with a helical nucleocapsid and a linear, single –stranded negative-polarity RNA. Rabies virus is the only important human pathogen.


12-Bunyaviruses These are enveloped viruses with a helical nucleocapsid and a single stranded circular negative-polarity RNA in three segments. Hanta viruses such as Sin nombre virus are important member of this family.

13-Delta virus:

13-Delta virus Hepatitis delta virus (HDV) is only member of this member of this genus. It is an enveloped virus with n RNA genome that is a single-stranded negative-polarity, covalently closed circle. It is defective virus because it cannot replicate unless hepatitis B virus (HBV) is present within same cell.

14-Corona viruses:

14-Corona viruses These are enveloped viruses with a helical nucleocapsid and a single stranded liner nonsegemented positive polarity RNA. Corona viruses causes respiratory tract infections, such as common cold and SARS (severe acute respiratory syndrome).

15-Filo viruses:

15-Filo viruses These are enveloped viruses with a helical nucleocpsid and single stranded,linear,non segmented negative polarity RNA. They are highly pleomorphic, long filaments that are 80 nm in diameter but can b of thousand nanometre long. Two important human pathogens are Ebol virus and Marburg.

Taxonomical Classification of Virus:

Taxonomical Classification of Virus FAMILY Character Typical members Diseases caused Poxviridae -Brick shaped particle -Largest virus Variola Vaccinia Small pox Labotary virus Herpesviridae -icosahedron capsid enclosed in an envelope Herpes simplex Cold, sores, genital infection Papovaviridae -72 capsomeres in capsid -Non-enveloped Human papilloma virus Warts, association with some cancers Paramyxoviridae Enveloped particles with “spikes” _Measles virus _Para influenza virus _Measles _Respiratory tract infections

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Family CHARACTER Typical members Diseases caused Orthomyxoviridae Enveloped particles with “spikes” Influenza virus Influenza Reoviridae Icosahedron, non enveloped Rotavirus Reovirus Infantile diarrhea, Respiratory tract infections, GE Picornaviridae Non enveloped Hepatitis-A virus Polio virus Infectious hepatitis poliomyelitis Rhabdoviridae Bullet-shaped enveloped particle Rabies virus Rabies Togaviridae Enveloped particle Arbovirus Yellow fever


VIRUSES THAT CAUSE DISEASES IN HUMANS Viral family Important species Size Envelopment Virion shape Diseases caused Parvoviridae Human bocavirus , Parvovirus B19 18-25 nm Non-enveloped Icosahedral Anemia in immunocompromised patients Adenoviridae Adenovirus 70-90 nm Non-enveloped Icosahedral Various respiratory infections

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Family Important species Size Envelopment Virion shape Diseases caused Calciviridae Norwalk virus 35-40nm Non-enveloped Icosahedral Gastro enteritis & human hepatitis Picornaviridae Coxsackie virus , Entero virus, hepatitis A virus , polio virus, rhinovirus 28-30 nm Non-enveloped Icosahedral Entero virus causes polio and rhinovirus causes common cold

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Family Important species Size Envelopment Virion shape Diseases caused Flaviviridae Hepatitis C virus , yellow fever virus , dengue virus, West Nile virus 40-50 nm Enveloped Icosahedral Causes yellow fever and dengue Coronaviridae Severe acute respiratory syndrome virus 160 nm Enveloped Helical Associated with upper respiratory tract infections and common cold

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Family Important species Size Envelopment Virion shape Diseases Caused Rhabdovirus Lyssavirus (Rabies virus) 80-160 nm Enveloped helical, bullet shaped Cause rabies Bunyaviridae Bunya virus, henta virus 90-120 nm Enveloped Helical Hantavirus cause hantavirus pulmonary syndrome

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Family Important species Size Envelopment Virion shape Diseases caused Arenaviridae Guanarito virus , Junín virus , Lassa virus , Sabiá virus, arena virus 110-130 nm Enveloped Helical Cause Lassa fever, lymphocytic choriomeningitis Retroviridae Oncoviruses, Lentiviruses 100-120 nm Oncovirus cause tumors in animals, Lentirus HIV cause AIDS

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Family Important species Size Envelopment Virion shape Diseases caused Filoviridae Ebola virus , Marburg virus, Filo virus 80-14000 nm Enveloped Helical Hepadnaviridae Hepatitis B virus 42 nm Enveloped Icosahedral Cause hepatitis B & liver tumor

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