Download Classification of Medically Important Viruses

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, positivepolarity 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, positivepolarity 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 singlestranded, 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, negativepolarity 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, negativepolarity 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.
-Hepatitis delta virus (HDV) is the only member of this genus.
It is an enveloped virus with an RNA genome that is a singlestranded, 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.