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Classification of Medically Important
Viruses
Prepared by :-
Mohammed Osama El-Ifranji
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).
Classification of DNA Viruses
Virus
Family
Envelope Capsid
Present
Symmetry
Particle DNA
size
MW
(nm)
(x106)
DNA
Structure
Medically
Important
Viruses
Parvovirus
No
Icosahedral
22
2
SS, linear
B19 virus
Papovavirus
No
Icosahedral
55
5–3
DS, circular,
supercoiled
Papillomavirus
Adenovirus
No
Icosahedral
75
23
DS, linear
Adenovirus
Hepadnavirus Yes
Icosahedral
42
1.5
DS,
incomplete
circular
Hepatitis B
virus
Herpesvirus
Yes
Icosahedral
100
–100
150
DS, linear
Herpes simplex
virus, varicellazoster virus,
cytomegaloviru
s, Epstein-Barr
virus
Poxvirus
Yes
Complex
250x
400
–125
185
DS, linear
Smallpox virus,
vaccinia virus
DNA Viruses
Parvoviruses
 These are very small (22 nm in diameter)
 naked icosahedra viruses with single-stranded
linear DNA.
 There are two types of parvoviruses -:
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
 best illustrated by B19 virus which
associated with :  aplastic crises in sickle cell anemia
patients
 erythema infectiosum ?!!
childhood disease characterized by a
"slapped-cheeks" rash, fever ,headach
and rash in (face,trunk,extremities )
Note / may confused with rubella.
Papovaviruses

These are naked icosahedra viruses (55 nm in
diameter)
 double-stranded circular supercoiled DNA
 "papova" is an acronym of papilloma, polyoma,
and simian vacuolating viruses
 Three human papovaviruses :
1. JC virus
progressive multifocal
leukoencephalopathy
2. BK virus : from the urine of
immunosuppressed kidney transplant patients
3. human papillomavirus .
Note/Polyomavirus (mice ), simian vacuolating
virus 40 (monkeys )
induce
malignant tumors
Adenoviruses
 These are naked icosahedral viruses (75
nm in diameter)
 double-stranded linear DNA.
 There are at least 40 antigenic types
 some of which cause sarcomas in animals
but no tumors in humans.
Note/ They cause pharyngitis, upper and
lower respiratory tract disease
Hepadnaviruses
 These are double-shelled viruses (42 nm
in diameter)
 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 .
e.g./Hepatitis B virus (human)
Herpesviruses
 These are enveloped viruses (100 nm in
diameter)
 icosahedra nucleocapsid and double-stranded
linear DNA.
 causing latent infections ‫؟؟‬
The ability of pathogenic virus to lie dormant within
a cell .
 The five important human pathogens are:
1. herpes simplex virus types 1
2. herpes simplex virus types 2
3. varicella-zoster virus
4. cytomegalovirus
5. Epstein-Barr virus (the cause of infectious
mononucleosis).??!!
infectious mononucleosis characteristic :
Increase mononuclear WBC .
Atypical lymphocyte.
Generalized lymphadenopathy .
Splenomegaly.
Occasional hepatomegaly by hepatitis .
Poxviruses
 These are the largest viruses
 bricklike shape
 an envelope with an unusual appearance
 complex capsid symmetry
 they cause skin lesions, or "pocks
 E.g./ Smallpox virus
vaccinia virus (used in the smallpox
vaccine. )
RNA Viruses
Picornaviruses
 These are the smallest (28 nm in
diameter) RNA viruses
 single-stranded, linear, nonsegmented,
positive-polarity RNA within a naked
icosahedral capsid
 Pico (small), RNA-containing
 There are two groups of human pathogens
1. enteroviruses : poliovirus, coxsackievirus,
echovirus, and hepatitis A virus
2. 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
1. Norwalk virus ?!
2. hepatitis E virus
Note/ Taxonomists have recently placed hepatitis
E virus into its own genus called hepevirus.
Norwalk virus ?!
First isolated in 1968 from the stools of
school children in Norwalk, Ohio who
were suffering from gastroenteritis .
Reoviruses
 These are naked viruses (75 nm in
diameter) with two icosahedral capsid
coats
 They have 10 segments of doublestranded 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
 E.g./rotavirus
diarrhea mainly in
infants
Flaviviruses
 E.g./St. Louis and Japanese encephalitis
viruses
 Return to table .
Togaviruses
 There are two major groups of human
pathogens:
1. The alphavirus group includes eastern
and western encephalitis viruses
2. the rubivirus group consists only of
rubella virus.
The young boy pictured here,
displayed the characteristic
maculopapular rash indicative of
rubella, otherwise known as
German measles, or 3-day
measles
•Rubella is a respiratory viral
infection characterized by mild
respiratory symptoms and lowgrade fever, followed by a
maculopapular rash lasting about
3 days
 Birth defects if acquired by a pregnant woman:
deafness, cataracts, heart defects, mental
retardation, and liver and spleen damage (at
least a 20% chance of damage to the fetus if a
woman is infected early in pregnancy (
 rubella vaccine be given as part of the
MMR vaccine (protecting against measles,
mumps, and rubella). MMR is
recommended at 12-15 months (not
earlier) and a second dose when the child
is 4-6 years old (before kindergarten or 1st
grade).
Retroviruses
 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 (HIV) and certain animal
pathogens, e.g., visna virus.
Note/ A third group, spumaviruses
Orthomyxoviruses
 The term "myxo" refers to the affinity of
these viruses for mucins
 "ortho" is added to distinguish them from
the paramyxoviruses
 Influenza virus is the main human
pathogen
Paramyxoviruses
 The important human pathogens are
measles, mumps, parainfluenza, and
respiratory syncytial viruses.
This child’s face is displaying diffuse lymphedema
of the neck due to
a mumps virus infection of the parotid salivary glands
 Measles is spread through respiration (contact with fluids
from an infected person's nose and mouth, either directly
or through aerosol transmission), and is highly
contagious.
Rhabdoviruses
 These are bullet-shaped enveloped viruses
("rhabdo" refers to the bullet shape )
 Rabies virus is the only important human
pathogen
This transmission electron
micrograph (TEM) revealed the
presence of numerous dark,
bullet-shaped rabies virions
within an infected tissue sample
Filoviruses
 The term "filo" means "thread" and refers
to the long filaments
 They are highly pleomorphic,
 long filaments that are 80 nm in diameter
but can be thousands of nanometers
long
 The two human pathogens are
1. Ebola virus
2. Marburg virus
Coronaviruses
 The term "corona" refers to the prominent
halo of spikes protruding from the
envelope
 Coronaviruses cause respiratory tract
infections in humans :
E.g./common cold
SARS (severe acute respiratory syndrome)
Arenaviruses
 The term "arena" means "sand" and
refers to granules on the virion surface
that are nonfunctional ribosomes
 Two human pathogens are
1. lymphocytic choriomeningitis virus
2. Lassa fever virus
Bunyaviruses
 Some bunyaviruses contain ambisense
RNA in their genome )unusual genome
like Arenaviruses)
 The term "bunya" refers to the prototype,
Bunyamwera virus, which is named for
the place in Africa where it was isolated
 These viruses cause :
1. encephalitis
2. fevers such as Korean hemorrhagic fever
 Hantaviruses, such as Sin Nombre virus
are members of this family
Deltavirus
 It is a defective virus because it cannot
replicate unless hepatitis B virus (HBV) is
present within the same cell.
 The RNA genome of HDV encodes only
one protein, the internal core protein called
delta antigen so
HBV is required because it encodes
hepatitis B surface antigen (HBsAg), which
serves as the outer protein coat of HDV
Note/ Hepatitis delta virus (HDV) is the only
member of this genus
.
Classification of RNA Viruses
Virus Family
Envelope
Present
Capsid
Symmetry
Particle
Size
(nm
RNA MW
)106x(
RNA
Structure
Medically
Important
Viruses
Picornavirus
No
Icosahedral
28
2.5
SS linear,
nonsegme
nted,
positive
polarity
Poliovirus,
rhinovirus,
hepatitis A
virus
Calicivirus
No
Icosahedral
38
2.7
SS linear,
nonsegme
nted,
positive
polarity
Norwalk
virus,
hepatitis E
virus
Reovirus
No
Icosahedral
75
15
DS linear,
10
segments
Rotavirus
Flavivirus
Yes
Icosahedral
45
4
SS linear,
nonsegment
ed, positive
polarity
Yellow fever
virus, dengue
virus, West
Nile virus,
hepatitis C
virus
60
4
SS linear,
nonsegmented,
positive polarity
Rubella
virus
72
SS linear, 2
identical strands
(diploid), positive
polarity
HIV,
human Tcell
leukemia
virus
–80
120
4
SS linear, 8
segments,
negative polarity
Influenza
virus
Helical
150
6
SS linear,
nonsegmented,
negative polarity
Measles
virus,
mumps
virus,
respiratory
syncytial
virus
Yes
Helical
75x
180
4
SS linear,
nonsegmented,
negative polarity
Rabies
virus
Yes
Helical
803
4
SS linear,
nonsegmented,
negative polarity
Ebola virus,
Marburg
virus
Togavirus
Yes
Icosahedral
Retrovirus
Yes
Icosahedral 100
Orthomyxovirus
Yes
Helical
Paramyxovirus
Yes
Rhabdovirus
Filovirus
Coronavirus
Yes
Helical
100
10
SS linear,
nonsegmented,
positive polarity
Coronavirus
Arenavirus
Yes
Helical
130–80
5
SS circular, 2
segments with
cohesive ends,
negative
polarity
Lymphocytic
choriomenin
gitis virus
Bunyavirus Yes
Helical
100
5
SS circular, 3
segments with
cohesive
ends, negative
polarity
California
encephaliti
s virus,
hantavirus
Deltavirus
Uncertain
37
0.5
SS circular,
closed circle,
negative
polarity
Hepatitis
delta virus
Yes