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POXVIRUSES
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POXVIRUSES
The family includes three viruses of medical
importance:
smallpox virus vaccinia virus
and molluscum contagiosum virus (MCV).
Poxviruses are the largest and most complex
viruses.
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SMALLPOXVIRUS
Disease
Smallpox virus, also called variola virus, is the agent of
smallpox,
It is the only disease that has been eradicated from the face of
the Earth.
Eradication is due to the vaccine. There is concern regarding the use of smallpox virus as agent of
bioterrorism
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Importantproperties
Poxviruses are brick-shaped particles containing
linear double-stranded DNA,
a disk-shaped core within a double membrane, and a lipoprotein envelope.
The Virion contains a DNA-dependent RNA polymerase.
This enzyme is required because the virus replicates in the
cytoplasm and does not have access to the cellular RNA
polymerase, which is located in the nucleus.
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Important properties
Smallpox virus has single, stable serotype, which is the
key to the success of the vaccine.
If the antigenicity varied as it does in influenza virus,
eradication would not have succeeded.
Smallpox virus infect only humans; there is no animalreservoir.
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Summary of Replicative Cycle
Vaccinia virus is nonpathogenic for humans,
is used for studies on poxvirus replication and as a vector
in certain gene therapy experiments.
After penetration of the cell and uncoating, the virion
DNA-dependent RNA polymerase synthesizerearly
mRNA, which is translated into early, nonstructural
proteins, mainly enzymes required for subsequent steps in
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Summary of Replicative Cycle
The viral DNA is replicated in typical semiconservative fashion,
after which late, structural proteins are synthesized that will
form the progeny virions. The virions are assembled and acquire their envelopes by
budding from the cell membrane as they are released from the
cell.
Note that all steps in replication occur in the cytoplasm, which
is unusual for a DNA virus.
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Transmission & Epidemiology
Before the disease was eradicated, smallpox virus was transmitted via
respiratory aerosol or by direct contact with virus either in the skin
lesions or on fomites such as bedding.
Prior to the 1960s, smallpox was widespread throughout large areas
of Africa;Asia and south America, and millions of people were
affected.
In 1967, the world Health Organization embarked on a vaccination
campaign that led to the eradication of smallpox.
The last naturally occurring case was in Somalia in 1977
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Pathogenesis & Immunity
Smallpox begins when the virus infects the upper
respiratory tract and local lymph nodes and than
enters the blood (primary viremia).
Internal organs are infected; then the virus reenters the
blood (secondary viremia) and spreads to the skin.
These events occur during the incubation period, when
the patient is still well.
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Pathogenesis & Immunity
The rash is the result of virus replication in the skin, but
there may be an immune component as well.
Immunity following smallpox disease is lifelong
Immunity following vaccination lasts about 10 years.
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Clinical findings
After an incubation period of 7-14 days, there is a sudden
onset of prodromal symptoms such as fever and malaise.
This is followed by the rash, which is worse on the face
and extremities than on the trunk(ie, it has a centrifugal
distribution).
The rash evolves through stages from macules to papules,
vesicles, pustules, and, finally, crusts in 2-3 weeks.
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Laboratory Diagnosis
In the past when the disease occurred, the
diagnosis was made :
either by growing the virus in cell culture or chick
embryos
or by detecting viral antigens in vesicular fluid byimmunofluorescence test
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Prevention
The disease was eradicated by global use of the vaccine,which contains live,attenuated vaccinia virus.
The success of the vaccine is dependent upon five criticalfactors
1) smallpox virus has a single, stable serotype(2) there is no animal reservoir, and humans are the only
hosts
(3) the antibody response is prompt: therefore, exposed
persons can be protected(4) the disease is easily recognized clinically; therefore,
exposed persons can be immunized promptly
(5) there is no carrier state or subclinical infection
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MOLLUSCUM CONTAGIOSUM VIRUS
Molluscum contagiosum virus (MCV) is a member of the poxvirus
family but is quite distinct from smallpox and vaccinia viruses.
It cause small, pink, papular, wartlike lesion of skin or mucosa
membranes.
The lesions have a characteristic cup-shaped crater with a white core.
Note that these lesions are different from warts, which are caused by
papillomavirus
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MOLLUSCUM CONTAGIOSUM
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MOLLUSCUM CONTAGIOSUM VIRUS
MCV is transmitted by close personal contact, including
sexually.
The disease is quite common in children, and the lesions
can be widespread in patients with reduced cellular
immunity.
In immunocompetent patients, the lesions are self-limited
but may last for months.
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MOLLUSCUM CONTAGIOSUM VIRUS
The diagnosis is typically made clinically
The virus is not isolated in the clinical laboratory, and antibody titers
are not helpful.
Removal of the lesions by curettage or with liquid nitrogen is often
effective.
There is no established antiviral therapy, but cidofovir may be useful
in the treatment of the extensive lesions that occur in
immunocompromised patients.
There is no vaccine.
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