Poxviruses

morphology of pox viruses, clinical features, diagnosis, treatment and vaccination

1. POXVIRUSES (MEMBERS OF THE FAMILY POXVIRIDAE )
ARE VIRUSES THAT CAN AS A FAMILY, INFECT BOTH
VERTEBRATE AND INVERTEBRATE ANIMALS.
THE SMALLPOX VIRUS REMAINS AS THE MOST
NOTABLE MEMBER OF THE FAMILY.
Poxviridae
Four genera of poxviruses may infect humans:
orthopox, parapox, yatapox,
molluscipox.

2. Taxonomy
•Subfamily Chordopoxvirinae
•Human pathogenic genera
•Genus Orthopoxvirus;
•diseases: cowpox, vaccinia, smallpox ,monkeypox
•Genus Parapoxvirus; type species: Orf virus
•Genus Molluscipoxvirus; type species: Molluscum
contagiosum virus
•Genus Yatapoxvirus; type species: Yaba monkey tumor
virus
•Nonpathogenic genera
•Genus Avipoxvirus; type species: Fowlpox virus
•Genus Capripoxvirus; type species: Sheeppox virus
•Genus Leporipoxvirus; type species: Myxoma virus
•Genus Suipoxvirus; type species: Swinepox virus
•Subfamily Entomopoxvirinae
•Genus Entomopoxvirus A
• Genus Entomopoxvirus B
•Genus Entomopoxvirus C

3. Structure
viral particles (virions) are generally enveloped (external
enveloped virion- EEV), though the intracellular mature
virion (IMV) form of the virus, which contains different
envelope and is also infectious.
generally shaped like a brick or as an oval form similar to a rounded brick.
size is around 200 nm in diameter and 300 nm in length and
carries its genome in a single, linear, double-stranded
segment of DNA.

4. Oval or "brick-shaped" particles 200-400nm long - can be
visualized by the best light microscopes. The external
surface is ridged in parallel rows, sometimes arranged
helically. The particles are extremely complex, containing
many proteins (more than 100) and detailed structure is
not known.
The extracellular forms contain 2 membranes (EEV -
extracellular enveloped virions), intracellular particles only
have an inner membrane (IMV - intracellular mature
virions).
Thin sections in E.M. reveal that the outer
surface is composed of lipid and protein which
surrounds the core, which is biconcave
(dumbbell-shaped), with two "lateral bodies"
(function unknown). The core is composed of a
tightly compressed nucleoprotein.

5. Replication
involves several stages
the virus does is to bind to a receptor on the host cell surface
After binding to the receptor, the virus enters the cell where it
uncoats. Uncoating of the virus is a two step process. Firstly the
outer membrane is removed as the particle enters the cell; secondly
the virus particle (without the outer membrane) is uncoated further
to release the core into the cytoplasm.
The pox viral genes are expressed in two phases. The early genes are expressed
first. These genes encode the non-structural protein, including proteins
necessary for replication of the viral genome, and are expressed before the
genome is replicated. The late genes are expressed after the genome has been
replicated and encode the structural proteins to make the virus particle.
Poxviruses are unique among DNA viruses in that they replicate
in the cytoplasm of the cell rather than in the nucleus. In order to
replicate, poxviruses produce a variety of specialized proteins not
produced by other DNA viruses, the most important of which is a
viral-associated DNA-dependent RNA polymerase.

6. replication is relatively quick taking approximately 12 hours
1. Entry
Intracellular mature virion (IMV) particles bind to unknown receptor(s)
and fuse with the cell membrane. Extracellular enveloped virion (EEV)
particles bind to unknown receptor(s) and are endocytosed into the cell.
2. Initial Uncoating
The viral core particle (CORE) containing the viral genome, the viral DNA-
dependent RNA polymerase, and other enzymes is released into the
cytoplasm.
3. Early Transcription
Early genes (including those coding for immunomodulatory proteins,
enzymes, and replication and transcription factors) are transcribed and
translated immediately upon core particle entry into the cytoplasm of the
cell.
4. Translocation
The viral core particle translocates to the outside of the cell nucleus.
5. Secondary Uncoating
The viral nucleoprotein (NP) complex, which contains the viral genome, is
released. At this point the viral genome is replicated as a concatemer and
transcription and translation of intermediate genes (mainly coding for
transcription factors) occurs.
6. Late Transcription
The viral late genes (coding for structural proteins, enzymes, and
transcription factors) are transcribed and translated.
7. Assembly
Concatemeric intermediates are resolved into linear double-stranded DNA
and packaged with late viral proteins into immature virions (IV).
8. Release
IVs mature into IMVs via an undescribed mechanism which may include
processing of the IV through the Golgi apparatus. The IMVs are transported
to the periphery of the cell where they are released in one of three ways.
IMVs released via cell lysis remain IMVs. Alternatively, IMVs can bud
through to the cell surface, picking up a viral envelope from the cell plasma
membrane. Lastly the IMV can bud through the plasma membrane picking
up an envelope and becoming an EEV.

7. Poviruses are unique among DNA viruses because they
replicate only in the cytoplasm of the host cell, outside
of the nucleus
IMV consists of a single lipoprotein membrane, while the EEV
is surrounded by two membrane layers
The IMV is the most abundant infectious form and is
thought to be responsible for spread between hosts and the
EEV is thought to be important for long range
dissemination within the host organism.

8. HUMAN INFECTION
Cowpox - is acquired by humans usually by milking cows; it then
manifests as ulcerative lesions (sometimes called "milkers nodules") on the
hands of dairy workers. It was noted to protect against smallpox and was
used by Jenner as a vaccine strain to protect persons against smallpox.
Despite its name, rodents are the main reservoir of cowpox; it spreads
secondarily to cows and domestic cats.
Molluscum contagiosum - is
a minor infectious warty papule
of the skin with a central
umbilication, transferred by
direct contact
Molluscum contagiosum
virion

9. Monkey pox - is a rare smallpox like disease of children in
central Africa. It is acquired from monkeys or wild squirrels, but
does occasionally spread from man to man in unvaccinated
communities. Antigenically cross-reacts with other poxviruses.
Sick monkeys have not been identified, but apparently healthy
animals have antibodies.
Pseudocowpox - occurs worldwide and is a disease primarily of
cattle. In humans it causes non-ulcerating "milker's nodes".
ORF - a worldwide occupational disease associated with handling
sheep and goats afflicted with "scabby mouth". In humans it
manifests as a single painless, papulo-vesicular lesion on the hand,
forearm or face.

10. Orf virus (parapox virus)
occurring primarily in sheep and goatsas
contagious pustular dermatitis
(CPD)
humans can contract this disorder through direct contact
with infected sheep and goats or with fomites carrying the
orf virus. It causes a purulent-appearing papule locally and
generally no systemic symptoms. Infected locations can
include the finger, hand, arm, face The papule may persist
for 7 to 10 weeks and spontaneously resolves
Orf pocks on thumb

11. Vaccinia virus
large, complex, enveloped virus, closey related to
the virus that causes cowpox
linear, double-stranded DNA genome
The dimensions of the virion are roughly 360 × 270 × 250 nm.
well-known for its role as a vaccine that eradicated the
smallpox disease(variola)
infection is very mild and is typically asymptomatic in
healthy individuals, but it may cause a mild rash and
fever.
Yaba monkey tumor virus: The type species of
yatapoxvirus, a tumor-producing DNA virus discovered in
monkeys in Yaba, Nigeria. It has been found to produce
histiocytomas (proliferation of tissue macrophages) in
monkeys and humans.

12. Smallpox (Orthopoxvirus infection )
an infectious disease unique to humans, caused by
either of two virus variants, Variola major and Variola
minor.
localizes in small blood vessels of the skin and in
the mouth and throat.
in the skin, this results in a characteristic
maculopapular rash, and later, raised fluid-filled
blisters. V. major produces a more serious disease
and has an overall mortality rate of 30–35%. V. minor
causes a milder form of disease

13. Long-term complications of V. major infection
include characteristic scars, commonly on the
face, which occur in 65–85% of survivors.
Blindness resulting from corneal
ulceration and scarring, and limb
deformities due to arthritis and
osteomyelitis are less common
complications, seen in about 2–5% of
cases.
Both enveloped and unenveloped virions are
infectious.

14. Transmission
through inhalation of airborne variola virus, usually droplets
expressed from the oral, nasal, or pharyngeal mucosa of an infected
person.
through direct contact with infected bodily fluids or
contaminated objects (fomites) such as bedding or clothing.
The virus can cross the placenta, but the incidence of
congenital smallpox is relatively low
Signs and symptoms
There are two clinical forms of smallpox
Variola major is the severe and most common form of
smallpox, with a more extensive rash and higher fever.
Variola minor is a less common presentation of smallpox, and
a much less severe disease, with historical death rates of 1% or
less.
[

15. The incubation period between contraction and the first obvious symptoms of the
disease is around 12 days. Once inhaled, variola virus invades the oropharyngeal
(mouth and throat) or the respiratory mucosa, migrates to regional lymph nodes,
and begins to multiply. In the initial growth phase the virus seems to move from
cell to cell, but around the 12th day, lysis of many infected cells occurs and the virus
is found in the bloodstream in large numbers (this is called viremia), and a second
wave of multiplication occurs in the spleen, bone marrow, and lymph nodes. The
initial or prodromal symptoms are similar to other viral diseases such as influenza
and the common cold: fever (at least 38.5 °C (101 °F)), muscle pain, malaise,
headache, prostration, and as the digestive tract is commonly involved, nausea and
vomiting and backache often occur. The prodrome, or preeruptive stage, usually
lasts 2–4 days. By days 12–15 the first visible lesions—small reddish spots called
enanthem—appear on mucous membranes of the mouth, tongue, palate, and
throat, and temperature falls to near normal. These lesions rapidly enlarge and
rupture, releasing large amounts of virus into the saliva.

16. By the sixth or seventh day, all the skin lesions have become
pustules. Between 7 and 10 days the pustules mature and reach
their maximum size. The pustules are sharply raised, typically
round, tense, and firm to the touch. The pustules are deeply
embedded in the dermis, giving them the feel of a small bead in the
skin. Fluid slowly leaks from the pustules, and by the end of the
second week the pustules deflate, and start to dry up, forming
crusts (or scabs). By day 16-20 scabs have formed over all the
lesions, which have started to flake off, leaving de-pigmented scars
the last natural case of smallpox was diagnosed in 1977

17. Flat small pox
In Flat-type smallpox (also called malignant
smallpox) the lesions remain almost flush with
the skin at the time when raised vesicles form in
ordinary-type smallpox. is nearly always fatal.
accounted for 5%–10% of cases
severe prodromal phase that
lasts 3–4 days, prolonged high
fever
The skin lesions mature very
slowly

18. Hemorrhagic
severe form of smallpox that is
accompanied by extensive bleeding
into the skin, mucous membranes, and
gastrointestinal tract. This form
developed in perhaps 2% of infections
and occurred mostly in adults. In
hemorrhagic smallpox the skin does
not blister, but remains smooth.
Instead, bleeding occurs under the
skin, making the skin look charred and
black, hence this form of the disease is
also known as black pox.
hemorrhaging appears on the second
or third day as sub-conjunctival
bleeding turns the whites of the eyes
deep red. hemorrhages in the spleen,
kidney, serosa, muscle, and, rarely, the
epicardium, liver, testes, ovaries and
bladder.

19. Diagnosis
poxviruses produce characteristic cytoplasmic inclusions, the most important
of which are known as Guarnieri bodies, and are the sites of viral replication.
Guarnieri bodies are readily identified in skin biopsies stained with
hematoxylin and eosin, and appear as pink blobs.
The diagnosis of an orthopoxvirus infection can also be made rapidly
by electron microscopic examination of pustular fluid or scabs.
However, all orthopoxviruses exhibit identical brick-shaped virions
by electron microscopy.
Definitive laboratory identification of variola virus involves growing
the virus on chorioallantoic membrane (part of a chicken embryo)
and examining the resulting pock lesions under defined temperature
conditions.
[

20. Smallpox virus pocks on the
chorioallantoic membrane of a
chick embryo

21. Strains may be characterized by polymerase chain reaction
(PCR) or restriction fragment length polymorphism (RFLP)
analysis
Serologic tests and enzyme linked immunosorbent assays
(ELISA), which measure variola virus-specific
immunoglobulin and antigen have also been developed to
assist in the diagnosis of infection.
[
Complications
most commonly in the respiratory system and range
from simple bronchitis to fatal pneumonia
Pustules can form on the eyelid, conjunctiva, and cornea,
leading to complications such as conjunctivitis, keratitis,
corneal ulcerHemorrhagic smallpox can cause
subconjunctival and retinal hemorrhages.

22. Treatment
Vaccination four to seven days after exposure likely offers
some protection from disease or may modify the severity of
disease
treatment of smallpox is primarily supportive,
such as wound care and infection controlNo drug
is currently approved for the treatment of
smallpox
antiviral drug cidofovir might be useful as a
therapeutic agent. The drug must be
administered intravenously, however, and
may cause serious renal toxicity.
[

23. Smallpox vaccine
The process of vaccination was discovered by
Edward Jenner in 1796, who acted upon his
observation that milkmaids who caught the
cowpox virus did not catch smallpox.
[
Before the introduction of
a vaccineA process called
inoculation, also known as
insufflation or variolation
was practiced in India as
early as 1000 BC
Vaccines that only contain attenuated
vaccinia viruses (an attenuated virus
is one in which the pathogenicity has
been decreased through serial
passage) have been proposed