Smallpox is a highly
contagious infection caused by the enveloped,
doubled-stranded DNA virus
smallpox, also known as variola virus (Figure
The virus ranges in size from 200 nm
(nanometres) to 400 nm and possesses a genome
that is 200 kilobase
pairs long, making it one of the largest animal
viruses. The pathogen is pleomorphic and
contains a nucleocapsid that is typically dumbbell-shaped.
Membrane proteins are arranged helically, and
are used to attach to host membrane receptors to
initiate entry. The pathogen belongs to the genus Orthopoxvirus
and the family Poxviridae. Smallpox virus
has two main variants, namely, variola
major and variola minor, both of
which infect humans only. Variola major is the causative agent of the deadly
infectious disease, while variola minor
is a much less common variant that produces mild, non-fatal
symptoms. The disease is commonly
spread by close interaction with an infected
person via the inhalation of
virion-contaminated aerosols. During the
infection, substantial amounts of the virus are
also found in the spleen, kidneys, bone marrow,
lymph nodes, and other viscera.
Figure 1. A
transmission electron micrograph of a tissue
section containing variola viruses.
The usual entry of smallpox virus is
through the respiratory tract with infection of
the oropharyngeal (mouth) or respiratory
(trachea and lung) mucosa. Secretions from the
mouth and nose, rather than scab material, are
the most important source of human-to-human
transmission. The initial infection in the
oropharynx or respiratory tract produces neither
symptoms nor local lesions, and patients are not
infectious until an oropharyngeal enanthem
appears at the end of the primary incubation
period. Transmission to others is generally
through coughing out of virions in oropharyngeal
secretions. Patients are most infectious during
the first week of rash. Scab material forms as
the rash dries and usually consists of large
fragments of cellular debris, with virions bound
within a dense, fibrous mesh containing a large
amount of the antiviral substance interferon.
Infectious virus is difficult to release from
scabs except by mechanical grinding.
There are two slightly different virions of
variola that are morphologically distinct,
called the intracellular mature virion (IMV) and
the extracellular enveloped virus (EEV). These
virions are surrounded by different numbers of
membranes, they have different proteins on their
surface, and they bind to different cell
receptors, which causes some confusion on how
exactly the variola virus enters the host cell.
The majority of IMV remain intracellular until
cell lysis, but some become wrapped by
additional membranes during a complex
morphogenesis pathway to form intracellular
enveloped virus (IEV). IEV are transported on
microtubules to the cell surface, where they
fuse with the plasma membrane to expose a virion
on the cell surface. This virion may be retained
on the cell surface as cell-associated enveloped
virus or released as EEV.
The binding proteins and
their functions for variola virus are unknown,
however it has been determined that only the IMV
virion has the proteins needed for fusion to the
cell. In this case, it is deduced that the outer
layer of an EEV is discarded before fusion to
the host cell. The specific entry path of the
virion depends on the virus strain or type; the
virus may enter the cell through either fusion
at the plasma membrane or through the endosomal
pathway. Finally, immediately before entering
the cell the outer membrane of the virion is
shed, which leaves only the dumbbell-shaped core
to enter the host cell.
Once the virus enters into
the cell, a host RNA polymerase is activated.
Along with other accessory enzymes, this
polymerase begins to be translated by the host
cell. This leads to the uncoating of the virus.
The dumbbell-shaped core of the virus is
revealed by the early mRNA. The nucleoproteins
pass through the core wall making it disappear.
The uncoating is a process completed by RNA and
protein synthesis. The DNA within the variola
virion is cut in half by the enzyme helicase.
DNA polymerase then matches the strand with each
unique pair, therefore creating a new strand of
identical DNA for the progeny virion. Once all
the essential proteins are transcribe and
translated, progeny virions are assembled.
Infection of the host cell typically results in
about 10000 copies of the genome per cell.
Symptoms: General symptom of the
infection are lesions on the skin (Figure 2).
Other symptoms include high fever, fatigue,
backache, and other flu-like symptoms. An
individual will remain symptom free for the
first 10 to 12 days. After this period, the virus
spreads throughout the body causing the onset of
the fever and rash. After the first week, the
virus is released through the respiratory tract.
An individual is most contagious when a rash and
fever have developed during the first week, when
the virus is released via the respiratory tract.
In the later stage of the disease when the scabs
start to fall off, the individual is only
moderately contagious since the shed skin is not
highly infectious. However, the virus can remain
stable for 2 to 4 months on scabs and can remain on
clothing for months to years. During the
incubation period, a person cannot infect
This infant was infected with the smallpox
virus, and on day four of the rash, shows the
typical lesions on his hand and forearm.
Smallpox can be identified
through various microscopic and laboratory
diagnosis. Smallpox can also be differentiated
from such viruses such as chickenpox via
comparative morphology of the scabs. For
instance, unlike smallpox, chickenpox lesions
are superficial, and on any one part of the body
there are lesions in different stages (i.e.:
papules, vesicles, and crusts) (Figure 3). Also,
chickenpox rarely has lesions on the palms and
soles, which is not the case with smallpox.
Initially the characteristic lesions first
appear on the head, then on the rear trunk, and
then the extremities.
This is a chickenpox scab (left), and smallpox
scab (right) viewed from above as a
demonstration in comparative morphology.
At the microscopic level,
Poxviruses form cytoplasmic inclusions, such as Guarnieri bodies in virus-infected epidermal
cells. Generally, small particles of insoluble
substances (i.e. lipids or calcium oxalate)
suspended in the cytosol are often referred to
as cytoplasmic inclusions. Microscopically, the
virus appears as pink blobs when stained with
eosin and hematoxylin (Riedel, 2005). In order
to precisely diagnose a smallpox infection,
virus must be grown on chorioallantoic membrane
- a vascular membrane found in bird eggs - and
then have the lesions examined (Figure 4). The
polymerase chain reaction and restriction
fragment length polymorphism analysis are also
used to identify the specific strains, while
ELISA and serologic tests measure the variola
virus-specific immunoglobin and antigen to aid
in the diagnosis of the virus.
Figure 4. A
photograph revealing smallpox virus pocks on the
chorioallantoic membrane of a developing
embryonic chick. Poxviruses are very easy to
isolate, and will grow in a variety of cell
cultures, producing characteristic hemorrhagic
pocks on the chick chorioallantoic membrane.
Smallpox vaccination will prevent or
significantly lessen the severity of smallpox
symptoms in the vast majority of people.
Vaccination four to seven days after the
exposure to smallpox virus will likely offer
some protection from disease or modify the
severity of disease.
Riedel, S. (2005). Smallpox
and biological warfare: a disease revisited.
Baylor University Medical Center Proceedings,