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Small pox (variola)
The last case of endemic smallpox was reported in 1977 from Somalia.
In 1980 the World Health Organization officially declared that smallpox
had been eliminated worldwide as a result of a global vaccination and
Important features that contributed to the unique success of this vaccine
program included (1) universal interest in eliminating this costly disease
with high morbidity and mortality, (2) the infection's long incubation
period and low level of communicability, (3) the ease of diagnosis of
skin lesions by characteristic histology or antigen detection, (4) the
fact that humans were the sole reservoir of the infection, (5) the absence
of a carrier state, and (6) the availability of an effective live-virus
vaccine that could readily be delivered to less developed countries because
of its resistance to chemicals, temperature changes, and drying. The only
known remaining repositories of smallpox virus reside in two research
laboratories (located in the United States and Russia), and the issue
of whether these last samples should be maintained or destroyed remains
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Before the eradication of smallpox, variola virus existed as two related
strains: variola major (smallpox), with a case-mortality rate of 20 to
50%, and variola minor (alastrim), which caused a clinically milder form
of smallpox with a mortality of <1%. The clinical presentation of smallpox
is now primarily of historic note. However, the threat of biologic terrorism
means that smallpox remains a remote possibility in the differential diagnosis
of a vesicular exanthem. Fever and macular rash appear after an average
incubation period of 12 days, with a progression to typical vesicular
and pustular lesions over 1 to 2 weeks. Rash generally appears first on
the face, oral mucosa, and arms, with relative sparing of the trunk. Smallpox
lesions may be confused with common chickenpox (varicella-zoster) infection
but tend to be more diffuse, peripheral, and uniform in their stage of
development. When the lesions heal they leave a scar.
Polymerase chain reaction promises to be more useful than traditional
electron microscopy or virus isolation for confirming variola or other
The origin of vaccinia virus -- the virus used for vaccination against
smallpox -- is uncertain, but it was probably derived from cowpox virus,
variola virus, or a hybrid of the two. It is now a laboratory virus with
no natural host. Experience has proven the effectiveness of live vaccinia
virus vaccine, although its efficacy and safety were not established in
controlled studies. Percutaneous administration of vaccinia virus vaccine
results in protective cellular and humoral immune responses in >95% of
primary vaccinees. Formation of a pustule and scab at the site of inoculation
is indicative of immunity; because immunity wanes after 10 to 20 years,
revaccination every 10 years is recommended for continued protection.
Routine smallpox vaccination was discontinued in 1971 and has not been
required for international travel since 1982.