|Scientific Name(s)||Variola virus, a member of the orthopoxvirus family|
|Physical Attributes||Large brick-shaped viruses with a double stranded DNA|
|Mode(s) of Transmission||Direct and fairly prolonged (typically more than 3 hours) face-to-face contact with a patient after fever has begun and during the first week of rash, when the virus is released via the respiratory tract|
|Likely BW Form(s)||Fomites, face-to-face and aerosol|
|Vector/Dormant Form||None; virus exists only in 2 laboratory repositories in the U.S. and Russia|
|Incubation Period||~12 days (ranges 7-17 days)|
|Vaccine||Live vaccinia virus, limited avaliability; 4 possible adverse side effects: eczema vaccinatum, progressive vaccinia, generalized vaccinia, postvaccinial encephalitis|
|Treatment||No effective treatment ever developed|
Smallpox is an acute, contagious, and sometimes fatal disease caused by the variola virus (an orthopoxvirus), and marked by fever and a distinctive progressive skin rash.
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. There are four types of variola major smallpox: ordinary (the most frequent type, accounting for 90% or more of cases); modified (mild and occurring in previously vaccinated persons); flat; and hemorrhagic (both rare and very severe). Historically, variola major has an overall fatality rate of about 30%; however, flat and hemorrhagic smallpox usually are fatal. Variola minor is a less common presentation of smallpox, and a much less severe disease, with death rates historically of 1% or less.
Generally, direct and fairly prolonged face-to-face contact is required to spread smallpox from one person to another. Smallpox also can be spread through direct contact with infected bodily fluids or contaminated objects such as bedding or clothing. Rarely, smallpox has been spread by virus carried in the air in enclosed settings such as buildings, buses, and trains. Humans are the only natural hosts of variola. Smallpox is not known to be transmitted by insects or animals.
The incubation period is typically 12 days (range, 7-17 days). The illness begins with a prodrome lasting 2-4 days, with generalized malaise, fever, headache and body aches, and sometimes vomiting. This is followed by a period of about 4 days when small red spots form in the tongue and in the mouth. These spots develop into sores that break open and spread large amounts of the virus into the mouth and throat. At this time, the person becomes most contagious. For the next 5 days, the bumps become pustules-sharply raised, usually round and firm to the touch as if there's a small round object under the skin. Then, the pustules begin to form a crust and then scab. About three weeks after the initial rash, the pustules scabs begin to fall off. Once the scabs have fallen, the person is no longer contagious. Permanent joint deformities and blindness may follow recovery.
The eruption of chickenpox (varicella) is typically centripetal in distribution (worse on trunk than face and extremities) and characterized by crops of lesions in different stages on development. Chickenpox papules are soft and superficial, compared to the firm and deep papules of smallpox. Chickenpox crusts fall off rapidly and usually leave no scar.
Monkeypox cannot be easily distinguished from smallpox clinically. Monkeypox occurs only in forested areas of West and Central Africa as a sporadic, zoonotic infection transmitted to humans from wild squirrels. Person-to-person spread is rare and ceases after 1-2 generations. Mortality is 15%. Other diseases that are sometimes confused with smallpox include typhus, secondary syphilis, and malignant measles. Skin samples (scrapings from papules, vesicular fluid, pus, or scabs) may provide a rapid identification of smallpox by direct electron microscopy, agar gel immunoprecipitation, or immunofluorescence.
There is no specific treatment available for smallpox although some evidence suggests that vaccinia-immune globulin may be of some value in treatment if given early in the course of the illness. Vaccination within 3 days of exposure will completely prevent or significantly modify smallpox in the vast majority of people. Vaccination 4 to 7 days after exposure likely offers some protection from disease or may modify the severity of disease.
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