Disclaimer: The opinions and assertions contained herein are the private views of the author, and are not to be construed as official or as reflecting the views of the US Department of the Army or the Department of Defense.
Smallpox is a contagious, sometimes fatal disease manifested most commonly by a severe febrile prodrome, followed by a characteristic vesicular-pustular rash.
Variola is a derivative of the Latin varius, meaning “spotted,” or varus, meaning “pimple.” There is no specific treatment for smallpox, but smallpox vaccination (vaccinia virus) is highly protective, even after exposure.
Smallpox was declared “eradicated” from nature in 1979, but remains a potential bioterrorism agent. The US Centers for Disease Control (US CDC) labels smallpox a Category A agent, a group of 13 highly lethal pathogens. Even a single case of smallpox would be considered an international health emergency.
What you should be alert for in the history
The incubation period of smallpox is typically about 12 days (range: 7 to 17 days), during which time there are no symptoms and the infection is not contagious.
The first stage of disease in smallpox is a severe febrile prodrome, which is a critical feature of the disease. Before skin lesions appear, the prodrome has an abrupt onset, lasting 2 to 4 days, with high fever (39ºC to 41ºC), headache, myalgias and prostration.
As the prodrome evolves, an evanescent enanthem develops on the tongue and palate, characterized by petechiae. On days 4 to 6, as the enanthem ulcerates, an exanthem begins, characterized by a centrifugal distribution, discussed below. The fever may lessen as skin lesions develop.
Relevant points in the history to form a differential diagnosis, discussed in more detail below:
– febrile prodrome (> 39ºC) for 2 to 4 days, followed by skin lesions
chronic skin disease, such as atopic dermatitis
– exposure to ill persons, farm animals, wild animals
– recent first or re-vaccination for smallpox
– varicella vaccination
In the United States, the last case of smallpox was in 1949. Smallpox vaccination of the American general population was stopped in 1972, but continued in military personnel until 1990.
More recently, because of bioterrorism concerns, first time or re-vaccination for smallpox is occurring in certain high-risk groups, such as health care workers and the US military. There are sufficient stockpiles of smallpox vaccine in the event larger numbers of people require vaccination. Smallpox vaccination is protective in more than 95% of people for at least 3 to 5 years. Partial protection lasts longer, which likely reduces disease severity.
Varicella vaccination prevents chickenpox and herpes zoster.
Characteristic findings on physical examination
A clinical diagnosis of smallpox involves the following:
– Prodrome: fever > 39°C, flu-like symptoms for 2 to 4 days
– Skin lesions (Figure 1, Figure 2, Figure 3, Figure 4, Figure 5)
– deep seated, firm vesicles, evolving to pustules
– centrifugal: concentrated on face and distal extremities
– all in same stage of development
– usually on palms and soles (distinguishes smallpox from varicella)
– Fever may continue during the rash, but reduced in comparison with the prodrome
– No other apparent causeOrdinary smallpox, rash evolution