Causative Agent

• Variola virus, a species of Orthopoxvirus

Incubation Period

• 12-14 days (range 7-17 days)

Infectious Period

• From fever onset (usually 2–4 days before rash) until last scab has separated; about three weeks.

Transmission

• Aerosols/droplets from nasopharyngeal lesions and contact with contaminated articles.

Epidemiology

• Last naturally acquired human case in the world occurred in Somalia in 1977; global eradication was certified two years later.

There are at least 2 strains, variola major and the variola minor.

• Variola major: the more severe form with case fatality rate up to 30-50% in susceptible populations.
• Variola minor: milder form of the disease with more diminutive pox lesions; case fatality rate of 1-2% in susceptible populations.

Clinical Features

• Characteristic rash appears 2-4 days after non-specific, flu-like prodrome (fever and headache).
• Maculopapular rash begins on mucosa of mouth and pharynx, face, hands, forearms and spreads to legs and centrally to trunk; lesions are more predominant on the face and extremities than on the trunk (centrifugal).
• Lesions progress synchronously on any given part of the body from macules to papules to vesicles to pustules and to crusty scabs.

Two rare forms of invariably fatal smallpox have been reported:

• Purpura variolosa or hemorrhagic type smallpox
• Flat type smallpox

Differential diagnosis

• Chickenpox, monkeypox, disseminated herpes zoster.

Clues to distinguish smallpox from chickenpox:

• Smallpox lesions are synchronous in their stage of development.
• Smallpox has many more lesions on the face and extremities than trunk (centrifugal spread)
• Smallpox lesions are more common on palms and soles
• Smallpox lesions are more deeply imbedded in the dermis compared with the superficial lesions of chickenpox