What is Smallpox and How Does the Clinical Presentation Differ From Chickenpox?

Smallpox is one of the most devastating diseases in the history of mankind. Believed to originate more than three millennia ago in India or Egypt, epidemics caused by smallpox have decimated populations across continents.

Thanks to a campaign of universal vaccination the last naturally occurring case of smallpox was in October 1977 in Somalia. Two years later the World Health Organization (WHO) certified it as eradicated. This was sanctioned by the World Health Assembly (WHA) on May 1980.

There were two types of smallpox recognized in the 20th century; variola minor which had a fatality rate of less than 1%, and variola major which had a fatality rate of around 30%.

Initially after infection with the virus, prior to getting the rash, there was a period of sudden onset of high fever, headache, backache and malaise. After 2-4 days, the rash would begin to appear.

The rash progressed through several stages; from raised bumps to fluid- filled, hard and firm vesicles with a depressed center resembling a belly button.

The lesions appear on the face and extremities, including the palms of the hands and the soles of the feet. The lesions are at the same stage of development in a given area.

Rarely, in 5-10% of smallpox patients, variola major would take a fulminant, hemorrhagic course where it was rapidly fatal. In such patients, the lesions are so densely confluent that the skin looks like crepe rubber; some patients’ exhibit bleeding into the skin and intestinal tract.

Smallpox is extremely contagious via respiratory secretions or direct contact with fluids with an incubation period of 12 days average. The illness usually lasts 4 weeks and the person would be contagious until all the lesions scabbed over and fell off in 3-4 weeks leaving pitted scars.

It is exclusively a human disease with no known animal or environmental reservoir hosts.

So how does the clinical presentation of smallpox differ from chickenpox? During the first 2 to 3 days of rash, it may be all but impossible to distinguish between the two. However after that:

Smallpox

Fever: 2 to 4 days before rash
Rash: Pocks at the same stage of development, deep seated
Distribution: More pocks on the extremities
Pocks on the palms and soles: Usually present
Death: 30%

Chickenpox

Fever: At the time of the rash
Rash: Pocks in several stages (crops), more superficial
Distribution: More pocks on trunk
Pocks on the palms and soles: Usually absent
Death: Rare