Distinguishing Smallpox from Chickenpox

Chickenpox (varicella), which infects millions of children each year in the United States, is the disease most frequently confused with smallpox. There are key differences between the two diseases:
SMALLPOX (Variola) CHICKENPOX (Varicella)
Smallpox incubation

7-17 days

Chickenpox  incubation

14-21 days

Smallpox prodrome

(illness prior to rash)

2-4 days

Chickenpox  prodrome

(illness prior to rash)

minimal/none

Smallpox distribution 1. Lesions initially tend to develop on the face and extremities, progressing to the trunk of the body.

 

2. Rash found on palms and soles.

Chickenpox  distribution 1. Lesions initially tend to develop on the trunk of the body, progressing to the face and extremities. Lesions also tend to be more abundant on trunk than on face and extremities.

2. Rash rarely found on palms and soles.

Smallpox depth of rash

Deeply embedded

Chickenpox  depth of rash

Superficial
Smallpox progression of rash Lesions develop and progress at the same rate. Chickenpox  progression of rash Lesions appear successively and progress at varying rates.
Smallpox scab formation 10-14 days after rash onset Chickenpox  scab formation 4-7 days after rash onset
Smallpox scab separation 14-28 days after rash onset Chickenpox  scab separation <14 days after rash onset
Smallpox communicable period From rash onset until all scabs have separated (3-4 weeks after onset of rash). Most infectious during the first week of rash, after prodrome. Chickenpox  communicable period As long as 5 days (but usually 1-2 days) before rash onset until all lesions are crusted (usually) about 5 days after rash onset). Most infectious 1-2 days before rash onset and for first few days of rash.

For further information regarding smallpox:
contact Orange County Public Health/Epidemiology: (714) 834-8180

or go to:

JAMA consensus article: Smallpox as a Biological Weapon:
http://jama.ama-assn.org/cgi/reprint/281/22/2127.pdf

Information adapted from: Los Angeles County Department of Health Services, Acute Communicable Disease Control