Distinguishing smallpox from other infections: a primer. (Initial Lesions Resemble Chickenpox).

Unless you've got your radar way up, smallpox early in its course is easy to confuse with other things, says Dr. Christopher J. Harrison of the University of Louisville (Ky).

The incubation period for smallpox averages 10-14 days, although it can be as short as a week or as long as 17 days. Early symptoms include abrupt onset of headache and fatigue, sometimes accompanied by intense muscle pain--particularly in the lower back. The high fever occurs 2-4 days from symptom onset; some patients experience nausea, vomiting, and abdominal pain. These symptoms may be indistinguishable from influenza.

This nonspecific "preeruptive phase" lasts about 3-5 days. Unfortunately, the disease can be contagious before external lesions appear, when intraoral lesions may precede skin rash by approximately 24 hours. Although virus is detectable in the preeruptive phase, data suggest that it may not be contagious until some lesions appear in the mouth or on the skin (JAMA 281[22]:2127-37, 1999).

Virus is shed in highest titers and is most contagious in the first week of rash. The high fever may decrease once pox appear on the skin.

Initial lesions can be confused with chickenpox; however, there are some key differences to keep in mind, Dr. Harrison says.

Varicella typically starts and dusters on the scalp within the hairline, neck, and the upper trunk, and then spreads outward to the legs, arms, hands, and feet. Smallpox goes the other way: It starts and is most dense on the face, hands, and legs, and then spreads inward to the trunk.

Smallpox lesions are harder, firmer, and more deeply embedded in the skin than varicella lesions. Both varicella and smallpox lesions evolve through similar stages, starting as flat, red macules, evolving to bumpy papules, then to vesicles, and finally becoming pustules.