Smallpox or chickenpox? How to make the diagnosis

Rash progression, location, will be different

Bioterrorism Watch

Smallpox or chickenpox? That clinical question has been long confined to the academic dustbin in the United States, where the last case of smallpox (variola) was diagnosed in 1949 in Texas. Smallpox has been vanquished yet is still feared; chickenpox (varicella) remains a fairly common pediatric infection. Continuing use of the varicella vaccine (recommended for use in the United States in 1996) should continue to reduce cases of chickenpox in the years to come. With bioterrorism a reality and a whole generation of medical students having never seen a case of smallpox, the Centers for Disease Control and Prevention (CDC) is again emphasizing the classic distinctions between the two poxes.

Though similar at onset, the two rash diseases take distinctly different progressions that provide more than a few telltale signs, says Lisa Rotz, MD, medical epidemiologist in the CDC bioterrorism response program. (See chart.) "The incubation period for both diseases spans similar time periods, but we do see a longer incubation period in the development of chickenpox as opposed to smallpox," she says. Usually symptoms such as high fever, malaise, and backache will proceed development of rash in smallpox cases. On the contrary, fever associated with chickenpox generally appears in conjunction with the first signs of rash. "You will also see a different distribution of lesions of the rash between the two diseases," Rotz says. "In general, smallpox lesions are much more numerous on the face and extremities."

Smallpox vs. Chickenpox
Variola Varicella

Incubation  7-17 days  14-21 days
Fever prodrome   2-4 days   minimal/none
Distribution  face/extremities   trunk/clusters
Progression  synchronous  synchronous
Scab formation  10-14 d p* rash  4-7 d p* rash
Scab separation  14-28 d p* rash  <14 d p* rash
Lesions soles/palms   yes  no
* d p = days after rash onset
Source: Centers for Disease Control and Prevention, Atlanta

In contrast, chickenpox lesions are more numerous on the trunk, and occur in clots or clusters. Moreover, as rash progresses in smallpox, the lesions in a particular area of the body progress along the same lines and appear similar. "Whereas in varicella in any one area of the body you may see lesions in different levels of progression," she says. "You might see a vesicle next to a scab. Also the rash of varicella progresses much more quickly and resolves more quickly than the rash of smallpox. So the overall illness has a much shorter course for chickenpox vs. smallpox."

As opposed to chickenpox, smallpox also will reveal itself through lesions on the soles and palms of those infected. Despite the disease names, chickenpox lesions are usually smaller than those created by smallpox. "It is difficult to distinguish early on between the two diseases, but they quickly diverge in their rash progression," Rotz says. "By day five a child with smallpox is showing increasing numbers of lesions still occurring on the face, while the child with chickenpox has about the same number of lesions on the face as appeared on day three. By day seven the rash is still progressing in the patient with smallpox but seems to be resolving in the child with chickenpox."

Though smallpox patient isolation measures are understandably more stringent, the patient isolation guidelines for the two diseases are actually very similar. The CDC recommends contact isolation for both (until scabs are gone) and airborne isolation measures for patients infected with either chickenpox or smallpox. Contact precautions include wearing gloves and a gown to enter the patient’s room; removing gloves and washing hands with an antimicrobial soap prior to leaving room; dedicating noncritical care items to individual patients; and taking extra care to clean the patient environment.

Airborne precautions call for placing the patient in a private room that has monitored negative air pressure in relation to the surrounding areas; six to 12 air changes per hour; and discharge of air outdoors or monitored high-efficiency filtration of room air before the air is circulated to other areas in the hospital. Keep the room door closed and the patient in the room, the CDC advises. Health care workers immune to chickenpox need not wear respiratory protection, but the CDC is calling for workers to wear N95 respirators — typically used for tuberculosis patients — when caring for smallpox patients.