Case definition for notification of smallpox under the IHR (2005)

Overview

States Parties to the IHR (2005) are required to immediately notify to WHO of any confirmed case of smallpox.

The case definition for a confirmed smallpox case includes the following:

Confirmed case of smallpox

• An individual of any age presenting with acute onset of fever (≥38.3°C/101°F), malaise, and severe prostration with headache and backache occurring 2 to 4 days before rash onset; and
• Subsequent development of a maculopapular rash starting on the face and forearms, then spreading to the trunk and legs, and evolving within 48 hours to deep-seated, firm/hard and round well-circumscribed vesicles and later pustules, which may become umbilicated or confluent; and
• Lesions that appear in the same stage of development (i.e. all are vesicles or all are pustules) on any given part of the body (e.g. the face or arm); and
• No alternative diagnosis explaining the illness; and
• Laboratory confirmation.

Note

In contrast to the varicella (chickenpox) infection with centripetal and more superficial lesions, the majority of smallpox cases present with a characteristic rash that evolves slowly over days (with each stage lasting 1-2 days) at the same rate and is centrifugal in distribution, i.e. predominantly concentrated on face and extremities with usual involvement of the palms and soles of the feet. More information and illustrative examples to differentiate smallpox from chickenpox can be found here.

The risk of not identifying atypical presentations of smallpox is weighed against the extreme low risk of reintroduction of the disease and the very high risk of obtaining a false-positive laboratory result. In view of this, laboratory tests to confirm smallpox should be limited to individuals that match the above clinical case definition. Should a single, laboratory confirmed case of smallpox ever occur, it would be then considered an outbreak since smallpox no longer exists as a naturally occurring disease.