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NOTE: A surveillance case definition is a set of uniform criteria used to define a disease for public health surveillance. Surveillance case definitions enable public health officials to classify and count cases consistently across reporting jurisdictions. Surveillance case definitions are not intended to be used by healthcare providers for making a clinical diagnosis or determining how to meet an individual patient’s health needs.

CSTE Position Statement(s)

  • 09-ID-49

Clinical Description

An illness with acute onset of fever ≥101°F (≥38.3°C) followed by a rash characterized by firm, deep seated vesicles or pustules in the same stage of development without other apparent cause. Clinically consistent cases are those presentations of smallpox that do not meet this classical clinical case definition: a) hemorrhagic type, b) flat type, and c) variola sine eruptione. (Detailed clinical description is available on the CDC web site, see URL:

Laboratory Criteria For Diagnosis

  • Polymerase chain reaction (PCR) identification of variola DNA in a clinical specimen, OR
  • Isolation of smallpox (variola) virus from a clinical specimen (CDC only; confirmed by variola PCR).

Note: Indications for laboratory testing of patients with suspected smallpox should be followed as described in detail on CDC's smallpox website at Diagnostic testing for variola virus is conducted in select LRN laboratories that meet variola testing facility requirements.

Case Classification


A case with a generalized, acute vesicular or pustular rash illness with fever preceding development of rash by 1-4 days.


A case that meets the classical clinical case definition, or a clinically consistent case that does not meet the clinical case definition and has an epidemiological link to a confirmed case of smallpox.


A case of smallpox that is laboratory confirmed, or a case that meets the classical clinical case definition that is epidemiologically linked to a laboratory confirmed case.

Other Criteria

Exclusion Criteria: A case may be excluded as a suspect or probable smallpox case if an alternative diagnosis fully explains the illness or appropriate clinical specimens are negative for laboratory criteria for smallpox.


Note: The smallpox case definition is to be used only during post-event surveillance. The case definition described on the CDC bioterrorism preparedness website (URL: includes different criteria for a suspected case than the smallpox case definition the Council of State and Territorial Epidemiologists approved for use in the National Notifiable Diseases Surveillance System (NNDSS). The smallpox case definition on the CDC bioterrorism web site is more sensitive and less specific than the case definition for the NNDSS, in that a "suspect" case is defined as: "a case with febrile rash illness with fever preceding the development of rash by 1-4 days."

This triage system allows a physician to immediately assess risk [PRE-EVENT], independent of epidemiologic case classification [EVOKED only POST-EVENT]. Patients that triage as ‘high risk’ would, by definition, fall into the first category of Probable smallpox cases (i.e., they will meet the classical clinical case definition.) Immediate –extremely urgent notification would be indicated. In the event that a patient does not meet the classical clinical case definition but has a clinically consistent illness and an epidemiologic link to a confirmed case (those in the 2nd category of Probable smallpox cases), the attending physician is advised to contact the Health Department, thus an immediate- extremely urgent notification would ensue if the Health Department deems it warranted.

Patients who triage [PRE-EVENT] as low to moderate risk would not meet the case definition for Probable smallpox; no notification would take place.

It is important to keep in mind that the triage system is part of the PRE-EVENT surveillance system; the CSTE case definitions apply only POST-EVENT. However, the 2 schemes are intrinsically consistent; patients meeting the Confirmed and Probable case definitions listed above would triage as high risk pre-event and would result in immediate-extremely urgent notification.

The 2004 case definition appearing on this page was re-published in the 2009 CSTE position statement 09-ID-49. Thus, the 2004 and 2010 versions of the case definition are identical.