Plague (Yersinia pestis)
2020 Case Definition

Background

The plague bacterium (Yersinia pestis) exists in enzootic cycles of rodents and their fleas in the western United States. People are infected with the plague bacterium through flea bites and direct contact with infected animal tissues or fluids. People are also infected by inhalation of droplets coughed by an infected human or animal. Plague is a febrile illness that typically manifests into one or more clinical syndromes, often reflecting the route of exposure to the bacterium. These clinical syndromes include bubonic, septicemic, and pneumonic plague. Several classes of antibiotics are effective against plague. Plague can be rapidly fatal if appropriate antimicrobial therapy is not initiated early in illness.

Clinical Criteria

An illness characterized by acute onset of fever as reported by the patient or healthcare provider with or without one or more of the following specific clinical manifestations:

• Regional lymphadenitis (bubonic plague)
• Septicemia (septicemic plague)
• Pneumonia (pneumonic plague)
• Pharyngitis with cervical lymphadenitis (pharyngeal plague)

Laboratory Criteria

Confirmatory laboratory evidence:

• Isolation of Y. pestis from a clinical specimen with culture identification validated by a secondary assay (e.g., bacteriophage lysis assay, direct fluorescent antibody assay) as performed by a CDC or Laboratory Response Network (LRN) laboratory,

OR

• Fourfold or greater change in paired serum antibody titer to Y. pestis F1 antigen

Presumptive laboratory evidence*:

• Elevated serum antibody titer(s) to Yersinia pestis fraction 1 (F1) antigen (without documented fourfold or greater change) in a patient with no history of plague vaccination,

OR

• Detection of Yersinia pestis specific DNA or antigens, including F1 antigen, in a clinical specimen by direct fluorescent antibody assay (DFA), immunohistochemical assay (IHC), or polymerase chain reaction (PCR)

*Other laboratory tests, including rapid bedside tests, are in use in some low resourced international settings but are not recommended as laboratory evidence of plague infection in the United States.

Epidemiologic Linkage

• Person that is epidemiologically linked to a person or animals with confirmatory laboratory evidence within the prior two weeks;
• Close contact with a confirmed pneumonic plague case, including but not limited to presence within two meters of a person with active cough due to pneumonic plague; or
• A person that lives in, or has traveled within two weeks of illness onset to a geographically-localized area with confirmed plague epizootic activity in fleas or animals as determined by the relevant local authorities

Criteria to Distinguish a New Case from an Existing Case

Serial or subsequent plague infections in one individual should only be counted if there is a new epidemiologically-compatible exposure and new onset of symptoms.

Suspect

• A clinically-compatible case with epidemiologic linkage without laboratory evidence, OR
• Confirmed or presumptive laboratory evidence without any associated clinical information.

Probable

• A clinically-compatible case with presumptive laboratory evidence without epidemiologic linkage in absence of an alternative diagnosis

Confirmed

• A clinically-compatible case with confirmatory laboratory evidence, OR
• A clinically-compatible case with presumptive laboratory evidence AND epidemiologic linkage.

• Plague (Yersinia pestis) | 1996 Case Definition
• Plague (Yersinia pestis) | 1990 Case Definition