Yersinia pestis infection laboratory tests

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Laboratory Findings

SUSPECTED PLAGUE SHOULD BE CONSIDERED IF THE FOLLOWING CONDITIONS ARE MET:

Clinical symptoms that are compatible with plague, i. e., fever and lymphadenopathy in a person who resides in or recently traveled to a plague-endemic area.

If small gram-negative and/or bipolar-staining coccobacilli are seen on a smear taken from affected tissues, e.g.:

• Bubo (bubonic plague)
• Blood (septicemic plague)
• Tracheal/lung aspirate (pneumonic plague

PRESUMPTIVE PLAGUE SHOULD BE CONSIDERED WHEN ONE OR BOTH OF THE FOLLOWING CONDITIONS ARE MET:

• If immunofluorescence stain of smear or material is positive for the presence of Yersinia pestis F1 antigen.
• If only a single serum specimen is tested and the anti-F1 antigen titer by agglutination is >1:10.*

CONFIRMED PLAGUE IS DIAGNOSED IF ONE OF THE FOLLOWING CONDITIONS IS MET:

• If a culture isolated is lysed by specific bacteriophage.
• If two serum specimens demonstrate a four fold anti-F1 antigen titer difference by agglutination testing.*
• If a single serum specimen tested by agglutination has a titer of >1:128 and the patient has no known previous plague exposure or vaccination history (Agglutination testing must be shown to be specific to Y. pestis F1 antigen by hemagglutination inhibition).

Human specimens

Appropriate specimens should be examined for evidence of plague if a person resides in, or has a recent travel history to, plague-infected areas; has been bitten by fleas; and presents with symptoms suggestive of plague (fever, lymphadenopathy). Specimens should be obtained from appropriate sites for isolating the bacteria. The preferred specimen for microscopic examination and isolation from a bubonic case is material from the affected bubo, which should contain numerous organisms. Blood cultures should be taken whenever possible. Organisms may be seen in blood smears if the patient is septicemic, while blood smears taken from suspected bubonic plague patients are usually negative for bacteria. Bacteria may be intermittently released from affected lymph nodes into the bloodstream; therefore, a series of blood specimens taken 10-30 minutes apart may be productive in the isolation of Y. pestis. Sputum/throat smears taken from pneumonic plague patients may contain too many other organisms to be of diagnostic value if only Wayson stain is used; these smears should be stained as well with the more specific fluorescent-antibody (FA) test. Bronchial/tracheal washing should be taken from suspected pneumonic plague patients; throat specimens are not ideal for isolation of plague since they often contain many other bacteria that can mask the presence of plague. In cases where live organisms are unculturable, e.g., in specimens taken postmortem, lymphoid tissues, lung and bone marrow samples may yield evidence of plague infection by FA test or by detection of Y. pestis DNA.

Specimens intended for culture should be taken before initiation of antibiotic treatment. Specimens are inoculated on general laboratory media and into laboratory mice for isolation; a thin smear is made from the remaining materials for examination by fluorescent microscopy. If a specimen is suspected to contain mixed flora, passage of the material through laboratory mice will increase the likelihood of recovery of a pure Y. pestis culture. Plague bacilli express a unique diagnostic envelope glycoprotein called the Fraction 1 (F1) antigen or capsular antigen at >33°C; this unique envelope antigen is the primary target antigen used for plague diagnostic FA and antibody tests. Plague bacilli are susceptible to lysis by a specific bacteriophage at both 25°C and 37°C. Plague bacilli are relatively inactive by standard enteric biochemical reactions; therefore, identification by biochemical profiles should be used as a supplemental diagnostic test. If a patient has been treated with a static antibiotic (e.g., tetracycline) for more than 4 days, bacterial cultures should be incubated for more than 5 days to give organisms a chance to recover. In case cultures yield negative results, serologic testing is advised. One serum specimen should be taken as early in the illness as possible to be followed by a second sample 1-4 months after antibiotic therapy has ceased.

Resources

CDC CDC Lab test criteria