Bubonic plague laboratory findings

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

Laboratory Findings

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.

Images

Animal/Flea Specimens

Likewise, appropriate tissues should be taken from animals for detection of Y. pestis. Lymphoid tissues should be removed for testing of the presence of F1 antigen by FA and by culture. Bone marrow from dessicated animal carcasses may yield positive results when other tissues are not available. In addition, serum and blood specimens may be taken for detection of antibody by agglutination. Fleas should be identified and may be placed in pools for tituration and examination. Titurated flea materials may be inoculated into laboratory mice for isolation of plague bacteria and for examination of mouse tissues by FA for expression of F1. Fleas or flea pool material may be directly examined by FA if the samples are pre-incubated at 37°C for 24 hours to encourage F1 antigen expression. The serum from inoculated laboratory mice may be examined for presence of antibody to F1. For serosurveillance of plague in animal populations, blood may be soaked and dried onto filter paper strips and sent to the laboratory for the detection of F1 antibody. Lastly, as with human specimens, in cases where no cultures or serum specimens are available for testing, both animal and flea material may be tested by polymerase chain reaction (PCR) to determine if plague DNA is present in the specimens.

Urinalysis

A urinalysis can also be useful for the determination of Y. Pestis. Urinalysis may show certain results such as:

• Red blood cell casts
• Proteinuria
• Gross hematuria

There has also been the development of rapid urine dipsticks to test for the Y. Pestis antigen.

• This type of development would be useful in the field if there were an outbreak for rapid identification.