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Tularemia: Differential Diagnoses & Workup
Updated: Aug 9, 2007
- Overview
- Differential Diagnoses & Workup
- Treatment & Medication
- Follow-up
- Multimedia
Differential Diagnoses
Other Problems to Be Considered
Psittacosis
Anthrax
Disseminated mycobacterial disease
Disseminated fungal disease
Lymphogranuloma venereum
Colorado tick fever
Rat-bite fever
Nontuberculous mycobacterial infections
Workup
Laboratory Studies
- Standard laboratory results are nonspecific.
- Serum transaminase levels are mildly elevated in about one half of patients.
- Urinalysis may show sterile pyuria in as many as one fourth of patients.
- The CBC may show an elevated WBC count in about one half of patients.
- Mild thrombocytopenia may be present.
- Hyponatremia is occasionally present.
- Elevation of creatine kinase values may be observed and is associated with rhabdomyolysis.
- CSF may show a mild elevation of protein concentration or pleocytosis.
- Although the organism has been cultured from sputum, pleural fluid, wounds, blood, lymph node biopsy samples, and gastric washings, the yield is extremely low and culture poses a danger to laboratory personnel. The plates must be sealed and handled by using a biosafety level-2 (BSL-2) facility, with further testing at BSL-3 facility after a presumptive identification of F tularensis.
- Other potentially effective media include chocolate, buffered charcoal yeast extract, and Thayer Martin agar.
- Specimens should be maintained for at least 10 days because the slow growth of the culture may require 48-72 hours to be identified.
- Blood cultures have poor sensitivity, which is probably due to the specific medium (cysteine-glucose-blood agar) needed to culture this organism.
- Serologic cross-reactivity is reported with Brucella species, Proteus Ox-19, and Yersinia species. Studies reveal the following:
- The diagnosis of tularemia is most often made with serologic testing. Antibodies may be measured by means of agglutination and enzyme-linked immunosorbent assay (ELISA).
- An agglutination titer greater than 1:160 is considered presumptively positive, and treatment may be started if this result is obtained.
- A second titer demonstrating a 4-fold increase in 2 weeks confirms the diagnosis. Note that although titers begin to rise within 7-10 days after exposure, early titers in the first 2 weeks of illness may be negative in the setting of infection. Detectable titers are identified in the second week of infection in more than 50% of cases.
- Titers achieve maximum levels between 4-8 weeks and may remain elevated for years after infection, causing an uncertainty in individuals with a remote history of tularemia exposure.
Imaging Studies
- Chest radiography
- Tularemia pneumonia may be present without respiratory symptoms.
- Chest radiography is indicated in any patient in whom the diagnosis of tularemia is suspected.
- Common findings in tularemia pneumonia include bilateral patchy infiltrates or lobar infiltrates (74%), cavitary lesions, which may be better visualized on chest CT, hilar lymphadenopathy (32%), or a pleural effusion (30%).
- The triad of oval opacities, hilar lymphadenopathy, and pleural effusion is more likely with tularemia than with other tick-borne diseases.
Other Tests
- Indirect fluorescent antibody test of suppurative material is rapid and specific.
- Microscopic examination of tissue and smear specimens is possible using fluorescent-labeled antibodies at reference laboratories, possibly providing rapid confirmation of disease.
- Polymerase chain reaction (PCR) has been used to detect F tularensis, even after initiation of antibiotic therapy. However, it is not yet available in most laboratories. PCR may provide rapid and specific confirmation, possibly including the disease phase.
- Capture ELISA is an advancement based on monoclonal antibodies specific for lipopolysaccharide of the virulent forms of F tularensis. In animal studies, capture ELISA was more sensitive and specific than routine ELISA and in fact rivals PCR without the time and expense associated with PCR.
- Other techniques available include antigen detection assays, immunoblotting, and pulsed-field gel electrophoresis. Unfortunately, availability of these techniques is largely limited to research or reference laboratories.
Histologic Findings
Caseating granulomas may be found on histologic examination of lymph nodes.
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 Differential Diagnoses & Workup: Tularemia |
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References
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Further Reading
Keywords
tularemia, Francisella tularensis, rabbit fever, rabbit skinners' disease, Amblyomma americanum, A americanum, Dermacentor andersoni, D andersoni, Dermacentor variabilis, D variabilis, Chrysops discalis, C discalis
