Rickettsialpox Workup

Updated: Jun 03, 2020
  • Author: Darvin Scott Smith, MD, MSc, DTM&H; Chief Editor: John L Brusch, MD, FACP  more...
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Laboratory Studies

Routine laboratory test results are nonspecific for rickettsialpox, but leukopenia with relative lymphocytosis and mild proteinuria are common. Thrombocytopenia is also a frequently reported finding.

A diagnosis of rickettsialpox is usually confirmed with a combination of clinical, epidemiological, and serological testing. The classical clinical triad of rickettsialpox is a black eschar, papular rash, and fever. [20] In the presence of compatible illness in the context of mite exposure, perform serologic tests for antibodies to the spotted-fever group of rickettsiae. Indirect immunofluorescence assay (IFA) is considered the reference method. Patients with rickettsialpox lack detectable antibodies for the first 7-10 days of illness, so a blood sample for IFA should be collected after this period for an accurate diagnosis.

If possible, send a biopsy specimen for immunohistochemical (IHC) assay, culture, and polymerase chain reaction (PCR). The eschar site is better than secondary papulovesicular lesions for sensitivity of the IHC assay. [21]

Blood smears are not effective since R akari is usually located inside cells. Weil-Felix test findings are negative.

If acute titer results are negative, obtain convalescent sera after 6-8 weeks.


Other Tests

Organism culture, immunohistochemical staining, protein gel electrophoresis, and molecular analysis via PCR may be performed, usually at the reference laboratory level. [22] A newer multiplex real-time PCR of skin biopsy specimens has been shown to yield higher sensitivity in the diagnosis of rickettsialpox. [23]

Giemsa stains of tissue specimens may reveal extremely small coccobacillary intracellular bacteria.


Histologic Findings

Skin biopsies are not routinely obtained to confirm a diagnosis of rickettsialpox. If collected, biopsy samples show epidermal infiltration by mononuclear cells and necrosis of the dermis and epidermis. Inflammation around blood vessels with thrombi and extravasation of red blood cells may also be observed. Vacuolar degeneration of basal cell layers and granulomatous inflammation is consistently present. [24]