Patient selection: febrile illness in an endemic region
Parameters:
(1) eschar
(2) IgM antibody in an ELISA assay for antibody on an initial specimen
(3) IgM antibody in an ELISA assay for antibody on a subsequent specimen (convalescent)
(4) response to appropriate therapy with doxycycline or other appropriate antibiotic
(5) exclusion of other causes for fever
(6) PCR or other molecular tests
(7) cell culture and immunofluorescence
The presence of an eschar at the site of chigger bite is a simple clinical test that can be valuable for diagnosis. It may be absent in patients but should be sought. Sites to examine include:
(1) axilla
(2) groin
(3) genitalia
(4) chest
(5) abdomen
(6) dorsum of feet
(7) cheek
(8) ear lobe
(9) other tissue folds
Criteria #1 for diagnosis – all of the following:
(1) presence of an eschar
(2) presence of IgM antibody
(3) exclusion of other explanations for fever (although there could be more than one cause)
Criteria #2 in the absence of eschar:
(1) presence of IgM antibody or appearance in subsequent specimen
(2) deferevescence within 48-hours of starting appropriate antibiotic therapy
(3) exclusion of other causes for fever
The use of PCR or other molecular test is very specific but the sensitivity tends to be low.
Cell culture is slow and requires reference laboratory with suitable biosafety precautions. Immunofluorescence for detection of antigen or antibody has been supplanted by the ELISA testing.
