Skin biopsies were taken to elucidate the underlying pathology. Although an arthropod assault was suspected, there also was concern for deep vessel vasculitis because of the presence of considerable petechiae and purpura (Figure 3). Histologically, leukocytoclasia was seen in deep dermal blood vessels. A mild eosinophilic spongiosis with a mixed dermal infiltrate was identified—strengthening our suspicion of an arthropod assault. Bacterial cultures for aerobes and anaerobes using material taken from the right shin showed no growth.
FIGURE 3. Erythematous edematous papules resembling petechial and purpuric lesions on the right shin.
Ten days after the initial biopsies, serum specimens were drawn and swabs of eschar were collected and sent to the Centers for Disease Control and Prevention for further testing. Serum was tested by immunofluorescence assay (IFA) for spotted fever group IgG to detect Rocky Mountain spotted fever and ATBF antibodies; both tests were negative. Swab material from eschar was tested again by IFA for spotted fever group IgG (Rocky Mountain spotted fever) and antibodies to ATBF and with bacterial culture isolation and nucleic acid amplification; the culture and amplification came back positive for R africae.
Because the specialized tests confirmed infection with R africae, the patient was given triamcinolone cream 0.1% to apply twice daily to the pruritic lesions for as long as 4 weeks; an additional 14-day course of oral doxycycline monohydrate (100 mg twice daily) was given. At follow-up, the lesions had fully resolved without evident scarring.
Various diagnostic techniques can detect R africae. Bacterial culture and the polymerase chain reaction are specific and therefore diagnostic. In addition, the diagnosis of rickettsiosis can be made with serology testing, in which disease-specific antibodies are detected by indirect IFA using disease-specific antigens.
Antigens from R rickettsii (the agent of Rocky Mountain spotted fever), R conorii (Mediterranean spotted fever), and R africae (ATBF) are commercially available for making the diagnosis of rickettsiosis. However, antigens from R conorii exhibit cross-reactivity with R africae, which can confound the diagnosis.1,10 Serologic IFA tests have been shown to be less sensitive, especially when performed after antibacterial treatment has started.
In a study, 17 of 65 (26%) ATBF-confirmed patients were seronegative (acute and convalescent-phase sera) against R africae; 14 had received doxycycline during the first week of clinical signs. The current hypothesis is that R africae is highly sensitive to doxycycline and that early exposure to the drug prevented development of detectable titers of reactive antibodies, thus producing a negative serology test.11
Furthermore, it has been shown that seroconversion of IgG and IgM antibodies in R africae–infected sera is delayed compared to what is observed with R conorii–infected sera. Typically, seroconversion of R conorii–infected sera can be detected within 2 weeks; seroconversion in R africae–infected sera can take 4 weeks or longer.11