Diagnosing Rickettsia: crimps, complications, and coups.

TO THE EDITOR—The recent publication by Denison et al [1] conducted a study on the detection of the 3 most common spotted fever group rickettsia (SFGR) species endemic to the United States: Ricksettsia rickettsii, Rickettsia parkeri, and Rickettsia akari. When patients present with fever and rash and SFGR is in the diagnosis, serology and skin biopsy are often done. Serology can be negative early, so skin biopsies are performed. Both immunohistochemical stain (IHC) and nested polymerase chain reaction (PCR) remain the standard tests. There are potential problems with IHC in a skin biopsy: (1) The rickettsial antigens are unevenly scattered in tissue so it can be missed, (2) the depth and size of the tissue may affect retrieval of the organisms, (3) the air or fixation method can affect the quality and amount of nucleic acid or PCR amplicons, and (4) the timing of the biopsy in a patient’s presentation and whether antimicrobial therapy were given can impact results [1]. Formalin-fixed, paraffin-embedded (FFPE) specimens are sent for the conventional nested PCR assays to the ompA gene and the 17-kDa antigen. The large segment gene targets take >8 hours to complete and are dependent on sampling and obtaining a sufficient number of organisms. Denison states that speciesspecific identification is difficult with FFPE specimens, so her team developed a multiplex PCR test that takes <3 hours, works on samples with fewer organisms, and can be performed on swabs and other sites as well on patients who have been treated with antibiotics [1]. In their study, there were 11 cases of rickettsialpox, 10 from New York. All tested positive with the multiplex test; none were positive with the nested PCR. In essence, rickettsialpox would not have been recognized given the current testing. In 2006, Paddock et al [2] described the culture and initial characterization of 5 isolates of R. akari from eschar biopsy specimens from New York City patients with rickettsialpox. In this study, fresh cutaneous biopsy specimens were obtained from 7 patients suspected of having rickettsialpox, presenting with eschar-associated febrile illnesses. SFGR were identified by IHC staining in eschar biopsy specimens of all patients; segment of the rickettsial 17-kD antigen gene of