Identification of Rickettsialpox and Rocky Mountain Spotted Fever in Philadelphia, 2011-2013

BACKGROUND:  Infection with Rickettsia akari (rickettsialpox) or Rickettsia rickettsii (Rocky Mountain spotted fever [RMSF]) can cause febrile, rash illness.  Despite identical treatment, these infections are transmitted by different vectors, impacting control efforts.  Rickettsialpox, transmitted by the house mouse mite, mostly occurs in urban settings, while RMSF, a tickborne illness, most frequently occurs in southeastern and south central states.   As R. akari and R. rickettsii exhibit serologic cross-reactivity, differentiation of these pathogens in clinical specimens is difficult without supporting clinical and epidemiologic evidence.   To assess for the presence of rickettsialpox and potential misclassification, we reviewed a series of rickettsialpox and RMSF cases reported to the Philadelphia Department of Public Health (PDPH). 

METHODS:  Rickettsialpox cases were defined as 1) fever accompanied by a rash or eschar; 2) lack of tick exposure or recent travel history; and 3) serologic evidence of elevated R. akari  IgG by immunofluorescent antibody assays (IFA).  Laboratory reports of rickettsial diseases are received by PDPH through passive surveillance.  PDPH interviewed case-patients for clinical information as well as recent tick and mouse exposures, outdoor activities, and travel history.  A descriptive analysis of rickettsialpox and RMSF cases from 2011–2013 was conducted.

RESULTS:  Three rickettsialpox cases were identified following commercial laboratory testing for RMSF or rickettsial spotted fever group (SFG).  Subsequent investigation determined that all patients were adults (30–63 years) with recent or ongoing exposure to mice, and no travel history.  All experienced fever and rash, with an eschar noted by two patients.  Serum collected 8–58 days after onset from the 3 cases demonstrated elevated R. akari IgG titers (≥1:128), although two had elevated RMSF IgG titers (≥1:256).  RMSF reports (n = 22) ranged in age from 13 to 70 years.  One patient exhibited a vesicular rash suggestive of R. akari and denied recent travel or outdoor exposure, thus representing possible misclassification of rickettsialpox as RMSF.  All other cases lacked an eschar or vesicular rash, or noted recent RMSF high risk exposures such as outdoor activity or travel. 

CONCLUSIONS:  This case series presents evidence for the presence of rickettsialpox in Philadelphia, as well as the need for epidemiologic and supportive laboratory information to distinguish rickettsialpox from RMSF.  In large US cities, reports with rickettsial SFG or RMSF-specific antibodies should be investigated for exposures to both ticks and rodents.  Increased clinician awareness is needed to support these efforts.  With proper identification of rickettsialpox, appropriate environmental assessments and remediation of rodent infestations can be implemented.