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Another Cause of Rickettsial-Spotted Fever in the United States
Abstract & Commentary
Synopsis: A fourth pathogen, Rickettsia parkeri, has been identified as a newly identified cause of rickettsial spotted fever in the United States.
Source: Paddock CD, et al. Rickettsia parkeri: a newly recognized cause of spotted fever rickettsiosis in the United States. Clin Infect Dis. 2004;38:805-811.
A 40-year-old man who lived adjacent to a tidal estuary in southeast Virginia presented with fever, headache, myalgias, arthralgias and multiple lower extremity eschars. His symptoms worsened despite treatment with amoxicillin/clavulanate, and he developed an erythematous maculopapular eruption beginning on his trunk and spreading to his extremities. A change in therapy to cephalexin was made without apparent benefit. After 9 days of illness he was seen at an infectious disease consultation, at which time his rash had extended to his face and involved his palms and soles and now included a few scattered pustules on an erythematous base. An enlarged inguinal lymph node was identified. His WBC was 3300/mm3 and there was mild transaminase elevation. A clinical diagnosis of rickesttsialpox was made and doxycycline was administered with subsequent resolution of illness.
IgG antibody to both Rickettsia ricketsii and Rickettsia akari were detected at a titer of 1:1024 in a serum specimen obtained on the 11th day of illness. DNA extracted from organisms recovered on Vero cells from an eschar biopsy was identified as being that of Rickettsia parkeri.
Comment by Stan Deresinski, MD, FACP
Three rickettsial species of the spotted fever group have been previously known to cause human infection in the United States Rickettsia akari, transmitted by mites, causes rickettsialpox, while Rickettsia felis causes cat flea typhus. Rickettsia ricketsii, transmitted by ticks, causes rocky mountain spotted fever. R. parkeri, first isolated from Gulf Coast ticks in Texas in 1939, can now be added to the list. Paddock and colleagues suggest that the likely vector of R. parkeri is the Gulf Coast tick, which is distributed primarily in coastal regions of southern and southeastern states bordering the Gulf of Mexico and the Atlantic Ocean, with foci also present in Kansas and Oklahoma.
R. parkeri is most closely genotypically related to Old World spotted fever group pathogens associated with eschar production. These include Rickettsi conorii and Rickettsia africae. Eschars are rarely identified in patients with Rocky Mountain spotted fever.
A number of rickettsia not known to cause human disease have been identified in ticks in the United States. It seems likely that 1 or more of these will likely be identified in the future as human pathogens, thus recapitulating the story of R. parkeri, for which more than 6 decades passed between its discovery in ticks and its association with human disease. The antigenic cross reactivity among the rickettsia of the spotted fever group makes the use of serologic testing for identification of the infecting species highly problematic.
As pointed out in an accompanying editorial by Didier Raoult, 8 new rickettsial species or diseases have been described since 1991 (most by Raoult himself). He points out the importance of investigation of cases with the use of cell culture and/or molecular techniques using biopsy specimens, particularly of eschar (see Table below).
Stan Deresinski, MD, FACP, Clinical Professor of Medicine, Stanford; Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Editor of Infectious Disease Alert