Rickettsia Parkeri

Tick-borne rickettsioses are caused by obligate intracellular, arthropod-borne bacteria belonging to the spotted fever group (SFG) of the genus Rickettsia within the family Rickettsiaceae in the order Rickettsiales [Raoult 1997]. Rickettsia parkeri, one of SFG rickettsiae, is named after Entomologist and Rickettsiologist Ralph Robinson Parker, who isolated the distinct rickettsial strain from Gulf Coast ticks (Amblyomma maculatum) collected from cattle in Liberty County, Texas in 1937 [Parker 1939]. But, since then, R. parkeri remained a relatively obscure rickettsia for the next several decades until recently it was found to be one of the pathogenic SFG rickettsiae [Paddock 2004, Paddock 2005, Venzal 2004]. R. parkeri is a small, rod-shaped bacterium, with an average size of 1.6 x 0.5 µm, found in the nucleus and cytoplasm of infected cells [Lackman 1965].

In 2004, Dr. Paddock reported the first case of R. parkeri infection in a 40-year-old man from coastal Virginia who had a febrile disease associated with headache, malaise, diffuse myalgia and arthralgias, maculopapular rash, and multiple eschars on his lower extremities [Paddock 2004]. R. parkeri was confirmed by molecular analyses and cell culture from punch biopsy specimen of the eschar. The patient was supposed to be infected by larval stage A. maculatum ticks as evidenced by absence of an identifiable tick bite by the patient and the occurrence of multiple eschars.   For the patient in our case, there is history of dog tick bite prior to the onset of symptoms and the event also occurred in Virginia beach. This means that R. parkeri could be infected by both larval and adult stage of A. maculatum ticks.

Studies have suggested a broad geographic distribution of A. maculatum ticks across the regions of central and South America although the actual distribution of R. parkeri and the frequency of occurrence in A. maculatum are not well known. R. parkeri has been isolated from Gulf Coast ticks in Alabama,...