Expanding Recognition of Rickettsia parkeri Rickettsiosis in Southern Arizona, 2016-2017

Rickettsia parkeri rickettsiosis is an emerging, tick-borne disease in the United States (US), transmitted by the bite of Amblyomma maculatum group ticks. Clinical manifestations include fever, headache, myalgia, maculopapular rash, and a characteristic eschar that forms at the site of the tick bite. Arizona's index case of R. parkeri rickettsiosis was reported in 2014. Seven additional confirmed and probable cases were identified during 2016-2017 through routine investigation of electronic laboratory reports and by self-reporting to public health authorities. Serum samples were evaluated for immunoglobulin G antibodies reactive with antigens of Rickettsia rickettsii (the agent of Rocky Mountain spotted fever [RMSF]) and R. parkeri using indirect immunofluorescence antibody tests. Eschar swab specimens were evaluated using Rickettsia genus-specific and R. parkeri-specific real-time PCR assays. Patients (six male, one female) ranged in age from 29 to 69 years (median of 41 years), and became ill between July 2016 and September 2017. Fever (6/7), myalgia (5/7), and arthralgia (5/7) were most commonly reported and 5/7 patients had a documented eschar. All patients reported a tick bite acquired in southern Arizona within 2-8 days before illness onset. Four patients worked as U.S. Border Patrol agents. Antibodies reactive to R. rickettsii, R. parkeri, or to both antigens were detected in all patients. Seroconversions between acute and convalescent-phase samples were identified for two patients and DNA of R. parkeri was identified in eschar swab samples from two patients. R. parkeri rickettsiosis is endemic to a region of the southwestern United States and presents an occupational risk that could be lessened by prevention messaging to Border Patrol agents. RMSF, a closely related and more severe spotted fever rickettsiosis, is also endemic to Arizona. Public health agencies can assist clinicians in distinguishing these two infections clinically through education and accessing species-specific diagnostic assays that can improve surveillance efforts for both diseases.