Rocky Mountain Spotted Fever Characterization and Comparison to Similar Illnesses in a Highly Endemic Area—Arizona, 2002–2011

2015 
Rocky Mountain spotted fever (RMSF), caused by the tick-borne pathogen Rickettsia rickettsii, was sporadically reported in Arizona prior to confirmation of a fatal case on an American Indian reservation in 2003, linked to an unexpected vector, Rhipicephalus sanguineus (the brown dog tick) [1, 2]. Through 2011, 219 human RMSF cases and 16 fatalities (case fatality rate, 7.3%) were reported from 4 Arizona reservations, and 2 additional reservations reported RMSF exposure in humans and/or dogs during 2012 [3, 4]. Affected tribes reported R. sanguineus infestation and large populations of free-roaming dogs [1, 2]. During the last decade, RMSF outbreaks caused by R. sanguineus have been documented in Mexico and South America [5, 6]. However, R. sanguineus ticks and the R. rickettsii organism found in Arizona are genetically distinct from those in Mexico, and the origin of the Arizona outbreak and reasons for its recent emergence remain unclear [5, 7, 8]. RMSF is easily treated with tetracyclines early in the illness, but other broad-spectrum antibiotics are not effective and doxycycline is the treatment of choice in patients of all ages [9–11]. The nonspecific clinical presentation of RMSF, lack of a sensitive early diagnostic test, and necessity of choosing an antibiotic not typically used for other common illnesses or sepsis make identification and management of cases challenging. Physicians need key information to guide early clinical decisions. Geographic patterns of infection and epidemiologic risk factors are important variables in these decisions. The Arizona RMSF outbreak is unusual because it occurred in association with a previously unrecognized tick vector in the United States, and emerged rapidly in a region where RMSF was not previously recognized. Its recent detection in tribal communities where multiple documented underlying health disparities exist [12, 13] lends importance and urgency to characterizing the epidemiology of this outbreak. The unique combination of host, vector, pathogen, and environmental variables within this outbreak suggest that important differences in the clinical manifestations and RMSF epidemiology may exist compared to the broader US experience [9, 14–17]. This study describes RMSF in this emerging setting to aid in differentiation of this potentially deadly disease from similar illnesses.
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