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

Abstract

Background Rocky Mountain spotted fever (RMSF) has emerged as a significant cause of morbidity and mortality since 2002 on tribal lands in Arizona. The explosive nature of this outbreak and the recognition of an unexpected tick vector, Rhipicephalus sanguineus, prompted an investigation to characterize RMSF in this unique setting and compare RMSF cases to similar illnesses. Methods We compared medical records of 205 patients with RMSF and 175 with non-RMSF illnesses that prompted RMSF testing during 2002–2011 from 2 Indian reservations in Arizona. Results RMSF cases in Arizona occurred year-round and peaked later (July–September) than RMSF cases reported from other US regions. Cases were younger (median age, 11 years) and reported fever and rash less frequently, compared to cases from other US regions. Fever was present in 81% of cases but not significantly different from that in patients with non-RMSF illnesses. Classic laboratory abnormalities such as low sodium and platelet counts had small and subtle differences between cases and patients with non-RMSF illnesses. Imaging studies reflected the variability and complexity of the illness but proved unhelpful in clarifying the early diagnosis. Conclusions RMSF epidemiology in this region appears different than RMSF elsewhere in the United States. No specific pattern of signs, symptoms, or laboratory findings occurred with enough frequency to consistently differentiate RMSF from other illnesses. Due to the nonspecific and variable nature of RMSF presentations, clinicians in this region should aggressively treat febrile illnesses and sepsis with doxycycline for suspected RMSF.

Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever and Other Spotted Fever Group Rickettsioses, Ehrlichioses, and Anaplasmosis — United States

“…sanguineus to remain active year-round (26). The majority of human cases of RMSF in Arizona occur during July-October after seasonal monsoon rains; however, cases have been reported every month of the year (25). Similar epidemiologic characteristics and transmission dynamics have been reported in parts of Mexico (27)(28)(29)(30).…”

“…Children aged <10 years represent more than half of reported cases in this region and are theorized to have higher rates of exposure to Rh. sanguineus ticks because of increased interaction with dogs and their habitats (16,25). On Arizona tribal lands, the warm climate and proximity of ticks to domiciles provide a suitable environment for Rh.…”

“…Although the recognition of a tick bite is helpful, unrecognized tick bites are common in patients who are later confirmed to have a tickborne rickettsial disease. A history of a tick bite within 14 days of illness onset is reported in only 55%-60% of RMSF cases (9,12,25) and 68% of ehrlichiosis cases (10). Therefore, absence of a recognized tick bite should never dissuade health care providers from considering tickborne rickettsial disease in the appropriate clinical context.…”

“…Activities that commonly result in contact with potential tick habitats include recreational pursuits (e.g., camping, hiking, fishing, hunting, gardening, and golfing) and occupational activities (e.g., forestry work, farming, landscaping, and military exercises). Although peak tick season is an important consideration, health care providers should remain aware that tickborne rickettsial illnesses have been reported in every month of the year, including winter (3)(4)(5)25,81). Climate differences and seasonal weather patterns can influence the duration and peak of tick season in a given geographic region and a given year.…”

“…Other early symptoms might include nausea or vomiting, abdominal pain, anorexia, and photophobia. A rash typically appears 2-4 days after the onset of fever; however, most patients initially seek health care before appearance of a rash (25,74,98). The classic triad of fever, rash, and reported tick bite is present in only a minority of patients during initial presentation to health care (6,25); therefore, health care providers should not wait for development of this triad before considering a diagnosis of RMSF.…”