Risk Factors for Fatal Outcome from Rocky Mountain Spotted Fever in a Highly Endemic Area-Arizona, 2002-2011

Background: Rocky Mountain spotted fever (RMSF) is a disease that now causes significant morbidity and mortality on several American Indian reservations in Arizona. Although the disease is treatable, reported RMSF case fatality rates from this region are high (7%) compared to the rest of the nation (<1%), suggesting a need to identify clinical points for intervention.

Methods: The first 205 cases from this region were reviewed and fatal RMSF cases were compared to nonfatal cases to determine clinical risk factors for fatal outcome.

Results: Doxycycline was initiated significantly later in fatal cases (median, day 7) than nonfatal cases (median, day 3), although both groups of case patients presented for care early (median, day 2). Multiple factors increased the risk of doxycycline delay and fatal outcome, such as early symptoms of nausea and diarrhea, history of alcoholism or chronic lung disease, and abnormal laboratory results such as elevated liver aminotransferases. Rash, history of tick bite, thrombocytopenia, and hyponatremia were often absent at initial presentation.

Conclusions: Earlier treatment with doxycycline can decrease morbidity and mortality from RMSF in this region. Recognition of risk factors associated with doxycycline delay and fatal outcome, such as early gastrointestinal symptoms and a history of alcoholism or chronic lung disease, may be useful in guiding early treatment decisions. Healthcare providers should have a low threshold for initiating doxycycline whenever treating febrile or potentially septic patients from tribal lands in Arizona, even if an alternative diagnosis seems more likely and classic findings of RMSF are absent.