Epidemiology

Only occurs in the western hemisphere and is the most common spotted fever group rickettsiosis. More than 5000 cases of spotted fever group rickettsiosis were reported in the US in 2019. It is unclear how many of these cases were Rocky Mountain spotted fever (RMSF) as cases are reported under the category of spotted fever rickettsiosis.[4]​ While the incidence has generally increased over time, the case fatality rate has decreased (currently approximately 0.5%). However, this includes data on less severe spotted fevers; clinical reviews suggest 5% to 10% of RMSF cases are fatal.​[5]​​

Although cases have been reported from most US states, Arkansas, Missouri, North Carolina, Tennessee, and Virginia account for over 50% of spotted fever rickettsiosis cases. RMSF is considered an emerging infection in Arizona, with nearly 470 cases reported between 2003 and 2019 and a case fatality rate of approximately 5%. Cases have been linked to communities with a large number of free-roaming dogs in Arizona.[5][6]​​

Most cases are reported from May through to August (when tick bites predominantly occur), although cases can occur at any time of the year. The incidence is higher in men compared to women, and in American Indians compared to other groups.[3]​​[5]​​​​ The highest number of cases is reported in adults over the age of 40 years, although it should be noted that while the incidence is lower in children <10 years of age, this age group represents the highest number of deaths.[5]​​

Cases have also been confirmed in Canada, Mexico, and Central and South America.[7]​​[8]​​

During 2008-2019, there had been a reemergence of RSMF in Mexicali, Mexico with at least 4290 people suspected to have RSMF with 779 confirmed cases and an 11-year case-fatality rate of 18%.[9]

Because RMSF is transmitted by ticks, it is often diagnosed in people who reside (or have recently spent time) in wooded or rural settings. Such tick exposures may occur in rural, suburban, or urban settings, including the yard or surrounding property of a home. It should be noted that cases may present in travelers returning from outdoor activities in an endemic region, such as northern Mexico or southwestern US.[1][10]​​​​[11]​ From July to December 2023, CDC and the California Department of Public Health recorded 5 patients with RSMF within 2 weeks following travel to Tecate, Mexico. Four of the patients were under 18 years of age, all 5 patients were hospitalized, and 3 died.[11]

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