Epidemiology
Only occurs in the western hemisphere and is the most common spotted fever group rickettsiosis. Approximately 1200 cases of spotted fever group rickettsiosis were reported in the US each year between 2020 and 2022. There was a peak of 6248 cases in 2017, with cases slightly lower in 2018 and 2019. The substantial drop in cases between 2020 and 2022 is possibly due to the coronavirus disease 2019 (COVID-19) pandemic and the changes made to the case definition for spotted fever group rickettsiosis in 2020. 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. While the incidence has generally increased over time (with the exception of 2020-2023), 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.[4]
Although cases have been reported from most US states, Alabama, Arkansas, Missouri, North Carolina, and Tennessee accounted for over 50% of spotted fever rickettsiosis cases between 2019 and 2023. RMSF is considered an emerging infection in Arizona, with around 600 cases reported between 2006 and 2023 and a case fatality rate of approximately 7%. Cases have been linked to communities with a large number of free-roaming dogs in Arizona.[4]
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][4] 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 reported deaths.[4]
Cases have also been confirmed in Canada, Mexico, and Central and South America.[5][6]
During 2008-2019, there had been a re-emergence of RMSF in Mexicali, Mexico with at least 4290 people suspected to have RMSF with 779 confirmed cases and an 11-year case-fatality rate of 18%.[7]
Because the infection 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 garden or surrounding property of a home. It should be noted that cases may present in travellers returning from outdoor activities in an endemic region, such as northern Mexico or southwestern US.[1][8] From July 2023 to January 2024, the Centers for Disease Control and Prevention (CDC) and the California Department of Public Health recorded 6 patients with RMSF within 2 weeks following travel to Tecate, Mexico, a municipality on the US border not previously described as a high-risk area for RMSF. Five of the patients were under 18 years of age, all patients were hospitalised, and 3 died.[9]
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