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Rocky Mountain spotted fever (RMSF) is a life-threatening tick-borne disease documented in North, Central, and South America. In California, RMSF is rare; nonetheless, recent fatal cases highlight ecological cycles of the two genera of ticks, Dermacentor and Rhipicephalus, known to transmit the disease. These ticks occur in completely different habitats (sylvatic and peridomestic, respectively) resulting in different exposure risks for humans. This study summarizes the demographic, exposure, and clinical aspects associated with the last 40 years of reported RMSF cases to the California Department of Public Health (CDPH). Seventy-eight RMSF cases with onsets from 1980 to 2019 were reviewed. The incidence of RMSF has risen in the last 20 years from 0.04 cases per million to 0.07 cases per million (a two-fold increase in reports), though the percentage of cases that were confirmed dropped significantly from 72% to 25% of all reported cases. Notably, Hispanic/Latino populations saw the greatest rise in incidence. Cases of RMSF in California result from autochthonous and out-of-state exposures. During the last 20 years, more cases reported exposure in Southern California or Mexico than in the previous 20 years. The driver of these epidemiologic changes is likely the establishment and expansion of Rhipicephalus sanguineus sensu lato ticks in Southern California and on-going outbreaks of RMSF in northern Mexico. Analysis of available electronically reported clinical data from 2011 to 2019 showed that 57% of reported cases presented with serious illness requiring hospitalization with a 7% mortality. The difficulty in recognizing RMSF is due to a non-specific clinical presentation; however, querying patients on the potential of tick exposure in both sylvatic and peridomestic environments may facilitate appropriate testing and treatment. Author summary: Our analysis of the last 40 years of reported Rocky Mountain spotted fever (RMSF) in California included cases that met specific inclusion criteria which highlighted new information. First, there are two distinct epidemiologic patterns which are illustrated with detailed descriptions of two fatal cases. The typical sylvatic cycle involves transmission by Dermacentor spp. ticks in grassland or woodland habitats. A peridomestic cycle exists that involves transmission by the brown dog tick, Rh. sanguineus, often in association with travel to hyper-endemic areas such as certain parts of northern Mexico or resulting from the increasing presence of Rh. sanguineus in Southern California. Second, the typical clinical picture of fever, headache, chills, and rash emphasized the non-specific nature of this life-threatening disease. Inquiries regarding recent tick exposure in both the sylvatic and peridomestic environments can help in recognition of the disease. Lastly, while RMSF remains rare in California, incidence is increasing, notably in the Hispanic/Latino population. Public health messaging about RMSF in California must focus on both sylvatic and peridomestic settings, particularly in Southern California, and should be culturally appropriate. [ABSTRACT FROM AUTHOR]

It says, "We used a strict RMSF case definition to specifically describe RMSF epidemiology in California, and did exclude three confirmed SFRG cases that presented with eschars as the principal clinical presentation since clinically those would be more typical of Pacific Coast Tick fever caused by Rickettsia 364D [40] and excluded 33 probable cases with titers <1:164 that did not increase or had IgM only testing." There is an error in the second sentence of the third paragraph of the Discussion section. [Extracted from the article]