Rocky Mountain Spotted Fever: California and Mexico
By Stan Deresinski, MD, FACP, FIDSA
Clinical Professor of Medicine, Stanford University
SYNOPSIS: The Centers for Disease Control and Prevention alerts clinicians to cases of Rocky Mountain spotted fever in southern California that had been acquired in Mexico.
SOURCE: Centers for Disease Control and Prevention. Emergency Preparedness and Response. Severe and fatal confirmed Rocky Mountain spotted fever among people with recent travel to Tecate, Mexico. Dec. 8, 2023.
Between late July and Dec. 8, 2023, five patients presented to hospitals in southern California with Rocky Mountain spotted fever (RMSF) and were reported to the Centers for Disease Control and Prevention (CDC) and the California Department of Public Health. Investigation revealed that each patient had become ill within two weeks of having traveled to or resided in the border town of Tecate, Mexico. Three of the five patients were residents of the United States, while two were residents of Mexico. Four of the five patients were younger than 18 years of age. All five were hospitalized, and three died.
The cases in this report acquired their infection on the Mexican side of the border with the United States in Tecate in the state of Baja California, only a one-hour drive from San Diego. RMSF is endemic in northern Mexico, including, besides Baja California, the states of Sonora, Chihuaha, Coahuilla, and Nuevo León.
In addition to exposures outside the state, some cases of RMSF in California may be acquired autochthonously.1 While only one to three confirmed cases of RMSF previously have been reported annually in California, the number has been increasing, probably the consequence of increasing numbers of the brown dog tick, Rhipicephalus sanguineus sensu lato, in southern California, together with outbreaks of RMSF in northern Mexico.2
The brown dog tick also is responsible for transmission of Rickettsia ricketsii in Arizona, where cases are associated with communities with large numbers of free-roaming dogs.2 Reported cases of RMSF have increased in Arizona, with more than 500 cases leading to 28 fatalities in 2002-2021.
Since 2010, RMSF cases are reported by the CDC only within the category Spotted Fever Ricketsiosis (SFR) — a consequence of the inability of commonly used serological tests to distinguish between RMSF, Rickettsia parkeri rickettsiosis, Pacific Coast fever, and rickettsialpox.3 Despite recent reports, California and Arizona do not even make it into the top five states (Arkansas, Missouri, North Carolina, Tennessee, and Virginia) reporting cases of SFR.
RMSF often is severe and may rapidly progress, in some cases to death — especially in young children. While it results from the bite of an infected tick, many patients do not recall being bitten, although an eschar may be detected. The initial symptoms may be mild and nonspecific, and a rash generally does not appear until two to four days after their onset. The macular eruption is faint initially but becomes petechial at day 5-6 — and some patients may never develop a rash. These characteristics make maintenance of a high level of suspicion and early intervention critical. As a consequence, the CDC states the following: “Consider initiating doxycycline based on presumptive clinical and epidemiologic findings, and do not delay treatment pending the result of a confirmatory laboratory test. Early treatment with doxycycline saves lives.”
- Kjemtrup AM, Padgett K, Paddock CD, et al. A forty-year review of Rocky Mountain spotted fever cases in California shows clinical and epidemiologic changes. PLoS Negl Trop Dis 2022;16:e0010738. (Erratum in: PLoS Negl Trop Dis 2023;17:e0011030.)
- California Department of Public Health. Spotted fever group rickettsia (Rocky Mountain spotted fever & Pacific Coast tick fever). Last updated Nov. 21, 2023.
- Centers for Disease Control and Prevention. Rocky Mountain Spotted Fever (RMSF). Epidemiology and Statistics. Page last reviewed Aug. 15, 2022. https://www.cdc.gov/rmsf/stats/index.html
The Centers for Disease Control and Prevention alerts clinicians to cases of Rocky Mountain spotted fever in southern California that had been acquired in Mexico.
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