Zazueta OE, Armstrong PA, Marquez-Elguea A, et al. Emerg Infect. Dis 2021;27:1567–1576.
Rocky Mountain spotted fever (RMSF), a severe and potentially deadly tickborne disease caused by Rickettsia rickettsii bacteria, occurs throughout the Americas. The classic epidemiology of RMSF is characterized by isolated and sporadic cases of disease that occur predominantly in rural or suburban settings. Occasionally, regional endemic foci of infection are described, which can persist for years, or sometimes decades. During the early 2000s, investigators identified multiple outbreaks of RMSF among several small communities in Arizona in the United States and in Sonora, Mexico. A feature common to each of these outbreaks has been the presence of large populations of stray and free-ranging dogs heavily infested with ticks. In these settings, canine populations can sustain and perpetuate massive numbers of brown dog ticks (Rhipicephalus sanguineus sensu lato), which serve as efficient vectors of R. rickettsii bacteria.
In December 2008, cases of RMSF were first recognized among residents of a neighborhood in Mexicali (population ≈700 000), the capital city of Baja California, Mexico, which is located at the Mexico-United States border, adjacent to the California town of Calexico. During the next few years, cases were identified in adjacent and distant neighborhoods. In contrast to almost all previously described outbreaks of RMSF, this epidemic emerged within a large metropolitan center, continues in the present day, and has affected hundreds of persons throughout the city. Cases of RMSF are now also reported beyond the city limits from several small communities in the Mexicali Valley (population ≈250 000). To more accurately characterize the epidemiology of RMSF in Mexicali, data available for all cases with serologic or molecular evidence of infection reported to the Secretariat of Health of Baja California (ISESALUD) during 2009–2019 were compiled and analyzed.
During 2009–2019, a total of 4290 persons in metropolitan Mexicali and the Mexicali Valley had clinical and epidemiologic features compatible with RMSF. Of the total probable cases identified by the surveillance case definition, 779 (18.5%) met the Directorate General of Epidemiology criteria for a confirmed case, 418 (53.66%) with a positive test result by immunofluorescence antibody and 361 (46.34%) by polymerase chain reaction. Among patients with laboratory-confirmed cases, the predominant signs and symptoms were fever (100%), headache (86.43%), myalgia (61.66%), arthralgia (53.1%), nausea (48.22%), abdominal pain (45.45%) and rash (43.27%).
Among all confirmed case-patients, the mean age was 23.89 years, which did not differ significantly between those cases confirmed by immunofluorescence antibody or polymerase chain reaction. Most patients (56.48%) were female. Cases occurred during each month of the year but were more frequent during the summer months. Overall, 140 patients died (11-year case-fatality rate 17.97%). Approximately, one-quarter of deaths occurred among children 15 years old and younger.
Comment: The Mexicali epidemic is unique from all previously described outbreaks of RMSF in terms of its magnitude, urban concentration and widespread persistence. The timing and origin of the introductory event that precipitated this multiyear outbreak remains unknown. However, the circumstances that propelled its expansion and eventual perpetuation across the city, including high-density, low-income neighborhoods with large numbers of free-roaming and stray dogs and abundant brown dog tick populations, exist within many other metropolitan areas across Mexico and Latin America.
In this context, similar urban outbreaks could plausibly originate elsewhere after local introduction of R. rickettsii. As noted in this report, during 2009–2019, surveillance activities by ISESALUD identified 779 patients with laboratory-supported diagnoses of RMSF in Mexicali and the Mexicali Valley. By comparison, the largest modern outbreak of RMSF in the United States, involving 466 confirmed and probable cases during 2003–2019, has affected predominantly rural tribal communities in Arizona (https://www.azdhs.gov/preparedness/epidemiology-disease-control/index.php#data-stats-past-years).
Urban foci of RMSF are described only rarely and sporadically in the United States and other countries of Latin America and are characteristically limited in size and duration. The longevity, remarkably high prevalence, and multifocal distribution of RMSF in a large metropolitan center poses unprecedented public health challenges.