Rocky Mountain Spotted Fever in a Large Metropolitan Center, Mexico-United States Border, 2009-2019

Abstract

Epidemic levels of Rocky Mountain spotted fe­­­ver (RMSF) have persisted in Mexicali, Mexico, since the initial outbreak was first reported in December 2008. We compared clinical and epidemiologic data of cases in Mexicali during 2009–2019 between patients with an IgG titer reactive with Rickettsia rickettsii bacteria by indirect immunofluorescence antibody (IFA) assay and those who demonstrated DNA of R. rickettsii in a whole blood sample when tested by PCR. We identified 4,290 patients with clinical and epidemiologic features compatible with RMSF; of these, 9.74% tested positive by IFA and 8.41% by PCR. Overall, 140 patients died (11-year case-fatality rate 17.97%). Substantial differences in the frequency of commonly recognized clinical characteristics of RMSF were identified between PCR-positive and IFA-positive cases. The Mexicali epidemic is unique in its size and urban centralization. Cases confirmed by PCR most accurately reflect the clinical profile of RMSF.

Keywords: Mexicali; Mexico; Rhipicephalus sanguineus; Rickettsia rickettsii; Rocky Mountain spotted fever; United States; bacteria; bacterial infections; epidemiology; rickettsial diseases; tickborne diseases; vector-borne infections; zoonoses.

Figure 1

Flowchart used to determine case status of patients in whom Rocky Mountain spotted fever was diagnosed, Mexicali, Mexico, 2009–2019. DGE, Directorate General of Epidemiology; IFA, indirect immunofluorescence antibody assay; RMSF, Rocky Mountain spotted fever.

Figure 2

Incidence and mortality rates of laboratory-confirmed Rocky Mountain spotted fever, Mexicali, Mexico, 2009–2019. Scales for the y-axes differ substantially to underscore patterns but do not permit direct comparisons.

Figure 3

Seasonal distribution of PCR- and IFA-positive cases of Rocky Mountain spotted fever, Mexicali, Mexico, 2009–2019. IFA, indirect immunofluorescence antibody assay.

Figure 4

Geographic distribution of all PCR-positive cases of Rocky Mountain spotted fever in Mexicali (A) and the Mexicali Valley (B), Mexico, 2009–2019. Outlined areas represent census-related Basic Geostatistical areas established by Mexico’s National Institute of Statistics and Geography. Numbers in circles represent the number of cases in each location.