Table 1.
| Period | No. (%) | Total no. cases | ||
|---|---|---|---|---|
| PCR-positive | IFA-positive | PCR- and IFA-positive | ||
| 2009–2011 | 73 (20.74) | 275 (78.13) | 4 (1.14) | 352 |
| 2012–2015 | 172 (56.95) | 114 (37.75) | 16 (5.30) | 302 |
| 2016–2019 | 96 (76.80) | 29 (23.20) | 0 (0) | 125 |
| Total | 341 (43.77) | 418 (53.66) | 20 (2.57) | 779 |
Table 1. Frequency of laboratory-confirmed cases of Rocky Mountain spotted fever by assay, Mexicali, Mexico, 2009–2019*
*IFA, indirect immunofluorescence antibody assay.
Table 2.
| Age group, y | No. (%) | |||
|---|---|---|---|---|
| PCR-positive | IFA-positive | PCR- and IFA-positive | Total | |
| ≤15 | 25 (20) | 6 (54.55) | 1 (25) | 32 (22.86) |
| 16–24 | 25 (20) | 0 | 0 | 25 (17.86) |
| 25–44 | 43 (34.40) | 3 (27.27) | 2 (50) | 48 (34.29) |
| 45–64 | 28 (22.40) | 2 (18.18) | 1 (25) | 31 (22.14) |
| ≥65 | 4 (3.20) | 0 | 0 | 4 (2.86) |
| Total | 129 (100) | 11 (100) | 4 (100) | 140 (100) |
Table 2. Age distribution of patients with PCR- and IFA-positive cases of fatal Rocky Mountain spotted fever, Mexicali, Mexico, 2009–2019*
*IFA, indirect immunofluorescence antibody assay.
Table 3.
| Characteristic | Total | PCR-positive | IFA-positive | p value |
|---|---|---|---|---|
| No. patients | 759 | 341 | 418 | |
| Sex | ||||
| F | 433 (57.05) | 170 (49.85) | 263 (62.92) | <0.001 |
| M | 326 (42.95) | 171 (50.15) | 155 (37.08) | |
| Age, y (mean ± SD) | 23.94 (± 17.67) | 24.38 (± 18.89) | 23.59 (± 16.62) | 0.540 |
| Hospitalized | 394 (51.91) | 271 (79.47) | 123 (29.43) | <0.001 |
| Died | 136 (17.92) | 125 (36.66) | 11 (2.63) | <0.001 |
| Signs and symptoms | ||||
| Fever | 759 (100) | 341 (100) | 418 (100) | 0.999 |
| Headache | 656 (86.43) | 288 (84.46) | 368 (88.04) | 0.166 |
| Myalgia | 468 (61.66) | 229 (67.16) | 239 (57.18) | 0.005 |
| Arthralgia | 403 (53.10) | 197 (57.77) | 206 (49.28) | 0.023 |
| Retro orbital pain | 82 (10.80) | 26 (7.04) | 58 (13.88) | 0.003 |
| Rash | 328 (43.27) | 181 (53.24) | 147 (35.17) | <0.001 |
| Pruritis | 139 (18.31) | 56 (16.42) | 83 (19.86) | 0.258 |
| Vomiting | 322 (42.42) | 188 (55.13) | 134 (32.06) | <0.001 |
| Nausea | 366 (48.22) | 206 (60.41) | 160 (38.28) | <0.001 |
| Chills | 274 (36.10) | 127 (37.24) | 147 (35.17) | 0.595 |
| Photophobia | 78 (10.28) | 29 (8.50) | 49 (11.72) | 0.152 |
| Abdominal pain | 345 (45.45) | 191 (56.01) | 154 (36.84) | <0.001 |
| Diarrhea | 188 (24.77) | 112 (32.84) | 76 (18.18) | <0.001 |
| Conjunctivitis | 110 (14.49) | 40 (11.73) | 70 (16.75) | 0.062 |
| Nasal congestion | 109 (14.36) | 34 (9.97) | 75 (17.94) | 0.002 |
| Cough | 189 (24.93) | 72 (21.18) | 117 (27.99) | 0.035 |
| Pharyngitis | 156 (20.58) | 69 (20.23) | 87 (20.86) | 0.857 |
| Rhinitis | 106 (13.97) | 34 (9.97) | 72 (17.22) | 0.004 |
| Hepatomegaly | 68 (8.96) | 44 (12.90) | 24 (5.74) | 0.001 |
| Splenomegaly | 31 (4.08) | 21 (6.16) | 10 (2.39) | 0.010 |
| Adenomegaly | 17 (2.24) | 7 (2.05) | 10 (2.39) | 0.810 |
| Jaundice | 40 (5.27) | 26 (7.62) | 14 (3.35) | 0.013 |
| Hemorrhage | 87 (11.46) | 60 (17.60) | 27 (6.46) | <0.001 |
| Seizures | 32 (4.22) | 30 (8.80) | 2 (0.48) | <0.001 |
Table 3. Demographic and clinical characteristics of patients with PCR- and IFA-positive cases of Rocky Mountain spotted fever, Mexicali, Mexico, 2009–2019*
*Values are no. (%) except as indicated. We excluded from these analyses 20 patients who were positive by both assays.
CME / ABIM MOC
Rocky Mountain Spotted Fever in a Large Metropolitan Center, Mexico–United States Border, 2009–2019
- Authors: Oscar E. Zazueta, MD; Paige A. Armstrong, MD, MHS; Adriana Márquez-Elguea, MD; Néstor Saúl Hernández Milán, MD, PhD; Amy E. Peterson, DVM, PhD; Diego F. Ovalle-Marroquín, MD; Maria Fierro, MD, MPH; Rodolfo Arroyo-Machado, RN; Moises Rodriguez-Lomeli, MD; Guillermo Trejo-Dozal, MD; Christopher D. Paddock, MD, MPHTM
- CME / ABIM MOC Released: 5/12/2021
- THIS ACTIVITY HAS EXPIRED FOR CREDIT
- Valid for credit through: 5/12/2022, 11:59 PM EST
Target Audience and Goal Statement
This activity is intended for primary care physicians, infectious disease specialists, and other physicians who care for patients with possible Rocky Mountain spotted fever (RMSF).
The goal of this activity is to evaluate the laboratory diagnosis, clinical presentation, and prognosis of RMSF.
Upon completion of this activity, participants will:
- Analyze symptoms of RMSF in the current study
- Assess the epidemiology of RMSF in the current study
- Evaluate the results of diagnostic testing for RMSF in the current study
- Distinguish the risk of mortality associated with RMSF in the current study
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Rocky Mountain Spotted Fever in a Large Metropolitan Center, Mexico–United States Border, 2009–2019
CME / ABIM MOC Released: 5/12/2021
Valid for credit through: 5/12/2022, 11:59 PM EST
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Abstract and Introduction
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.
Introduction
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[1]. Occasionally, regional endemic foci of infection are described, which can persist for years, or sometimes decades[2]. During the early 2000s, investigators identified multiple outbreaks of RMSF among several small communities in Arizona in the United States and in Sonora, Mexico[3–6]. 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, the capital city of Baja California, Mexico[7,8]. 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[9,10].
The ongoing epidemic of RMSF in Mexicali resembles past and present outbreaks in Arizona and northern Mexico. Cases of disease occur primarily in impoverished neighborhoods, where the presence of large populations of stray dogs infested with infected brown dog ticks greatly increase the human risk for exposure to the pathogen[10–13]. Efforts to document the scope and magnitude of RMSF in Mexicali have been hampered by limited access to sensitive and specific diagnostic techniques, the relatively nonspecific clinical findings observed during the early stages of illness, and incomplete awareness among many residents and local health care providers of the regional risk and scope of the epidemic[10]. To more accurately characterize the epidemiology of RMSF in Mexicali, we compiled and analyzed data available for all cases with serologic or molecular evidence of infection that were reported to the Secretariat of Health of Baja California (ISESALUD) during 2009–2019.
