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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
  • Valid for credit through: 5/12/2022
Start Activity


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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Faculty

  • Oscar E. Zazueta, MD

    Instituto de Servicios de Salud Pública del Estado de Baja California
    Mexicali, Baja California, México

    Disclosures

    Disclosure: Oscar E. Zazueta, MD, has disclosed no relevant financial relationships.

  • Paige A. Armstrong, MD, MHS

    Rickettsial Zoonoses Branch, Centers for Disease Control and Prevention
    Atlanta, Georgia, United States

    Disclosures

    Disclosure: Paige A. Armstrong, MD, MHS, has disclosed no relevant financial relationships.

  • Adriana Márquez-Elguea, MD

    Instituto de Servicios de Salud Pública del Estado de Baja California
    Mexicali, Baja California, México

    Disclosures

    Disclosure: Adriana Márquez-Elguea, MD, has disclosed no relevant financial relationships.

  • Néstor Saúl Hernández Milán, MD, PhD

    Instituto de Servicios de Salud Pública del Estado de Baja California
    Mexicali, Baja California, México

    Disclosures

    Disclosure: Néstor Saúl Hernández Milán, MD, PhD, has disclosed no relevant financial relationships.

  • Amy E. Peterson, DVM, PhD

    Rickettsial Zoonoses Branch
    Centers for Disease Control and Prevention
    Atlanta, Georgia, United States

    Disclosures

    Disclosure: Amy E. Peterson, DVM, PhD, has disclosed no relevant financial relationships.

  • Diego F. Ovalle-Marroquín, MD

    Instituto de Servicios de Salud Pública del Estado de Baja California
    Mexicali, Baja California, México

    Disclosures

    Disclosure: Diego F. Ovalle-Marroquín, MD, has disclosed no relevant financial relationships.

  • Maria Fierro, MD, MPH

    Imperial Country Public Health Department
    El Centro, California, United States

    Disclosures

    Disclosure: Maria Fierro, MD, MPH, has disclosed no relevant financial relationships.

  • Rodolfo Arroyo-Machado, RN

    Instituto de Servicios de Salud Pública del Estado de Baja California
    Mexicali, Baja California, México

    Disclosures

    Disclosure: Rodolfo Arroyo-Machado, RN, has disclosed no relevant financial relationships.

  • Moises Rodriguez-Lomeli, MD

    Instituto de Servicios de Salud Pública del Estado de Baja California
    Mexicali, Baja California, México

    Disclosures

    Disclosure: Moises Rodriguez-Lomeli, MD, has disclosed no relevant financial relationships.

  • Guillermo Trejo-Dozal, MD

    Instituto de Servicios de Salud Pública del Estado de Baja California
    Mexicali, Baja California, México

    Disclosures

    Disclosure: Guillermo Trejo-Dozal, MD, has disclosed no relevant financial relationships.

  • Christopher D. Paddock, MD, MPHTM

    Rickettsial Zoonoses Branch
    Centers for Disease Control and Prevention
    Atlanta, Georgia, United States

    Disclosures

    Disclosure: Christopher D. Paddock, MD, MPHTM, has disclosed no relevant financial relationships.

CME Author

  • Charles P. Vega, MD

    Health Sciences Clinical Professor of Family Medicine
    University of California, Irvine School of Medicine

    Disclosures

    Disclosure: Charles P. Vega, MD, has disclosed the following relevant financial relationships:
    Served as an advisor or consultant for: GlaxoSmithKline

Editor

  • Terie A. Grant, BS

    Copyeditor
    Emerging Infectious Diseases

    Disclosures

    Disclosure: Terie A. Grant, BS, has disclosed no relevant financial relationships.

CME Reviewer

  • Esther Nyarko, PharmD

    Associate Director, Accreditation and Compliance
    Medscape, LLC

    Disclosures

    Disclosure: Esther Nyarko, PharmD, has disclosed no relevant financial relationships.

Medscape, LLC staff have disclosed that they have no relevant financial relationships.


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    For Physicians

  • Medscape, LLC designates this Journal-based CME activity for a maximum of 1.0 AMA PRA Category 1 Credit(s)™ . Physicians should claim only the credit commensurate with the extent of their participation in the activity.

    Successful completion of this CME activity, which includes participation in the evaluation component, enables the participant to earn up to 1.0 MOC points in the American Board of Internal Medicine's (ABIM) Maintenance of Certification (MOC) program. Participants will earn MOC points equivalent to the amount of CME credits claimed for the activity. It is the CME activity provider's responsibility to submit participant completion information to ACCME for the purpose of granting ABIM MOC credit.

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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, MPHTMFaculty and Disclosures
THIS ACTIVITY HAS EXPIRED

CME / ABIM MOC Released: 5/12/2021

Valid for credit through: 5/12/2022

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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.