Summary
- Already known: Rocky Mountain spotted fever (RMSF) is a tickborne disease found across the Americas. Northern Mexico has persistently high rates, sustained by brown dog ticks (Rhipicephalus sanguineus sensu lato) and free-roaming dogs.
- This report: from July 2023 to January 2024, six RMSF cases in people exposed in Tecate, Mexico — a border town not previously known as high-risk — were reported to the California Department of Public Health (CDPH); three died.
- Implication: doctors on both sides of the border need to recognize RMSF and treat it promptly.
How the cases were found
In October 2023, CDPH learned that a Karius Test — a commercial test for microbial cell-free DNA in blood — had detected Rickettsia rickettsii in a patient who died of suspected RMSF; CDPH's lab confirmed it by RT-PCR. A week later came a second fatal case, confirmed by CDC on autopsy tissue. More cases were found by asking border-county hospitals and alerting public health officials.
The patients
The six ranged from 17 months to 65 years; all but one were male, and three lived in the U.S. All had been in Tecate within 8 days of falling ill and had contact with dogs there; one reported a tick bite.
| Patient | Age | Course | Outcome |
|---|---|---|---|
| A | 17 months | fever and presumed stomach bug treated in Tecate; rash; seen twice in San Diego before a petechial rash on palms and soles prompted doxycycline on day 10 | recovered after >2 weeks in hospital |
| B | 4.5 years | rash thought to be chickenpox, lethargy, diarrhea, breathing trouble | died of cardiac failure on the way to care, day 5 |
| C | 3 years (B's sibling; a tick removed 2 days before illness) | doxycycline started in the ED on suspicion; brain MRI showed the "starry sky" pattern linked to RMSF | survived |
| D | 65 years | abdominal pain, fever; very low platelets; rash on day 7 prompted doxycycline | died the same day |
| E | 17 years | sore throat, aches, headaches, then sepsis; antibiotics without doxycycline until a Karius Test on day 9 | died on day 16 |
| F | 13 years | cough and fever; a dark scab a week earlier, then a widespread rash; doxycycline on suspicion | recovered after 35 days in hospital and 2 weeks of rehabilitation |
Autopsy tissue from a child who died, showing damaged, inflamed small blood vessels and rickettsial antigens (stained). CDC Infectious Diseases Pathology Branch.
The response
CDPH, CDC and Baja California officials confirmed RMSF was rising in Tecate. Alerts followed: a San Diego County health advisory (November 3, 2023), a national Health Alert Network advisory (December 8, 2023) and a Travel Health Notice (March 12, 2024). In May 2024, Baja California's public health institute and CDC trained about 200 health workers at Tecate's two main hospitals. Since January 2024, no more Tecate cases had been reported in California (one was linked to Mexicali).
Why it matters
- Unusual numbers: California averaged one confirmed RMSF case a year in 2011–2019; six in 6 months is striking. Since the start of the 21st century, very high RMSF rates have emerged on tribal lands in the U.S. Southwest and across northern Mexico, including border cities in Baja California.
- Hard to diagnose: early fever, cough or abdominal pain mimic other illnesses, especially where RMSF is rare. There's no rapid test; molecular tests help — the Karius Test was key for four of these patients — but RMSF can turn deadly fast.
- Treat on suspicion: start doxycycline as soon as RMSF is suspected, without waiting for confirmation.
- Work across the border: home inspections by vector control helped show exposures happened in Mexico, and continued binational surveillance and communication will guide prevention.
Sources
Based on "Severe and Fatal Rocky Mountain Spotted Fever After Exposure in Tecate, Mexico — California, July 2023–January 2024," by Anne M. Kjemtrup and others, of the California Department of Public Health, San Diego, San Bernardino and Imperial County health departments, and CDC, Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report's brain images of two children, credited to San Diego County, are left out.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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