Infectious Disease · PANCE / PANRE

Rocky Mountain Spotted Fever (RMSF)

Written & clinically reviewed by Ryan Busch, PA-C · Updated August 2026 · Report an error

Tick-borne rickettsial vasculitis with fever, headache, and centripetal petechial rash — empiric doxycycline at first suspicion saves lives.

Also known as: RMSF, Rickettsia rickettsii, spotted fever rickettsiosis

Overview

Acute systemic illness caused by Rickettsia rickettsii, an obligate intracellular gram-negative coccobacillus, transmitted by Dermacentor variabilis (American dog tick, eastern US), D. andersoni (Rocky Mountain wood tick, western US), and Rhipicephalus sanguineus (brown dog tick, Arizona).

Epidemiology

Despite the name, most US cases occur in the South Atlantic and Central states (North Carolina, Tennessee, Oklahoma, Arkansas, Missouri). Peak May-September. Case-fatality up to 20-25% untreated, <1% with early doxycycline. Children and immunocompromised at highest risk.

Try two board-style Rocky Mountain Spotted Fever questions

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Question 1Infectious DiseaseMedium
A 9-year-old boy from North Carolina presents in late June with 5 days of high fever, severe headache, and myalgias. A blanching macular rash began on his wrists and ankles and has spread centrally to involve his palms and soles. His mother removed an engorged tick from his scalp about 7 days ago. Platelets are 78,000/microL, sodium is 128 mEq/L, and AST is 140 U/L. If effective treatment is delayed, which of the following is the most likely serious complication?
  • AComplete atrioventricular block
  • BVasculitis with multiorgan failure
  • CBilateral facial nerve palsy
  • DChronic monoarticular arthritis
Reveal answer & full explanation
Correct answer: B — Vasculitis with multiorgan failure
  • AComplete atrioventricular block
  • BVasculitis with multiorgan failure✓
  • CBilateral facial nerve palsy
  • DChronic monoarticular arthritis

Why vasculitis with multiorgan failure is correct

  • A rash beginning on the wrists and ankles and spreading to the palms and soles, a Dermacentor tick exposure in an endemic state, and the triad of thrombocytopenia, hyponatremia, and transaminitis identify Rocky Mountain spotted fever (Rickettsia rickettsii)
  • R. rickettsii invades vascular endothelium and produces a systemic small-vessel vasculitis
  • Without prompt doxycycline (the treatment of choice at any age), the vasculitis progresses to noncardiogenic pulmonary edema, encephalitis, acute kidney injury, DIC, and shock — multiorgan failure with a case fatality approaching 20–25%

Why the others are wrong

  • Complete atrioventricular block — high-grade AV block is a complication of Lyme carditis (Borrelia burgdorferi), not rickettsial disease (confused-with: another tick-borne illness)
  • Bilateral facial nerve palsy — cranial neuropathy is a feature of early disseminated Lyme disease, not RMSF (confused-with Lyme)
  • Chronic monoarticular arthritis — persistent large-joint arthritis characterizes late Lyme disease, not RMSF (confused-with Lyme)
Question 2Infectious DiseaseMedium
A 33-year-old hiker in North Carolina develops fever, headache, myalgias, thrombocytopenia, hyponatremia, and elevated aminotransferases. Which of the following is the most appropriate treatment?
  • AOral nitrofurantoin therapy
  • BOral fluconazole therapy
  • COral doxycycline therapy
  • DOral amoxicillin therapy
Reveal answer & full explanation
Correct answer: C — Oral doxycycline therapy
  • AOral nitrofurantoin therapy
  • BOral fluconazole therapy
  • COral doxycycline therapy✓
  • DOral amoxicillin therapy

Why Oral doxycycline therapy is correct

  • Fever, headache, thrombocytopenia, hyponatremia, and transaminitis in a Southeastern tick-exposed patient point to Rocky Mountain spotted fever.
  • Doxycycline is first-line and must start empirically, since delay raises mortality.
  • It is used at every age because the survival benefit outweighs dental concerns.

Why the others are wrong

  • Oral amoxicillin therapy — Beta-lactams have no activity against intracellular Rickettsia; a familiar-antibiotic trap.
  • Oral fluconazole therapy — Fluconazole is an antifungal and does nothing for a rickettsial illness.
  • Oral nitrofurantoin therapy — Nitrofurantoin concentrates in urine for cystitis and cannot treat systemic tickborne disease.
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Risk factors

  • Outdoor exposure in endemic regions during warm months
  • Tick attachment (transmission can occur within 6-10 hours)
  • Contact with dogs in tick-endemic areas
  • G6PD deficiency associated with fulminant disease

Pathophysiology

Rickettsia rickettsii invades vascular endothelial cells, multiplying intracellularly and spreading cell-to-cell via actin-based motility. The resulting widespread small-vessel vasculitis causes increased vascular permeability, microhemorrhage (petechial rash), tissue edema, hypovolemia, and end-organ damage (encephalitis, ARDS, AKI, DIC).

Clinical presentation

Symptoms

  • Abrupt fever, severe headache, myalgia, malaise (2-14 days post-tick bite)
  • Nausea, vomiting, abdominal pain (can mimic acute abdomen)
  • Rash appears day 2-5: blanching macules → maculopapular → petechial; starts at wrists/ankles → spreads centripetally to trunk; involves palms and soles in ~50%
  • Confusion, lethargy, focal neuro deficits in severe disease

Signs / physical exam

  • Petechial rash on wrists/ankles, palms/soles (~half of patients; appears late)
  • Conjunctival injection, periorbital edema
  • Hypotension, oliguria in advanced disease
  • Hepatosplenomegaly, jaundice

Classic findings

Fever + headache + centripetal rash (wrists/ankles spreading inward, including palms/soles) in a summer outdoor exposure — but rash may be absent or appear late in 10-20%; DO NOT wait for rash to treat.

Differential diagnosis

  • Meningococcemia — Rapidly progressive petechiae/purpura, hypotension, meningitis; blood culture; treat empirically with ceftriaxone + doxycycline overlap until clarified
  • Ehrlichiosis/anaplasmosis — Tick-borne, similar prodrome, rash less common; leukopenia, thrombocytopenia, elevated LFTs; same doxycycline coverage
  • Measles — Cough, coryza, conjunctivitis, Koplik spots; rash starts on face and spreads caudally
  • Secondary syphilis — Rash includes palms/soles but not petechial; positive RPR
  • Drug reaction (TEN, DRESS) — Recent drug exposure, mucosal involvement, eosinophilia; biopsy
  • Idiopathic thrombocytopenic purpura (ITP) — Isolated thrombocytopenia, no fever, no systemic illness
  • Viral exanthem (enterovirus, parvovirus) — Usually self-limited; less toxic appearance; supportive care

Diagnostic workup

Diagnostic criteria

Clinical diagnosis with serologic confirmation. Treatment should NEVER be delayed for laboratory confirmation.

Labs

  • CBC (thrombocytopenia, normal or low WBC), CMP (hyponatremia, elevated LFTs, elevated Cr)
  • Coags (prolonged PT/PTT in severe disease)
  • Rickettsia rickettsii IgM and IgG indirect immunofluorescence assay (IFA) — paired acute and convalescent (2-4 weeks later) titers, 4-fold rise diagnostic
  • Skin biopsy with immunohistochemistry/PCR — highest sensitivity within 24 hours of rash onset
  • Blood PCR — variable sensitivity; useful early

Imaging

  • CXR if respiratory symptoms (pulmonary edema/ARDS)
  • CT/MRI brain for altered mental status or focal deficits

Diagnostic algorithm

flowchart TD
  A[Fever + headache<br/>summer + outdoor exposure] --> B{Tickborne illness<br/>suspected?}
  B -->|Yes| C[Start doxycycline<br/>do NOT delay for labs]
  C --> D[Obtain: CBC, CMP,<br/>IFA serology, skin bx if rash]
  D --> E{Rash develops?}
  E -->|Yes| F[Centripetal pattern,<br/>palms/soles → RMSF likely]
  E -->|No rash| G[Consider ehrlichiosis,<br/>anaplasmosis — doxy covers all]
  D --> H[Continue doxy<br/>≥3 days after afebrile]
Empiric management of suspected tickborne rickettsial illness — initiate doxycycline first, confirm later.

Treatment

First-line

  • Doxycycline 100 mg PO/IV BID for all ages — including children <8 (CDC and AAP — benefit far exceeds tooth discoloration risk for short courses)
  • Treat for at least 3 days after defervescence; typical course 5-7 days
  • Pediatric dosing: 2.2 mg/kg/dose BID up to 100 mg

Second-line / adjunct

  • Chloramphenicol — only for confirmed doxycycline allergy or pregnancy (debated; doxycycline preferred in pregnancy if severe)
  • Avoid sulfonamides — may worsen rickettsial illness
  • Fluoroquinolones and beta-lactams ineffective

Complications

  • Encephalitis with seizures, coma, focal deficits
  • ARDS, myocarditis, AKI requiring dialysis
  • DIC with gangrene of digits/extremities (amputation in survivors)
  • Permanent neurologic sequelae (hearing loss, paralysis, cognitive impairment) in delayed-treatment survivors
  • Death (mortality 20-25% untreated, <1% if doxycycline within 5 days of symptom onset)

PANCE pearls

  • Triad of fever, headache, and rash is present in <50% at initial presentation — treat empirically based on exposure and clinical picture.
  • Mortality climbs sharply when doxycycline is delayed beyond day 5 of illness — never withhold for serology.
  • Doxycycline is the drug of choice in children regardless of age (CDC 2017 update).
  • Hyponatremia, thrombocytopenia, and transaminitis form a useful early laboratory triad.
  • Tick avoidance: permethrin-treated clothing, DEET on skin, daily tick checks, prompt removal with fine forceps at skin level.

References

  • CDC 2016 — Diagnosis and Management of Tickborne Rickettsial Diseases (MMWR Recommendations and Reports)
  • AAP Red Book — Doxycycline use in children — Rocky Mountain Spotted Fever and other tickborne rickettsioses
  • IDSA — Guidelines for selected rickettsial diseases

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Differentials & related conditions

Measles (Rubeola)Immune Thrombocytopenic Purpura (ITP)

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