Rocky Mountain Spotted Fever (RMSF)

BASICS

Definition: Tick-borne disease caused by Rickettsia rickettsii; most severe/fatal US spotted fever rickettsiosis (SFR).
Key Symptoms: Fever, headache, and rash (typically wrists/ankles, spreading to palms/soles and trunk).
Systems affected: Cardiovascular, skin, CNS, renal, hepatic, pulmonary.

Vectors: Ticks (American dog tick Dermacentor variabilis—most common; Rocky Mountain wood tick, brown dog tick).
Distribution: All US states except Hawaii/Alaska; most common in southeast/southcentral US (Arkansas, Missouri, NC, TN, VA).
Incidence: US SFR incidence: 1.7/million (2000) → 13.2/million (2016); 5,207 cases (2019).
Season: Peak May–August.
Risk Groups: Highest incidence >40 years, highest mortality in children <10 years.

Organism: R. rickettsii released from tick salivary glands after 6–10 hr feeding.
Pathology: Vascular endothelial infection → small/medium vessel injury, disseminated inflammation, ↑ vascular permeability → edema, hyponatremia, end-organ damage (e.g., pulmonary/cerebral edema, ARF, shock).
Transmission: Mostly via tick bite; rarely by direct inoculation into wounds/conjunctiva or inhalation.

Clinical suspicion essential—do not wait for lab confirmation to start treatment!
Typical history: Acute febrile illness, rash (may be absent!), possible tick exposure, May–August.
Symptoms (First 1–4 days): Fever, severe headache, malaise, myalgia, N/V, photophobia.
Rash: 2–4 days after fever onset; starts as blanching pink macules on wrists/ankles, spreads to trunk, may become petechial. Palms/soles involved by days 5–6 (advanced disease). Rash spares face, not pruritic, may be absent (esp. adults).
Other findings: Conjunctival injection, mental status changes, arthralgia, periorbital/peripheral edema, calf pain, hearing loss.

Fever >102°F
Erythematous/maculopapular rash (1–5 mm), can become petechial, purpuric, necrotic in severe cases.
May have AMS, focal neurologic deficits, lymphadenopathy, hepatosplenomegaly, edema.

Serology (IFA): Gold standard (4-fold rise in IgG). Sensitivity low in first 10–12 days, so treat empirically.
PCR: Blood or skin biopsy, more specific.
Labs: Thrombocytopenia (60%), hyponatremia, ↑ LFTs, anemia, ↑ BUN/creatinine, elevated PT/PTT.
CSF: Lymphocytic pleocytosis, ↑ protein, normal glucose.
Imaging: Rarely helpful.

Doxycycline (adults and children, including <8 years):
45 kg: 100 mg PO/IV q12h
<45 kg: 2.2 mg/kg PO/IV q12h
Continue at least 3 days after fever subsides (minimum 5–7 days).
Adverse effects: GI upset, photosensitivity; no dental staining risk in children <8 yrs.
Severe allergy: Desensitization may be considered in life-threatening illness.

Short-term doxycycline is appropriate for RMSF, including in pregnancy/breastfeeding, if suspicion high.
Chloramphenicol only if absolutely necessary.

CNS dysfunction, vomiting, immunocompromised, organ failure, oral therapy not possible, severe allergy.

Follow-up: Every 2–3 days until symptom resolution.
Hospitalize moderate/severe cases; mild disease can be outpatient with close follow-up.
Monitor: CBC, LFTs, electrolytes if needed.
Prognosis: Excellent if treated promptly; mortality ↑ with delayed therapy (>5 days).
No lifelong immunity—reinfection possible.

Encephalopathy, seizures, focal neurologic deficit, renal/hepatic failure, CHF, respiratory failure, DIC, tissue necrosis.
Highest risk: Children <10 years, adults >70, G6PD deficiency (fulminant, fatal cases possible in ≤5 days).

Diagnosis requires a high index of suspicion.
Tick bite is often not noticed; rash may be absent.
Start doxycycline immediately if RMSF suspected—do not wait for lab confirmation.
No evidence for antibiotic prophylaxis after tick bite.