Lyme disease vs Rocky Mountain spotted fever
How to tell Lyme disease and Rocky Mountain spotted fever apart when a board question makes them look alike.
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Rash involving palms and soles that started at wrists and ankles is the fastest clue for Rocky Mountain spotted fever.
Side by side
Expanding erythema migrans lesion after Ixodes exposure in endemic areas with early flu-like symptoms indicates early Lyme disease.
High fever with severe headache and rash that begins at wrists and ankles and includes palms and soles indicates Rocky Mountain spotted fever.
Borrelia burgdorferi transmitted by Ixodes ticks disseminates from skin and triggers multisystem inflammatory manifestations.
Rickettsia rickettsii infects endothelial cells, causing vasculitis with capillary leak, thrombocytopenia, and hyponatremia.
Early localized Lyme is usually a clinical diagnosis with erythema migrans; two-tier serology can be negative in the first weeks and should not delay treatment when rash is classic.
RMSF often shows thrombocytopenia, sodium below 135 mEq/L, mildly elevated transaminases, and diagnosis is clinical because early serology may be negative.
The trap
Waiting for confirmatory serology in suspected RMSF is dangerous because early tests are frequently negative and mortality rises with delay; start doxycycline immediately when the rash pattern and systemic toxicity fit.
The repair move
Next best step: if RMSF is clinically suspected, start doxycycline 100 mg orally or IV every 12 hours immediately, including in children and pregnant patients when severe disease is suspected. Do not wait for serology. For classic early erythema migrans without systemic instability, treat Lyme disease with doxycycline 100 mg twice daily for 10 days, or amoxicillin 500 mg three times daily for 14 days if doxycycline is contraindicated.
Try a free question
CoreA 9-year-old boy returns from camping with 3 days of fever, severe headache, and a new petechial rash on wrists and ankles that now includes palms. BP 96/58 mm Hg, HR 124/min, temp 39.1 C. Platelets are 95,000/uL and sodium is 130 mEq/L. What is the next best step in management?
Show answer and explanation
Answer: A. Start doxycycline immediately. This is classic Rocky Mountain spotted fever and doxycycline should be started at first clinical suspicion to reduce mortality.
Why the other choices are tempting
- B. Wrong because delayed treatment while awaiting serology worsens outcomes.
- C. Wrong because cephalexin does not cover rickettsial infection.
- D. Wrong because Lyme therapy choice here misses the rash pattern and systemic toxicity of Rocky Mountain spotted fever.
- E. Wrong because this child has high-risk features requiring urgent antimicrobial therapy.
Flashcards
Tap a card to flip it.
Which rash distribution most strongly indicates Rocky Mountain spotted fever rather than Lyme disease?
A rash that begins on wrists and ankles and includes palms and soles is classic for Rocky Mountain spotted fever.
Why is erythema migrans usually treated without waiting for serology?
Early antibody tests can be negative, and a classic expanding lesion in an exposed patient is sufficient for clinical treatment.
Quick answers
What is the fastest way to tell Lyme disease from Rocky Mountain spotted fever?
Rash involving palms and soles that started at wrists and ankles is the fastest clue for Rocky Mountain spotted fever.
What is the key clue for Lyme disease?
Expanding erythema migrans lesion after Ixodes exposure in endemic areas with early flu-like symptoms indicates early Lyme disease.
What is the key clue for Rocky Mountain spotted fever?
High fever with severe headache and rash that begins at wrists and ankles and includes palms and soles indicates Rocky Mountain spotted fever.
What is the common trap with Lyme disease vs Rocky Mountain spotted fever?
Waiting for confirmatory serology in suspected RMSF is dangerous because early tests are frequently negative and mortality rises with delay; start doxycycline immediately when the rash pattern and systemic toxicity fit.