7.4 Tick-Borne Diseases (Lyme, Rocky Mountain Spotted Fever)

7.4 Tick-Borne Diseases (Lyme, Rocky Mountain Spotted Fever)

Why This Matters for the PANCE

Tick-borne illnesses are heavily tested on the PANCE, particularly Lyme disease and Rocky Mountain Spotted Fever (RMSF). Candidates must be able to recognize the characteristic rash patterns (erythema migrans vs. a centripetal macular-to-petechial rash), identify the causative pathogens and vectors, understand the diagnostic limitations of early serologic testing, and select appropriate pharmacotherapy. Pay close attention to drug-choice exceptions in pregnancy and pediatric populations, which are classic PANCE traps.

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Lyme Disease

Pathophysiology

Lyme disease is the most common vector-borne disease in the United States. It is caused by the spirochete Borrelia burgdorferi and transmitted by the bite of the Ixodes scapularis (deer tick) in the Northeast and Upper Midwest. The tick must remain attached for at least 24 to 36 hours to transmit a sufficient bacterial load.

Clinical Stages

1. Stage 1: Early Localized (3 to 30 days post-bite): Characterized by Erythema Migrans (EM). This is an expanding red rash that often develops a "bull's-eye" appearance with central clearing, though it can remain solidly erythematous. The rash is typically warm but non-painful and non-pruritic. It is often accompanied by flu-like symptoms (fatigue, headache, myalgias, arthralgias).
2. Stage 2: Early Disseminated (weeks to months post-bite): Systemic spread leads to:
   • Multiple secondary erythema migrans lesions.
   • Neurologic symptoms: Bilateral facial nerve palsy (Bell's palsy) is highly classic for Lyme disease; aseptic meningitis; peripheral neuropathy.
   • Cardiac symptoms: Lyme Carditis, characterized by fluctuating atrioventricular (AV) blocks (first-degree, Mobitz, or complete third-degree heart block).
3. Stage 3: Late Disseminated (months to years post-bite): Presents as chronic monoarticular or oligoarticular arthritis affecting large joints (especially the knee), and mild cognitive encephalopathy.

Diagnostic Workup

• Early Localized Stage: Clinical diagnosis! If a patient presents with a classic erythema migrans rash and a history of tick exposure in an endemic area, initiate treatment immediately. Do not order serologic testing in this stage, as antibodies take 2-4 weeks to form, leading to false negatives.
• Disseminated/Late Stages: Two-tier serologic testing:
  1. ELISA Screening: If positive or equivocal, proceed to:
  2. Western Blot Confirmation: Assesses IgM and IgG antibodies. (Only IgG is used if symptoms have lasted > 30 days to avoid false positives).

Treatment & Prophylaxis

• First-line treatment: Doxycycline (100 mg PO twice daily for 10–21 days) is the drug of choice.
• Pregnancy & Lactation: Oral Amoxicillin (500 mg TID for 14–21 days) is first-line. Doxycycline is contraindicated due to risks of fetal dental discoloration and bone growth restriction.
• Pediatric Patients: Amoxicillin is appropriate for children under 8 years. However, current CDC and AAP guidelines state that short-term courses of doxycycline (up to 21 days) are safe in children of all ages and do not cause significant tooth staining.
• Severe Manifestations (Meningitis, severe carditis/AV block): IV Ceftriaxone.
• Post-Exposure Prophylaxis (PEP): A single dose of Doxycycline 200 mg may be given within 72 hours of tick removal if the tick is identified as an Ixodes scapularis tick attached for ≥ 36 hours in an endemic area.

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Rocky Mountain Spotted Fever (RMSF)

Pathophysiology

Rocky Mountain Spotted Fever is a severe, potentially fatal tick-borne infection caused by Rickettsia rickettsii, an obligate intracellular Gram-negative bacterium. It is transmitted by the Dermacentor variabilis (American dog tick) and Dermacentor andersoni (Rocky Mountain wood tick). Despite the name, it is highly endemic in the Southeastern and South-Central United States (e.g., North Carolina, Oklahoma, Tennessee, Arkansas).

Clinical Presentation

Symptoms begin 3–12 days after a tick bite. The classic clinical triad consists of fever, headache, and a rash (though many patients do not recall a tick bite).

• The Rash: Appears 2–5 days after the onset of fever. It begins as small, blanching, erythematous macules on the wrists and ankles and then spreads centripetally (inward) to the palms, soles, and extremities, eventually reaching the trunk. Over time, the rash becomes petechial and non-blanching.
• Systemic Toxicity: R. rickettsii infects vascular endothelial cells, causing systemic vasculitis. This leads to vascular leak, non-cardiogenic pulmonary edema (ARDS), acute kidney injury, shock, gangrene, and DIC. Untreated RMSF carries a mortality rate of up to 20–30%.

Diagnostic Workup & Management

• CRITICAL RULE: Do NOT delay treatment for laboratory confirmation. RMSF is rapidly progressive. Antibody titers (IFA) take 7-10 days to become positive. Initiate treatment immediately based on clinical suspicion.
• First-line Treatment for ALL Patients: Doxycycline (100 mg PO/IV twice daily) is the drug of choice for all patients, regardless of age, including children and pregnant women.
  • Pregnancy Exception: In pregnant patients with suspected RMSF, the risk of maternal and fetal death from the infection far outweighs the theoretical risk of fetal tooth staining from a short course of doxycycline. (Chloramphenicol is a second-line alternative but is less effective and highly toxic).

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Comparison of Lyme Disease and Rocky Mountain Spotted Fever