Red Migrating Rash with Lower Extremity Pain in Child
The most likely diagnosis is Lyme disease (erythema migrans), and first-line therapy is oral amoxicillin or doxycycline (if ≥8 years old) for 10–14 days, initiated immediately based on clinical findings without waiting for serologic confirmation. 1
Diagnostic Reasoning
Clinical Features Strongly Suggesting Lyme Disease
Erythema migrans is characterized by an expanding reddish or bluish-red patch with an advancing, distinct, intensely colored edge that is typically not markedly elevated, localized around the point of tick inoculation. 1
The rash often presents with central clearing (classic "bull's-eye"), though early erythema migrans commonly has homogeneous or central redness rather than peripheral erythema with partial central clearing. 2
The expanding, migrating nature of the rash is pathognomonic for Lyme disease when occurring after outdoor exposure in tick-endemic areas. 1, 3
Lower extremity pain (arthralgia) is one of the most common associated symptoms, along with myalgia, fatigue, headache, and low-grade fever. 4, 2
Critical Differential: Rocky Mountain Spotted Fever (RMSF)
While RMSF must be considered in any child with rash and tick exposure, several features make it less likely here:
RMSF rash begins as small (1-5 mm) blanching pink macules on ankles, wrists, or forearms 2-4 days after fever onset, then evolves to maculopapular with central petechiae. 5
The rash spreads centripetally (from extremities to trunk), eventually involving palms and soles, while generally sparing the face. 5
Less than 50% of RMSF patients have rash in the first 3 days of illness, and up to 20% never develop a rash. 5
RMSF is associated with high fever, severe headache, and systemic toxicity, with a 5-10% case-fatality rate if untreated. 6, 5
If the rash is petechial, rapidly progressive, or accompanied by high fever and systemic toxicity, immediately initiate doxycycline empirically for presumed RMSF without waiting for confirmation. 6, 7
Diagnostic Approach
Clinical Diagnosis is Sufficient
Diagnosis of erythema migrans is based on clinical findings in a patient with possible or confirmed recent tick bite; serological tests are NOT useful at this early stage of infection. 8, 1
Laboratory evidence is NOT essential for diagnosis when the major clinical criteria (expanding reddish patch with distinct advancing edge) are present. 1
Only 65% of patients with microbiologically confirmed erythema migrans develop positive IgM or IgG antibody responses by convalescence, making early serology unreliable. 2
Key Historical Features to Elicit
Recent outdoor exposure in tick-endemic areas (particularly grassy, wooded areas during April-September). 1
History of tick bite at the same location as the rash (though absence does not exclude diagnosis, as 40% of patients do not recall tick exposure). 1, 6
Timing: erythema migrans usually appears within 2 weeks after an infected tick bite, with median presentation 3 days after symptom onset. 3, 2
Physical Examination Findings
Expanding annular rash with distinct borders, often with central clearing, though homogeneous redness is common early. 1, 2
Associated symptoms include malaise, fatigue, headache, low-grade fever, regional lymphadenopathy, and migratory musculoskeletal pain. 4
First-Line Treatment
Antibiotic Selection
For children <8 years old: Amoxicillin is the first-line agent. 1, 8
For children ≥8 years old: Either amoxicillin or doxycycline is appropriate. 1, 8
Doxycycline should NOT be used in children <8 years old due to risk of tooth and bone disorders. 8
In randomized trials, amoxicillin, doxycycline, cefuroxime, and ceftriaxone had similar efficacy, clearing signs and symptoms in about 90% of patients with relapse rates <5% at 6 months. 8
Macrolide antibiotics (azithromycin, clarithromycin, erythromycin) have lower efficacy and should be avoided. 8
Treatment Duration and Expected Response
Most patients respond promptly to antibiotic treatment, with excellent prognosis when therapy is administered soon after symptom onset. 9, 2
Treatment prevents progression to potentially severe later stages of Lyme disease, including neurological, articular, and cardiac complications. 8
Critical Pitfalls to Avoid
Do Not Wait for Laboratory Confirmation
Initiating treatment based on clinical diagnosis alone is appropriate and prevents delays that could lead to disseminated disease. 1, 8
Serologic testing in early Lyme disease is unreliable and should not guide initial treatment decisions. 8, 2
Do Not Dismiss Based on Absent Tick Bite History
The majority of patients will not recall or recognize an attached tick because tick bites are typically painless and nymphal ticks are only 1-2 mm in size. 1
Only 60% of RMSF cases and 68% of ehrlichiosis cases report definite tick bite history. 1
Recognize When RMSF Cannot Be Excluded
If ANY of the following are present, initiate doxycycline immediately for presumed RMSF (even in children <8 years old): 6, 7
- Fever + rash + headache in tick-endemic area
- Petechial or purpuric rash elements
- Rash involving palms and soles
- Systemic toxicity (high fever, altered mental status, hypotension)
- Thrombocytopenia or hyponatremia on laboratory testing
Add intramuscular ceftriaxone if meningococcemia cannot be excluded based on clinical presentation. 6, 7