Clues Pointing TO This Diagnosis

Expanding red circular rash on the thigh reaching several inches — erythema migrans expands gradually over days to >5 cm and is the hallmark early sign
Classic bull's-eye pattern with central clearing and an expanding outer ring — central clearing producing a "target"/"bull's-eye" appearance is characteristic of erythema migrans
Recent tick removal from the leg days before the rash — EM appears at the tick-bite inoculation site 3–30 days after the bite
Frequent weekend hiking April–November with repeated tick exposure — outdoor exposure in tick habitat is the key epidemiologic risk factor
Accompanying fatigue and malaise — constitutional symptoms accompany EM in roughly two-thirds of US patients
Evening fever/chills — fever and chills are typical early-localized systemic features
Myalgias and arthralgias — achiness and joint pain commonly accompany early Lyme disease
Migratory ("wandering") knee joint pain — migratory arthralgia/arthritis of large joints is a recognized Lyme manifestation
Rash that persists and enlarges rather than resolving — a slowly enlarging lesion over days distinguishes EM from a transient reaction
Rash reportedly warm but not itchy or painful — EM may feel warm but is rarely pruritic or painful

Why It's NOT the Similar Diseases

NOT a local tick-bite hypersensitivity reaction — a bite reaction appears within hours, stays small (usually <5 cm), and resolves in a day or two, whereas Milton's lesion expanded over days to several inches
NOT cellulitis — bacterial cellulitis is warm, tender, and often painful with a poorly defined advancing edge, rather than the painless, gradually expanding annular lesion with central clearing seen here
NOT tinea corporis (ringworm) — tinea has a raised scaly active border and is usually itchy, unlike the smooth, non-scaling, minimally pruritic erythema migrans
NOT a spider bite/arthropod reaction — those produce a localized painful papule or necrotic center, not a large expanding target lesion with systemic symptoms after a documented tick exposure
NOT nummular eczema — eczema forms chronic scaly coin-shaped plaques without the acute expansion, tick-exposure history, or accompanying fever and arthralgias

Differential Diagnosis List

Local tick-bite reaction — ruled out by rash expanding over days to several inches rather than resolving quickly
Cellulitis — ruled out by the painless, annular, centrally clearing lesion and lack of a tender ill-defined border
Tinea corporis — ruled out by absence of a scaly active border and minimal pruritus
Southern tick-associated rash illness (STARI) — ruled out by exposure pattern and typical Lyme presentation; STARI follows Amblyomma (lone star) bites and is diagnosed clinically
Arthropod/spider bite — ruled out by the expanding target morphology and accompanying systemic symptoms after documented tick exposure

Key Distinguishing Test

Early Lyme disease presenting with erythema migrans in a patient with plausible tick exposure in an endemic area is a clinical diagnosis; serologic testing is not required and is often falsely negative in the first 1–2 weeks because antibodies have not yet developed.
For extracutaneous or disseminated manifestations (neurologic, cardiac, arthritis) or for atypical lesions, confirm with standard or modified two-tiered serologic testing (an initial EIA followed by immunoblots or a second EIA).

Board Note

Erythema migrans occurs in 70–80% of infected patients and is the earliest and most common sign; when present in an endemic area, diagnose and treat clinically without waiting for serology
First-line treatment for erythema migrans is oral doxycycline, amoxicillin, or cefuroxime axetil; azithromycin is a second-line agent for patients who cannot take the others
Untreated infection disseminates in roughly 60% of patients over weeks to months, causing multiple EM lesions, cranial neuritis (most often facial/Bell's palsy, can be bilateral), lymphocytic meningitis, radiculoneuritis, Lyme carditis (AV block), and later oligoarticular large-joint arthritis
Lyme carditis classically causes fluctuating AV nodal block; suspect it with syncope, palpitations, or a prolonged PR interval, and monitor/refer accordingly
Prevent bites with permethrin-treated clothing, EPA-approved skin repellents (DEET, picaridin), long clothing, prompt tick checks, and showering within 2 hours; transmission risk is low if an Ixodes tick is attached <36 hours
Post-exposure doxycycline prophylaxis is appropriate mainly when an identified Ixodes tick was attached ≥36 hours, treatment can start within 72 hours, and local infection rates are high
The following figure from the New England Journal of Medicine review shows the branching approach to a patient with possible early Lyme disease, including the principle that erythema migrans alone warrants treatment without awaiting serology:
Figure 1. Initial Assessment of a Patient in a High-Incidence Area of Exposure with Possible Early Lyme Disease.
How Do I Approach the Evaluation and Treatment of Early Lyme Disease?. NEJM Evid. July 31, 2024.
Used under license from The New England Journal of Medicine.

“Milton’s Story”