A Case of Atypical Erythema Migrans in a High-Risk Individual

Case at a Glance

A 25-year-old female from a tick-endemic region presented with a single, expanding rash but no other symptoms. Despite the atypical appearance of the rash and no memory of a tick bite, her high-risk exposure and a recent Lyme diagnosis in a close contact raised strong suspicion for early localized Lyme disease, prompting a clinical diagnosis and treatment.

Patient's Story

The patient is a 25-year-old female residing in a heavily wooded area known for high tick prevalence. She presented to her primary care physician after noticing a large, red rash on her trunk that had appeared the previous day. An avid outdoors enthusiast, she reported frequent tick exposure, having removed more than 20 crawling ticks and 3 to 5 attached ticks from her body over the past two months. She denied any memory of a tick bite at the site of the rash and was otherwise asymptomatic, reporting no fever, chills, fatigue, headache, or arthralgia. Her concern was heightened because her live-in partner had recently been diagnosed with Lyme disease after presenting with a similar rash and was currently undergoing treatment with doxycycline.

Initial Assessment

On physical examination, the patient was afebrile and appeared well. A single, non-tender, non-pruritic, erythematous, annular patch measuring approximately 6 cm in diameter was noted on her right flank. The lesion had a slightly raised, well-demarcated border with some central clearing, though it did not present as a classic 'bull's-eye' (erythema migrans). The remainder of the physical examination was unremarkable. Given the clinical presentation and history, the primary differential diagnosis included early localized Lyme disease, tinea corporis, nummular eczema, and a localized contact dermatitis or insect bite reaction.

The Diagnostic Journey

The patient's residence in a Lyme-endemic area, significant history of tick exposure, and the expanding nature of the rash created a high clinical suspicion for Lyme disease. It was noted that a lack of memory of a specific tick bite is common, as nymphal Ixodes ticks are very small and their bites are often painless. The physician initially expressed some uncertainty due to the atypical appearance of the rash and the patient's complete lack of systemic symptoms. Serologic testing (Lyme titers) was discussed but ultimately deferred, as it is often negative in the early stages of infection and is not recommended for diagnosis when a suspicious rash is present in a high-risk individual. The diagnosis was therefore based on epidemiological risk factors and the clinical presentation of the rash.

Final Diagnosis

Early Localized Lyme Disease.

Treatment Plan

Despite the initial uncertainty, the decision was made to treat empirically for Lyme disease to prevent potential progression to later-stage disseminated disease. The patient was prescribed a 14-day course of Doxycycline 100 mg twice daily. She was counseled on the importance of completing the full course of antibiotics, even if the rash resolved quickly. Additionally, she received education on tick-bite prevention strategies, including the use of insect repellent, wearing protective clothing, and performing daily, thorough tick checks.

Outcome and Follow-up

The patient was advised to monitor the rash, which was expected to begin resolving within several days of starting treatment. She was instructed to follow up immediately if she developed any new symptoms, such as fever, headache, joint pain, or neurological signs. A follow-up appointment was scheduled for two weeks after the completion of her antibiotic course to ensure complete resolution of the rash and to confirm the absence of any new clinical developments.