What is the appropriate treatment for a healthy adult patient with a bullseye bite, potentially from a tick, and no significant medical history?

Treatment of Bullseye Rash (Erythema Migrans)

A bullseye rash following a tick bite is erythema migrans, the hallmark of Lyme disease, and should be treated immediately with oral antibiotics based on clinical diagnosis alone—do not wait for laboratory confirmation. 1

Immediate Clinical Diagnosis

• Make the diagnosis clinically without laboratory testing when a patient in a Lyme-endemic area presents with an expanding erythematous skin lesion at a tick bite site. 1
• Erythema migrans typically appears 7-14 days (range 3-30 days) after tick detachment and is at least 5 cm in diameter. 2
• The rash may appear homogeneously erythematous or have central clearing with a target-like "bullseye" appearance. 2
• Laboratory testing is not recommended for typical erythema migrans presentations, as early serologic tests are often negative and treatment should not be delayed. 1

First-Line Treatment Regimen

Doxycycline is the preferred antibiotic for all adults and children ≥8 years old:

• Adults: Doxycycline 100 mg orally twice daily for 10 days 1, 2
• Children ≥8 years: Doxycycline 4 mg/kg per day in 2 divided doses (maximum 100 mg per dose) for 10 days 2
• This shorter 10-day course is equally effective as longer regimens and is strongly recommended over extended treatment. 1

Alternative Regimens (When Doxycycline Contraindicated)

For pregnant women, children <8 years, or patients with doxycycline contraindications:

• Amoxicillin: 500 mg orally three times daily for 14 days (adults) 1, 2
  • Children: 50 mg/kg/day in 3 divided doses (maximum 500 mg per dose) for 14 days 2
• Cefuroxime axetil: 500 mg orally twice daily for 14 days (adults) 1, 2
  • Children: 30 mg/kg/day in 2 divided doses (maximum 500 mg per dose) for 14 days 2
• Azithromycin is less effective and should only be used if the patient cannot tolerate any of the above options; if used, give for 7-10 days. 1

Critical Monitoring Parameters

• Fever should resolve within 24-48 hours of starting doxycycline; lack of response suggests alternative diagnosis or coinfection. 2
• Monitor for signs of disseminated disease: multiple erythema migrans lesions, facial nerve palsy, meningitis symptoms, heart block, or joint swelling. 1
• Watch for Jarisch-Herxheimer reaction (temporary worsening of symptoms within 24 hours of starting antibiotics). 2

When to Hospitalize

Admit the patient if any of the following develop:

• Evidence of organ dysfunction (renal or hepatic impairment) 3
• Severe thrombocytopenia on laboratory testing 3
• Mental status changes or altered consciousness 3
• Cardiac conduction abnormalities (heart block) 1
• Severe neurologic manifestations 1

Common Pitfalls to Avoid

• Do not delay treatment while waiting for tick identification or serologic confirmation—early serology is frequently negative and treatment delay worsens outcomes. 3
• Do not order serologic testing for typical erythema migrans presentations, as it adds no value and may cause confusion if negative. 1
• Do not treat for longer than recommended durations (10-14 days)—there is no evidence that extended courses improve outcomes. 1
• Do not withhold doxycycline in children <8 years if rickettsial disease (Rocky Mountain spotted fever) is also in the differential, as short courses do not cause tooth staining and mortality risk outweighs this concern. 3
• Remember that up to 30-50% of patients do not recall a tick bite, so absence of tick bite history should not dissuade you from diagnosing Lyme disease. 1

Tick Removal (If Still Attached)

• Remove the tick immediately by grasping with fine-tipped tweezers close to the skin and pulling upward with steady, even pressure. 1, 3
• Never burn the tick or apply petroleum jelly, nail polish, or other substances to coax detachment. 1, 3
• Disinfect the bite wound after removal. 1
• Submit the tick for species identification to confirm it is an Ixodes species. 1, 3

Prevention of Future Tick Bites

• Use EPA-registered repellents (DEET 20-30%, picaridin, permethrin on clothing). 1
• Wear protective clothing and perform full-body tick checks after outdoor activities in endemic areas. 1, 4
• Remove attached ticks within 36 hours to prevent transmission, as Borrelia burgdorferi typically requires ≥36 hours of attachment for transmission. 1, 4

Prognosis

With appropriate antibiotic treatment for 10-14 days, the prognosis is excellent and progression to late disseminated Lyme disease is prevented. 2, 5