What is the treatment and recommended laboratory tests for a patient with a suspected tick bite on the abdomen, presenting with a red, itchy lesion approximately the size of a baseball, noticed 3 days ago?

Management of Suspected Tick Bite with Erythema Migrans

For a patient with a suspected tick bite presenting with a baseball-sized red, itchy lesion on the abdomen that has been present for 3 days, immediate treatment with doxycycline is recommended as this presentation is consistent with erythema migrans, the hallmark of early Lyme disease.

Clinical Assessment

The patient's presentation has several key features suggesting erythema migrans (EM):

• Large size (baseball-sized or approximately 7-8 cm)
• Red, expanding lesion
• Central location on abdomen (common site for EM)
• Recent onset (3 days)
• Itching (though typically EM is not intensely pruritic)

Differential Diagnosis

• Erythema migrans (early Lyme disease)
• Tick bite hypersensitivity reaction (less likely given the size)
• Cellulitis (less likely given location on abdomen)
• Other dermatologic conditions

Treatment Recommendations

First-line Treatment

• Oral doxycycline 100 mg twice daily for 10 days 1
• For children ≥8 years: 4 mg/kg per day divided into 2 doses (maximum 100 mg per dose)
• For children <8 years: amoxicillin 50 mg/kg/day divided into 3 doses (maximum 500 mg per dose) for 14 days

Alternative Regimens (if doxycycline contraindicated)

• Amoxicillin 500 mg three times daily for 14 days
• Cefuroxime axetil 500 mg twice daily for 14 days

Laboratory Testing

Recommended Tests

• Clinical diagnosis is sufficient for typical erythema migrans; laboratory testing is not necessary for initial management 1
• Consider marking the borders of the lesion with ink to monitor expansion

Additional Testing to Consider

• If symptoms suggest disseminated disease (fever, arthralgia, headache, multiple EM lesions):
  • Complete blood count
  • Comprehensive metabolic panel
  • Two-tier serologic testing for Lyme disease (ELISA followed by Western blot if positive)
  • Consider testing for co-infections if in endemic area:
    • Anaplasmosis: PCR and/or serology
    • Babesiosis: Blood smear, PCR, serology

Follow-up Recommendations

• Clinical reassessment within 48-72 hours to ensure response to therapy
• Patients should be informed that:
  1. Fever typically subsides within 24-48 hours after treatment initiation
  2. Lack of response within 48 hours suggests alternative diagnosis or co-infection
  3. The rash may take several days to resolve even with appropriate therapy

Prevention Counseling

Advise the patient on tick prevention measures:

• Use of EPA-registered repellents containing DEET, picaridin, IR3535, oil of lemon eucalyptus, or permethrin on clothing
• Perform thorough tick checks after outdoor activities
• Proper tick removal technique using fine-tipped tweezers
• Avoid wooded and brushy areas with high grass when possible

Important Considerations and Pitfalls

1. Do not delay treatment while awaiting laboratory confirmation - early treatment is critical to prevent progression to disseminated disease 1

2. Consider geographic location - treatment approach may vary based on local prevalence of Lyme disease and co-infections

3. Watch for co-infections - patients with tick bites may be simultaneously infected with multiple tick-borne pathogens requiring different treatment approaches

4. Doxycycline is now considered safe for children of all ages for short courses of therapy as recommended by the American Academy of Pediatrics and CDC 1

5. Erythema migrans is a clinical diagnosis - negative serology early in the course of illness does not rule out Lyme disease 1