What is the recommended doxycycline (antibiotic) dose for prophylaxis against Lyme disease after a tick bite in a patient with a history of Lyme disease?

Doxycycline Prophylaxis for Tick Bite in Patients with Prior Lyme Disease

The recommended dose is a single 200 mg dose of doxycycline for adults (or 4 mg/kg up to 200 mg maximum for children ≥8 years), regardless of prior Lyme disease history, as previous infection does not provide protective immunity against reinfection. 1, 2

Key Principle: No Protective Immunity from Prior Infection

• Prior Lyme disease does not protect against reinfection from subsequent tick bites. 1
• The dosing recommendation remains identical whether or not the patient has had previous Lyme disease. 1, 2
• Similarly, prior Lyme disease vaccination (OspA vaccine) does not alter prophylaxis recommendations, as protective immunity is not long-lasting. 1

Specific Dosing Regimen

Adults

• Single dose of 200 mg doxycycline orally 1, 2

Children ≥8 years

• Single dose of 4 mg/kg (maximum 200 mg) 1, 2

Contraindications

• Doxycycline is relatively contraindicated in pregnant women and children <8 years old. 1, 2
• Do not substitute amoxicillin for prophylaxis in patients with doxycycline contraindications, as there is no evidence for effective short-course prophylaxis with alternative agents. 1

Strict Criteria for Prophylaxis (All Must Be Met)

Prophylaxis should only be given when ALL of the following conditions are satisfied: 1, 2

1. Tick identification: The tick is reliably identified as an adult or nymphal Ixodes scapularis (blacklegged tick). 1, 2

2. Attachment duration: The tick was attached for ≥36 hours, estimated by degree of engorgement or certainty about exposure time. 1, 2

3. Timing: Prophylaxis can be started within 72 hours of tick removal. 1, 2

4. Geographic endemicity: The bite occurred in a highly endemic area where ≥20% of ticks are infected with Borrelia burgdorferi (parts of New England, mid-Atlantic states, Minnesota, Wisconsin). 1, 2

5. No contraindications: Doxycycline is not contraindicated for the patient. 1, 2

Critical Timing Considerations

• The 72-hour window is crucial for efficacy. 1, 2
• Animal studies suggest prophylaxis effectiveness drops dramatically after 24 hours post-tick removal, with complete ineffectiveness ≥2 days after removal. 3
• The narrow therapeutic window emphasizes the importance of prompt treatment. 3

When Prophylaxis Is NOT Recommended

• If any of the above criteria are not met, observation is recommended instead of prophylaxis. 1
• Routine prophylaxis for all tick bites is not recommended. 1, 2
• A wait-and-watch approach is appropriate when the tick bite cannot be classified with high certainty as high-risk. 2
• Ixodes pacificus bites generally do not require prophylaxis due to low infection rates (<20%) in most endemic regions. 1

Post-Prophylaxis Monitoring

• Monitor for signs of Lyme disease for 30 days, particularly erythema migrans (expanding rash) at the bite site or elsewhere. 2, 4
• Watch for flu-like symptoms including fever, chills, fatigue, body aches, headache, and swollen lymph nodes. 4
• Seek immediate medical attention if an expanding rash or systemic symptoms develop, as the single prophylactic dose does not guarantee prevention. 4
• Serologic testing is not recommended in asymptomatic individuals following prophylaxis. 4

Efficacy Evidence

• Clinical trials demonstrate 87% efficacy in preventing Lyme disease when given appropriately. 3
• A European trial showed 67% relative risk reduction with a number-needed-to-treat of 51. 5
• The single 200 mg dose is the complete prophylactic regimen; no additional doses are required. 4

Common Pitfall to Avoid

Do not assume prior Lyme disease provides immunity. This is a critical misconception that could lead to withholding appropriate prophylaxis. Patients with previous Lyme disease remain fully susceptible to reinfection and should receive the same prophylactic approach as those without prior infection. 1