Lyme Disease Prophylaxis for a 9-Year-Old Girl
A single dose of doxycycline (4 mg/kg up to a maximum of 200 mg) should be offered to this 9-year-old girl, but only if all four strict criteria are met: the tick is identified as an adult or nymphal Ixodes scapularis, it was attached for ≥36 hours, prophylaxis can be given within 72 hours of removal, and the bite occurred in a highly endemic area (≥20% tick infection rate). 1
Age-Appropriate Dosing
- For children ≥8 years old (which includes this 9-year-old), doxycycline is the recommended agent at 4 mg/kg up to a maximum dose of 200 mg as a single dose 1, 2, 3
- This dosing is identical whether or not the child has had prior Lyme disease, as previous infection does not confer protective immunity 2
All Four Criteria Must Be Met
Prophylaxis should only be given when every single criterion is satisfied 1, 3:
Tick identification: Must be reliably identified as an adult or nymphal Ixodes scapularis tick (not I. pacificus, which generally does not require prophylaxis) 1, 2
Attachment duration: Estimated to have been attached for ≥36 hours based on degree of engorgement or certainty about exposure time 1, 3
Timing: Prophylaxis must be started within 72 hours of tick removal—this window is critical for efficacy 1, 2, 3
Geographic endemicity: The bite must have occurred in an area where ≥20% of ticks are infected with Borrelia burgdorferi (parts of New England, mid-Atlantic states, Minnesota, and Wisconsin) 1, 3
Critical Pitfall to Avoid
Do not substitute amoxicillin for doxycycline in this 9-year-old child 1. The guidelines explicitly recommend against using amoxicillin for prophylaxis because there is no evidence for an effective short-course regimen, it would require a multi-day course with associated adverse effects, and the risk of serious complications from a recognized tick bite is extremely low given the excellent efficacy of treatment if infection develops 1.
When Prophylaxis Is NOT Recommended
- If any of the four criteria above are not met, observation is the appropriate management rather than prophylaxis 2, 3
- Routine prophylaxis for all tick bites is explicitly not recommended 1, 3
- Routine serologic testing after a tick bite is also not recommended 1
Post-Exposure Monitoring (Essential Regardless of Prophylaxis)
- Monitor the child closely for 30 days for signs of tick-borne diseases, particularly watching for an expanding skin lesion (erythema migrans) at the bite site 1, 2
- Seek immediate medical attention if an expanding rash or viral-like illness develops, as the single prophylactic dose does not guarantee prevention 2
- Even children who receive prophylaxis require this monitoring period 1, 2
Evidence Quality
The recommendation for single-dose doxycycline prophylaxis is graded as B-I (moderately in favor, with evidence from properly randomized controlled trials) 1. Recent meta-analyses confirm efficacy with a pooled risk ratio of 0.38 for unfavorable events and 0.29 specifically for single-dose 200 mg doxycycline 4. A 2021 European trial demonstrated 67% relative risk reduction with a number-needed-to-treat of 51 5.
Important Nuance About Timing
Animal models suggest the window for effective prophylaxis is even narrower than 72 hours—protection dropped from 74% when given on the day of removal to 47% at 24 hours and was ineffective at ≥48 hours 6. While human pharmacokinetics differ, this emphasizes the importance of prompt administration within the 72-hour window 6.