How to Describe the Area of a Tick Bite
When documenting a tick bite, describe the size (measure the largest diameter in centimeters or millimeters), shape (round, oval, irregular), color (erythematous, blanching, petechial, purpuric), presence or absence of central features (clearing, vesicles, pustules, eschar), borders (well-demarcated vs. diffuse), and anatomic location, while noting whether the tick is still attached and the timing relative to tick removal. 1
Key Documentation Elements
Size Measurement
- Measure and document the largest diameter in centimeters or millimeters 1
- Lesions ≥5 cm are significant for diagnosing erythema migrans (Lyme disease), while smaller lesions (<5 cm) appearing within 48 hours of tick removal are more likely hypersensitivity reactions 1
- Mark the borders with ink and reassess in 1-2 days to document expansion or resolution, which helps differentiate infectious from allergic processes 1
Timing Relative to Tick Exposure
- Document when the lesion appeared relative to tick attachment or removal 1
- Lesions present while the tick is still attached or developing within 48 hours are typically hypersensitivity reactions (non-infectious) 1
- Erythema migrans typically appears 7-14 days (range 3-30 days) after tick detachment 1
- Rocky Mountain spotted fever rash appears 2-4 days after fever onset 1
Morphologic Features
Color and appearance:
- Erythematous (red), blanching vs. non-blanching 1
- Maculopapular (flat with raised bumps) 1
- Petechial (pinpoint hemorrhages) or purpuric (larger hemorrhages), particularly on lower extremities 1
- Homogeneously red vs. target-like with central clearing 1
Central features:
- Presence of vesicles or pustules (seen in ~5% of erythema migrans cases) 1
- Eschar formation (dark scab-like plaque over shallow ulcer) suggests certain rickettsial diseases, though rarely seen with Rocky Mountain spotted fever 1
- Note: vesicular lesions from tick bites lack significant pruritus, unlike contact dermatitis 1
Border characteristics:
- Well-demarcated vs. diffuse edges 1
- Expanding vs. stable 1
- Urticarial (hive-like) appearance suggests hypersensitivity 1
Anatomic Location
- Document the specific body site (scalp, abdomen, axilla, groin, popliteal fossa, ankles, wrists, forearms, palms, soles) 1
- Erythema migrans often occurs in sites unusual for bacterial cellulitis (axilla, popliteal fossa, abdomen) 1
- Rocky Mountain spotted fever classically begins on ankles, wrists, or forearms and spreads centrally, including palms and soles 1
Associated Features
- Presence or absence of scaling (erythema migrans is not scaly unless long-standing or treated with topical corticosteroids) 1
- Whether the tick is still attached or has been removed 1
- Presence of multiple lesions (secondary erythema migrans from hematogenous dissemination) 1
Common Pitfalls to Avoid
Do not dismiss small lesions appearing immediately after tick removal as they may represent the initial presentation before expansion occurs; serial measurements over 24-48 hours are critical 1
Do not assume all erythematous lesions at tick bite sites are infectious—hypersensitivity reactions typically begin to disappear within 24-48 hours, while infectious processes expand 1
Do not wait for the "classic" appearance (target lesion with central clearing) as many erythema migrans lesions are homogeneously erythematous 1
Absence of a rash does not exclude serious tick-borne disease—less than 50% of Rocky Mountain spotted fever patients have a rash in the first 3 days, and some never develop one 1