Insect Bite of Unknown Type: Diagnosis, Treatment, and Red Flag Monitoring (Bee, Wasp, Hornet, Ant, Mosquito, Flea, Bedbug, Rove beetle, Centipede, Spider, Tick, Mite, Louse, Scorpion)

Category	Drug	Dose & Route	Frequency	Duration	Notes
Topical steroid (first-line)	Hydrocortisone 1% cream	Thin layer over lesion	bid	5–7 days	Safe, low potency; exam-preferred
Topical steroid (if severe inflammation)	Betamethasone 0.1% cream (15 g)	Thin layer	bid	≤5 days	Higher potency; short course only
Antihistamine (oral)	Loratadine	10 mg po	od	prn or up to 7–10 days	Non-sedating
	Cetirizine	10 mg po	od	prn or up to 7–10 days	Alternative
Analgesic	Paracetamol	500–1000 mg po	q6h prn (max 4 g/day)	As needed	For pain/fever
Antibiotic (if cellulitis, OPD)	Amoxicillin-Clavulanate	875/125 mg po	bid pc	5–7 days	First-line per Thai CPG/IDSA
	Clindamycin	300 mg po	tid pc	5–7 days	Use if penicillin allergy
Severe allergy / anaphylaxis	Epinephrine (IM, 1:1000)	0.3–0.5 mg IM (adult)	Once, repeat q5–15 min if needed	Emergency only	First-line for anaphylaxis
	Prednisolone (optional adjunct in anaphylaxis)	40–60 mg po/iv	od	3–5 days	To prevent biphasic reaction (not first-line)

📚 Memory Pearls

• 🟢 HC 1% cream + Loratadine = standard insect bite Rx

• 🟡 Add antibiotic only if infection signs (cellulitis, pus).

• 🔴 Epinephrine IM if systemic allergic reaction.

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🩺 Diagnosis (Clinical, not lab-based)

1. History

• Recent outdoor/indoor exposure.

• Onset: hours–days after bite.

• Symptoms: pain, erythema, swelling, itching, blistering.

• Progression: worsening redness, pus, fever (→ suspect secondary infection).

• Allergic history: prior insect sting reactions, anaphylaxis.
  

2. Physical Examination

• Local findings:

  • Red, swollen papule/wheal ± central punctum.

  • Pruritus vs pain.

  • Bullae/vesicles (e.g., rove beetle / แมลงก้นกระดก).

  • Necrosis or ulcer (rare, severe bites).

• Systemic signs:

  • Fever, lymphangitis → cellulitis.

  • Hypotension, urticaria, bronchospasm → anaphylaxis.
    

3. Rule Out Other Conditions

• Cellulitis from skin trauma.

• Allergic contact dermatitis.

• Necrotizing fasciitis (rapid progression, severe pain).

⚠️ No special lab test confirms “insect bite.” Labs only needed if infection/sepsis suspected (CBC, CRP, blood culture).

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⚕️ Management (When Insect Type Unknown)

Step 1. General Wound Care

• Wash with soap + water.

• Cold compress 10–15 min intervals.

• Avoid scratching → prevents secondary infection.
  

Step 2. Symptom Relief

• Topical low-potency steroid: Hydrocortisone 1% cream bid × 5–7 days.

  • (If severe inflammation → short course of Betamethasone 0.1% acceptable, but Hydrocortisone preferred in exams ✅).

• Oral antihistamine: Loratadine (10 mg) 1×1 po qd for itch.

• Analgesia: Paracetamol 500 mg q6h prn pain.
  

Step 3. Prevent/Treat Secondary Infection

• If mild, no pus → no antibiotics needed.

• If cellulitis/pus/rapid spread → start oral antibiotic:

  • Amoxicillin-clavulanate (875/125 mg) 1 tab bid pc × 7 days.

  • Alternative: Clindamycin 300 mg tid × 7 days (if penicillin allergy).
    

Step 4. Monitor for Red Flags

• Admit/IPD if:

  • Systemic signs (fever, hypotension, tachycardia).

  • Necrotic ulcer, spreading rapidly.

  • Anaphylaxis → give IM epinephrine (0.3–0.5 mg 1:1000), oxygen, IV fluids.

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📅 Follow-Up

• Recheck in 48 hours for spread or infection.

• Return immediately if fever, pus, necrosis, or systemic symptoms.

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📚 Guideline Sources

• Thai Clinical Practice Guidelines for Skin/Soft Tissue Infection.

• CDC: “Insect Bite and Sting Care” (2021).

• IDSA guidelines for Cellulitis (2014).
  

💡 Exam Pearls

• ❌ Do not use systemic corticosteroids unless anaphylaxis.

• ✅ Topical steroid + antihistamine = first-line for unknown insect bite.

• ✅ Antibiotics only for clear bacterial infection.