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)

🟢 Hydrocortisone 1% + Antihistamine = first-line treatment 🟡 Add antibiotics ONLY if infection (cellulitis, pus) 🔴 Epinephrine IM = first-line for anaphylaxis

💊 Treatment Summary Table

Category	Drug	Dose & Route	Frequency	Duration	Notes
Topical steroid (first-line)	Hydrocortisone 1% cream	Apply thin layer	bid	5–7 days	✅ Safe, exam-preferred
Topical steroid (severe inflammation)	Betamethasone 0.1% cream	Thin layer	bid	≤5 days	Short course only
Antihistamine (oral)	Loratadine	10 mg po	od	prn (≤7–10 days)	Non-sedating
	Cetirizine	10 mg po	od	prn	Alternative
Analgesic	Paracetamol	500–1000 mg po	q6h prn	As needed	Max 4 g/day

🦠 Antibiotic (ONLY if infected insect bite)

Drug	Dose & Route	Frequency	Duration	Notes
Dicloxacillin ✅	500 mg po	1×4 ac	5–7 days	⭐ First-line (MSSA + Strep)
Cephalexin	500 mg po	1×4	5–7 days	Alternative
Amoxicillin-clavulanate	875/125 mg po	1×2 pc	5–7 days	Use if contaminated / polymicrobial risk
Clindamycin	300 mg po	1×3 pc	5–7 days	Penicillin allergy

🚨 Severe Allergy / Anaphylaxis

Drug	Dose	Frequency	Notes
Epinephrine (IM 1:1000)	0.3–0.5 mg IM	Repeat q5–15 min	⭐ First-line
Prednisolone (adjunct)	40–60 mg po/iv	od × 3–5 days	Not first-line

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🩺 Diagnosis (Clinical — NO routine labs)

1. History

• Recent exposure (outdoor/indoor)

• Onset: hours–days after bite

• Symptoms:

  • Itching (histamine reaction)

  • Pain, swelling, erythema

  • Blister (rove beetle)

• Red flags:

  • Increasing redness, pus → infection

  • Dyspnea, urticaria → anaphylaxis
    

2. Physical Examination

Local findings:

• Erythematous papule/wheal ± central punctum

• Pruritus > pain → allergic reaction

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

• Necrosis (rare, severe)

Systemic findings:

• Fever, lymphangitis → cellulitis

• Hypotension, wheezing → anaphylaxis
  

3. Differential Diagnosis

• Cellulitis (bacterial)

• Allergic contact dermatitis

• Necrotizing fasciitis (⚠️ severe pain, rapid progression)
  

🧪 Investigation

❌ No test confirms “insect bite.”

Only if severe:

• CBC, CRP → infection

• Blood culture → sepsis suspicion

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⚕️ Management (Unknown Insect Bite)

✅ Step 1. General Care

• Wash with soap + water

• Cold compress (10–15 min)

• Avoid scratching ❗ (prevent infection)
  

✅ Step 2. Symptom Relief (FIRST-LINE)

• Hydrocortisone 1% cream bid × 5–7 days

• Loratadine (10 mg) 1×1 po od

• Paracetamol (500 mg) q6h prn

👉 Exam answer = topical steroid + antihistamine

⚠️ Step 3. Treat Infection (ONLY if present)

❌ No infection:

→ NO antibiotic (RDU principle)

✅ Mild cellulitis:

• Dicloxacillin (500 mg) 1×4 po ac × 5–7 days

⚠️ Complicated / contaminated wound:

• Amoxicillin-clavulanate (875/125 mg) 1×2 po pc × 5–7 days

❗ Penicillin allergy:

• Clindamycin (300 mg) 1×3 po × 5–7 days
  

🚨 Step 4. Emergency (Anaphylaxis)

• Epinephrine IM immediately

• Oxygen + IV fluids

• Admit (IPD)

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🔴 Red Flag Signs (MUST MONITOR)

👉 Return immediately if:

• Fever

• Rapidly spreading redness

• Pus / abscess

• Necrosis / ulcer

• Hypotension / dyspnea

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📅 Follow-Up

• Re-evaluate in 48 hours

• Assess:

  • Spread of erythema

  • Response to treatment

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📚 Guideline References

• IDSA Skin & Soft Tissue Infection (2014)

• Thai CPG: Cellulitis

• CDC Insect Bite & Sting Care (2021)

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🎯 Exam Pearls

✅ Topical steroid + antihistamine = first-line ✅ Dicloxacillin = first-line antibiotic (if cellulitis) ❌ Do NOT give antibiotics routinely ❌ Do NOT use systemic steroids unless anaphylaxis

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🧠 Final Clinical Insight

👉 Always ask:

“Is this allergic reaction or infection?”

• Allergic → antihistamine + steroid

• Infection → antibiotic

• Severe allergy → epinephrine