🟢 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

🩺 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

⚕️ 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)

🔴 Red Flag Signs (MUST MONITOR)

👉 Return immediately if:

Fever
Rapidly spreading redness
Pus / abscess
Necrosis / ulcer
Hypotension / dyspnea

📅 Follow-Up

Re-evaluate in 48 hours
Assess:
- Spread of erythema
- Response to treatment

📚 Guideline References

IDSA Skin & Soft Tissue Infection (2014)
Thai CPG: Cellulitis
CDC Insect Bite & Sting Care (2021)

🎯 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

🧠 Final Clinical Insight

👉 Always ask:

“Is this allergic reaction or infection?”

Allergic → antihistamine + steroid
Infection → antibiotic
Severe allergy → epinephrine

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)