Three days after a centipede bite, the patient has generalized itching and a urticarial rash; what is the likely diagnosis and appropriate treatment?

Diagnosis: Delayed Hypersensitivity Reaction (Cutaneous Systemic Reaction) to Centipede Bite

This patient is experiencing a delayed hypersensitivity reaction to centipede venom, manifesting as generalized urticaria and pruritus 3 days post-bite—treat immediately with oral antihistamines and consider a short course of oral corticosteroids.

Clinical Reasoning

The presentation of generalized itching and urticarial rash appearing 3 days after centipede bite, rather than at the bite site itself, indicates a systemic cutaneous reaction rather than a simple local reaction. This delayed hypersensitivity pattern is well-documented with centipede envenomation 1, 2. The key distinguishing feature is that symptoms are not contiguous with the bite site—this defines it as systemic rather than a large local reaction 3.

Centipede venom contains allergenic proteins that can trigger IgE-mediated responses 4, 5. While most centipede bites cause only local pain and swelling, delayed hypersensitivity reactions with generalized pruritus and recurrent swelling occurring days after the bite are recognized complications 1.

Immediate Treatment Algorithm

First-Line Therapy

• Oral H1-antihistamines (second-generation preferred):
  • Loratadine 10 mg daily, OR
  • Cetirizine 10 mg daily
  • These reduce itching and urticarial rash 6, 7

Second-Line (Add if inadequate response within 24-48 hours)

• Oral corticosteroids:
  • Prednisolone 40-50 mg daily for 3 days in adults 6, 7
  • Can extend to tapering course over 3-4 weeks if severe 6
  • This addresses the allergic inflammation driving the systemic reaction 3

Symptomatic Adjuncts

• Cold compresses for any residual local symptoms
• Oral analgesics if needed 3

Critical Assessment Points

Monitor for progression to anaphylaxis, though unlikely at 3 days post-bite. Warning signs include:

• Respiratory symptoms (dyspnea, throat tightness, wheezing)
• Cardiovascular symptoms (hypotension, dizziness)
• Gastrointestinal symptoms (nausea, vomiting, abdominal pain)
• Facial/tongue swelling 3, 2

If any of these develop, this becomes an anaphylactic emergency requiring intramuscular epinephrine 0.5 mL of 1:1000 (500 µg) and emergency department transport 3, 6.

Common Pitfalls to Avoid

1. Do NOT prescribe antibiotics unless clear evidence of secondary infection exists. The swelling and inflammation are from mediator release, not infection—antibiotics are commonly misprescribed in this scenario 3.

2. Do NOT dismiss as simple local reaction. Five patients in one case series re-attended specifically for delayed pruritus and recurrent swelling 1, indicating this is a recognized pattern requiring treatment.

3. Recognize cross-reactivity potential: Patients with centipede allergy may show positive IgE to hymenoptera (wasp/bee) venom, though the clinical significance is unclear 4. This doesn't change acute management but is relevant for future risk assessment.

Follow-Up Considerations

Consider prescribing an epinephrine auto-injector for future use if:

• Patient lives in centipede-endemic area with high re-exposure risk
• History suggests potential for more severe reactions
• Patient develops any respiratory or cardiovascular symptoms 3, 2

While most patients with purely cutaneous systemic reactions have low risk of life-threatening anaphylaxis 3, the decision should account for exposure risk and patient anxiety. Recent case reports document true anaphylaxis from centipede bites, though rare 2, 5.

Venom-specific IgE testing is not standardized for centipedes and not necessary for acute management 2. Focus on symptomatic treatment and patient education about avoiding future exposures and recognizing warning signs of anaphylaxis.

The prognosis is excellent with appropriate antihistamine therapy, with most delayed hypersensitivity reactions resolving within days to weeks 7, 1.