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Bites and stings

Specialties > Dermatology

Differential Diagnosis Schema 🧠

Local Reactions

  • Urticaria: Well-demarcated wheals, pruritic, often transient, typically resolve within 24 hours.
  • Angioedema: Localised, non-pitting swelling of subcutaneous tissues, usually in the face, lips, or oropharynx.
  • Cellulitis: Erythema, warmth, and tenderness spreading from the site of the bite or sting; may have systemic symptoms like fever.

Systemic Reactions

  • Anaphylaxis: Rapid onset of symptoms including urticaria, angioedema, respiratory distress, and hypotension; requires immediate treatment.
  • Toxic Reactions: Symptoms depend on the venom type; can include nausea, vomiting, dizziness, and muscle cramps.
  • Serum Sickness: Delayed reaction, occurring days to weeks post-exposure; fever, rash, arthralgia, and lymphadenopathy.

Infectious Complications

  • Lyme Disease: Erythema migrans (bullseye rash), flu-like symptoms, joint pain, associated with tick bites.
  • Rocky Mountain Spotted Fever: Fever, headache, and petechial rash starting on wrists and ankles; associated with tick bites.
  • Tetanus: Muscle stiffness and spasms, especially in the jaw, neck, and abdomen; consider if the wound is deep or contaminated.

Neurological Complications

  • Guillain-Barré Syndrome: Ascending muscle weakness and paralysis; associated with certain infections following a bite or sting.
  • Botulism: Descending flaccid paralysis, often with cranial nerve involvement; consider if exposure to toxin-producing species.
  • Encephalitis: Fever, headache, and altered mental status; may be seen with certain arthropod-borne viruses.

Key Points in History 🥼

Presenting Symptoms

  • Onset and timing: Immediate onset suggests an allergic or toxic reaction; delayed onset may suggest infection or serum sickness.
  • Location and appearance of lesion: Helps differentiate between local and systemic reactions; a bullseye rash is suggestive of Lyme disease.
  • Associated symptoms: Dyspnoea and hypotension suggest anaphylaxis; fever and malaise may suggest infection.
  • Pain severity: Intense pain may suggest a severe envenomation; less severe pain might suggest a mild local reaction.

Background

  • Past Medical History: Known allergies or previous severe reactions to bites or stings are significant.
  • Drug History: Immunosuppressants can increase the risk of infection; beta-blockers may mask the signs of anaphylaxis.
  • Family History: Atopic conditions may predispose individuals to severe allergic reactions.
  • Social History: Occupational or recreational exposure to certain environments may increase risk (e.g., hikers, gardeners).

Possible Investigations 🌡️

Blood Tests

  • Full Blood Count (FBC): Leukocytosis may indicate infection; eosinophilia may suggest a parasitic infection or allergy.
  • Urea and Electrolytes (U&E): To assess renal function, especially if systemic involvement or dehydration is suspected.
  • C-reactive protein (CRP): Raised levels may indicate an acute inflammatory or infectious process.

Imaging

  • Ultrasound: Useful in detecting retained foreign bodies or abscess formation at the site of the bite or sting.
  • X-ray: May be required if there’s suspicion of a foreign body, particularly in deep puncture wounds.
  • CT or MRI: Indicated if there is concern for more extensive tissue involvement or neurological complications.

Microbiological Tests

  • Wound Swab: For culture and sensitivity in cases of suspected secondary infection.
  • Blood Cultures: Should be considered in patients with systemic symptoms such as fever, indicating possible bacteraemia.
  • Serology: Useful in diagnosing specific infections like Lyme disease or other vector-borne illnesses.

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Dermatology