Differential Diagnosis for a 7-Month-Old with a Possible Spider Bite

• Single Most Likely Diagnosis
  • Insect bite reaction: This is the most common cause of a skin lesion in a child presenting with a possible spider bite. Most spider bites cause minor reactions and can be treated symptomatically.
• Other Likely Diagnoses
  • Cellulitis: A bacterial infection of the skin that can occur after a bite or scratch, presenting with redness, swelling, and warmth.
  • Folliculitis: An inflammation of the hair follicles, which can be caused by an insect bite or other skin irritation.
  • Contact dermatitis: A skin reaction that occurs after contact with an allergen or irritant, which could be related to the spider's venom or other environmental factors.
• Do Not Miss Diagnoses
  • Necrotic arachnidism (Brown Recluse spider bite): Although rare, a bite from a Brown Recluse spider can cause severe necrotic lesions and systemic symptoms.
  • Latrodectism (Black Widow spider bite): A bite from a Black Widow spider can cause severe pain, muscle cramps, and systemic symptoms, including abdominal pain and vomiting.
  • Sepsis: Any skin lesion can become infected and lead to sepsis, especially in young children, so it's crucial to monitor for signs of infection.
• Rare Diagnoses
  • Lyme disease: Although more commonly associated with tick bites, it's essential to consider in areas where Lyme disease is prevalent.
  • Erythema migrans: A rash that can occur after a tick bite, causing a distinctive "bull's-eye" lesion.
  • Other arachnid bites (e.g., Tarantula, Wolf Spider): While rare, bites from these spiders can cause unique symptoms and reactions.