Differential Diagnosis for Insect Bite with Swelling and Erythematous Area

• Single most likely diagnosis:
  • Insect bite reaction (e.g., mosquito, flea, or ant bite): This is the most common cause of a localized erythematous reaction with swelling following an insect bite.
• Other Likely diagnoses:
  • Cellulitis: A bacterial infection of the skin that can cause erythema, swelling, and warmth, potentially following an insect bite that becomes secondarily infected.
  • Contact dermatitis: An allergic or irritant reaction to substances like insect repellents, soap, or plant allergens that could cause erythema and swelling.
  • Urticaria: Hives can sometimes be caused by insect bites and can present with erythematous, swollen lesions.
• Do Not Miss diagnoses:
  • Anaphylaxis: Although rare, anaphylactic reactions to insect bites (e.g., bee stings) can be life-threatening and require immediate medical attention. Symptoms can include widespread erythema, swelling, difficulty breathing, and cardiovascular collapse.
  • Necrotizing fasciitis: A severe bacterial infection that can start with a minor skin injury, including an insect bite, and rapidly progress to cause severe damage to tissue.
• Rare diagnoses:
  • Erythema migrans (Lyme disease): Caused by the bite of an infected tick, it can present with a characteristic "bull's-eye" rash, although not all cases have this appearance.
  • Leishmaniasis: Transmitted by the bite of an infected sandfly, it can cause skin lesions that may resemble an insect bite reaction but can progress to more serious disease.
  • Tularemia: Also known as rabbit fever, it is caused by the bacterium Francisella tularensis and can be transmitted by the bite of an infected tick or deer fly, presenting with a skin ulcer and swollen lymph nodes.