Differential Diagnosis for a Single Papular Rash from an Insect Bite

• Single Most Likely Diagnosis
  • Insect bite reaction: This is the most straightforward diagnosis given the presentation of a single papular rash following an insect bite. The body's immune response to the saliva or venom from the insect bite can cause such a reaction.
• Other Likely Diagnoses
  • Contact dermatitis: This could be considered if the rash appears after exposure to a new substance or plant, aside from the insect bite itself.
  • Folliculitis: If the rash is centered around a hair follicle, it could be folliculitis, an inflammation of the follicle often caused by bacterial infection.
  • Urticaria (hives): While typically more widespread, a single large hive could be mistaken for a papular rash and can be caused by an allergic reaction, including to insect bites.
• Do Not Miss Diagnoses
  • Erythema migrans (Lyme disease): Although less common, a single papular rash can be the initial presentation of Lyme disease, transmitted by a tick bite. Missing this diagnosis could lead to serious long-term health issues.
  • Southern tick-associated rash illness (STARI): Similar to erythema migrans, STARI presents with a rash and is associated with the bite of the lone star tick. While its long-term effects are less clear than Lyme disease, it's a consideration in endemic areas.
• Rare Diagnoses
  • Tularemia: Caused by the bacterium Francisella tularensis, tularemia can be transmitted by the bite of an infected tick or deer fly, leading to a skin ulcer or papule at the site of the bite.
  • Rickettsialpox: This is caused by the bacterium Rickettsia akari, transmitted by the bite of an infected mite. It presents with a vesicle or papule at the bite site, followed by a rash.