Differential Diagnosis for Spider Bite Red and Inflamed

• Single Most Likely Diagnosis
  • Cellulitis: This is the most likely diagnosis due to the redness and inflammation, which are common symptoms of a bacterial infection of the skin and subcutaneous tissues, potentially resulting from a spider bite.
• Other Likely Diagnoses
  • Spider bite reaction: Many spider bites can cause a local reaction that includes redness, swelling, and inflammation, making this a plausible diagnosis.
  • Insect bite reaction: Other insects like mosquitoes, fleas, or ticks can also cause similar reactions, making this a consideration.
  • Folliculitis: An infection of the hair follicles, which could be secondary to the bite, is another possible diagnosis.
• Do Not Miss Diagnoses
  • Necrotic arachnidism (Brown Recluse Spider Bite): Although less common, a bite from a brown recluse spider can cause necrotic lesions and severe systemic symptoms, making it crucial not to miss this diagnosis.
  • Sepsis: If the bite becomes infected and the infection spreads, it could lead to sepsis, a life-threatening condition that requires immediate medical attention.
• Rare Diagnoses
  • Erythema migrans (Lyme disease): While more commonly associated with tick bites, it's a rare consideration if the spider bite is in an area where Lyme disease is prevalent and the patient has been exposed to ticks.
  • Leishmaniasis: In endemic areas, sand fly bites can cause leishmaniasis, which might present with skin lesions, although this is quite rare and more associated with specific geographic locations.