Differential Diagnosis for Bullseye Rash

• Single most likely diagnosis
  • Lyme disease: This is the most common cause of a bullseye rash, also known as erythema migrans. It is a tick-borne illness caused by the bacterium Borrelia burgdorferi. The rash is often accompanied by flu-like symptoms and can appear within a week of a tick bite.
• Other Likely diagnoses
  • Southern tick-associated rash illness (STARI): This condition is caused by the bite of the lone star tick and can also produce a bullseye rash. It is similar to Lyme disease but is not caused by the same bacterium.
  • Ehrlichiosis: This is a bacterial infection caused by the bite of an infected tick. It can cause a rash, fever, and other symptoms, although the rash is not always a bullseye pattern.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Rocky Mountain spotted fever (RMSF): This is a tick-borne illness that can cause a rash, fever, and other symptoms. If left untreated, it can be fatal. The rash may not always be a bullseye pattern, but it is a critical diagnosis to consider.
  • Tularemia: This is a rare bacterial infection caused by the bite of an infected tick or deer fly. It can cause a rash, fever, and other symptoms, and can be life-threatening if left untreated.
• Rare diagnoses
  • Tick-borne relapsing fever (TBRF): This is a rare bacterial infection caused by the bite of an infected tick. It can cause a rash, fever, and other symptoms, although the rash is not always a bullseye pattern.
  • African tick-bite fever: This is a rare rickettsial infection caused by the bite of an infected tick. It can cause a rash, fever, and other symptoms, although it is typically seen in travelers to sub-Saharan Africa.