Differential Diagnosis for Neck Stiffness and Erythema Migrans

• Single Most Likely Diagnosis

  • Lyme disease: This is the most likely diagnosis given the combination of neck stiffness and erythema migrans, which is a characteristic rash of Lyme disease. Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted to humans through the bite of infected blacklegged ticks.

• Other Likely Diagnoses

  • Southern Tick-Associated Rash Illness (STARI): This condition presents with a similar rash to erythema migrans but is caused by a different tick bite. It's less common than Lyme disease but should be considered in the differential diagnosis.
  • Rocky Mountain Spotted Fever (RMSF): Although RMSF typically presents with a different type of rash, it can sometimes cause a similar rash to erythema migrans. RMSF is a serious disease caused by Rickettsia rickettsii and is also transmitted by ticks.

• Do Not Miss Diagnoses

  • Meningitis: While less likely, meningitis (infection of the membranes covering the brain and spinal cord) can present with neck stiffness and a rash. It's crucial to consider this diagnosis due to its high morbidity and mortality if not promptly treated.
  • Tick-Borne Relapsing Fever (TBRF): This disease, caused by various Borrelia species, can present with fever, headache, and a rash, and is transmitted by soft ticks. It's less common but can be severe if not treated.

• Rare Diagnoses

  • Tularemia: Caused by Francisella tularensis, tularemia can present with a rash and systemic symptoms, including neck stiffness, after the bite of an infected tick. It's rare but should be considered in endemic areas.
  • Anaplasmosis: This tick-borne disease, caused by Anaplasma phagocytophilum, can present with fever, headache, and rash, but neck stiffness is less commonly reported. It's a rare consideration but important in areas with high tick activity.