Differential Diagnosis for a Tick on the Scalp

• Single most likely diagnosis
  • Tick bite: This is the most straightforward diagnosis, given the presence of a tick on the scalp. The tick could be attached and feeding, or it could have already detached, leaving behind a bite mark.
• Other Likely diagnoses
  • Lyme disease: If the tick is identified as a black-legged tick (Ixodes scapularis) or another species known to transmit Lyme disease, this diagnosis becomes more likely, especially if the patient is in an endemic area.
  • Southern tick-associated rash illness (STARI): This condition, also known as Masters disease, is associated with the bite of the lone star tick (Amblyomma americanum) and presents with a rash similar to that of Lyme disease.
  • Ehrlichiosis/Anaplasmosis: These are bacterial infections transmitted by ticks, with symptoms that can include fever, headache, and rash, making them plausible diagnoses if the patient is showing such symptoms after a tick bite.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Rocky Mountain spotted fever (RMSF): Although less common, RMSF is a potentially fatal disease transmitted by ticks. Early recognition and treatment are crucial, making it a "do not miss" diagnosis.
  • Tularemia: This bacterial infection, also known as rabbit fever, can be transmitted by tick bites and has a high mortality rate if not treated promptly.
  • Babesiosis: A parasitic disease transmitted by ticks, babesiosis can be severe, especially in individuals without a spleen or with weakened immune systems.
• Rare diagnoses
  • Tick paralysis: A rare but potentially life-threatening condition caused by a toxin in the saliva of certain ticks, leading to ascending paralysis.
  • Powassan virus disease: A rare but serious tick-borne illness that can cause encephalitis and meningitis, with a high mortality rate.
  • Relapsing fever: Transmitted by soft ticks, this disease is characterized by recurring episodes of fever and can be severe if not treated properly.

Each of these diagnoses has a brief justification based on the clinical presentation of a tick on the scalp and the potential diseases that ticks can transmit. The likelihood and severity of each condition vary, but all should be considered in the differential diagnosis to ensure proper management and treatment.