Differential Diagnosis for a Red Lesion 2 Weeks After a Tick Bite

Single Most Likely Diagnosis

• Lyme Disease (Erythema Migrans): This is the most common diagnosis for a red lesion appearing after a tick bite. Erythema migrans, the characteristic rash of Lyme disease, typically appears within 3 to 30 days after the bite, often around 7-10 days. It starts as a small red spot that expands over time, sometimes clearing in the center to form a "bull's-eye" pattern.

Other Likely Diagnoses

• Southern Tick-Associated Rash Illness (STARI): This condition, also known as Masters disease, presents with a similar rash to erythema migrans but is not caused by the bacteria that cause Lyme disease. It is associated with the bite of the lone star tick.
• Tick-Borne Relapsing Fever: While this condition is more commonly associated with fever, headache, and muscle aches, a rash can occur. It's caused by various species of Borrelia spirochetes transmitted by soft ticks.
• Rocky Mountain Spotted Fever (RMSF): Although RMSF typically presents with fever, headache, and a characteristic petechial rash, the rash can sometimes appear as red spots or patches. It's transmitted by the American dog tick, Rocky Mountain wood tick, and the brown dog tick.

Do Not Miss Diagnoses

• Anaplasmosis: This bacterial infection, transmitted by the blacklegged tick and western blacklegged tick, can cause fever, headache, and sometimes a rash. It's crucial to diagnose promptly as it can lead to severe complications if not treated.
• Babesiosis: A parasitic disease transmitted by the blacklegged tick, babesiosis can cause hemolytic anemia, among other symptoms. While a rash is not a primary symptom, it can occasionally occur, making it a "do not miss" diagnosis due to its potential severity.
• Ehrlichiosis: Similar to anaplasmosis, ehrlichiosis is a bacterial infection that can cause fever, headache, and sometimes rash. It's transmitted by the lone star tick and the blacklegged tick.

Rare Diagnoses

• Tularemia: Also known as rabbit fever, tularemia is a rare infectious disease caused by Francisella tularensis. It can be transmitted by the bite of an infected tick and may cause a skin ulcer at the site of the bite, along with other systemic symptoms.
• Tick-Borne Encephalitis: This viral infection is primarily found in Europe and Asia and is transmitted by the bite of an infected tick. While rare, it can cause severe neurological symptoms and should be considered in travelers to endemic areas who have been bitten by ticks.