Differential Diagnosis for a 50-year-old with Skin Lesions, Fever, and "Brain Fog" after Tick Bites

Single Most Likely Diagnosis

• Anaplasmosis: This diagnosis is the most likely due to the high titer of A. phagocytophilum IgG (1:128), which indicates a recent or current infection. The patient's symptoms of fever, "brain fog," and history of tick bites 6 weeks prior are consistent with anaplasmosis. The fact that the patient has been treated with doxycycline, which is the recommended treatment for anaplasmosis, further supports this diagnosis.

Other Likely Diagnoses

• Lyme Disease: Although the patient is being treated with doxycycline, which is also used for Lyme disease, the high titer for A. phagocytophilum IgG points more towards anaplasmosis. However, Lyme disease can present with similar symptoms and should be considered, especially given the tick bite history.
• Southern Tick-Associated Rash Illness (STARI): This condition, caused by the bite of the lone star tick, can present with a rash and systemic symptoms like fever. However, the presence of "brain fog" and the specific serological findings make anaplasmosis more likely.
• Ehrlichiosis: Another tick-borne illness that can present with fever and systemic symptoms. The differentiation between ehrlichiosis and anaplasmosis can be challenging without specific diagnostic tests, but the high titer for A. phagocytophilum IgG supports anaplasmosis.

Do Not Miss Diagnoses

• Rocky Mountain Spotted Fever (RMSF): Although less likely given the serological results, RMSF is a potentially deadly disease that can present with fever, headache, and rash after a tick bite. It requires prompt treatment with doxycycline, which the patient is already receiving.
• Babesiosis: A parasitic disease transmitted by ticks, babesiosis can cause fever, fatigue, and hemolytic anemia. It's more common in certain geographic areas and in individuals with splenectomy or immunocompromised states.

Rare Diagnoses

• Tick-Borne Relapsing Fever (TBRF): Caused by various Borrelia species, TBRF is characterized by recurring episodes of fever and can be accompanied by headache and rash. It's less common and typically associated with soft ticks.
• Powassan Virus Disease: A rare but potentially severe tick-borne viral disease that can cause encephalitis or meningitis. The presentation can include fever, headache, and altered mental status, but it is much less common than anaplasmosis or Lyme disease.