Differential Diagnosis for Tick Bite Reaction

• Single most likely diagnosis
  • Localized hypersensitivity reaction: This is the most likely diagnosis given the symptoms of redness, raised skin, itchiness, and a central open lesion at the site of the tick bite. The reaction is likely due to the body's immune response to the tick's saliva or other substances introduced during the bite.
• Other Likely diagnoses
  • Infection at the bite site: Although the patient took doxycycline, which is commonly used to prevent Lyme disease, there is still a possibility of infection, especially if the tick was infected with another type of bacteria.
  • Contact dermatitis: The patient may have developed contact dermatitis due to the tick's saliva or other substances, which could cause an allergic reaction.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Lyme disease: Although the patient took doxycycline, it is essential to consider Lyme disease, especially if the tick was infected. The absence of a bullseye rash does not rule out Lyme disease, as not all patients develop this characteristic rash.
  • Southern tick-associated rash illness (STARI): This condition is similar to Lyme disease but is caused by a different tick-borne illness. It can cause a rash similar to the one described, although it is typically smaller than the bullseye rash associated with Lyme disease.
  • Rocky Mountain spotted fever (RMSF): This is a tick-borne illness that can cause a rash, although it is typically more widespread than the localized reaction described. RMSF can be deadly if left untreated, making it a crucial diagnosis not to miss.
• Rare diagnoses
  • Ehrlichiosis: This is a tick-borne illness that can cause a rash, although it is less common than other tick-borne diseases. Ehrlichiosis can be severe, especially in people with weakened immune systems.
  • Anaplasmosis: Similar to ehrlichiosis, anaplasmosis is a tick-borne illness that can cause a rash, although it is relatively rare. It can be severe and even life-threatening if left untreated.