Differential Diagnosis for Recurrent Fevers

Given the patient's symptoms of recurrent fevers occurring once daily and the fact that they have been treated with doxycycline and Bactrim, the following differential diagnoses can be considered:

• Single Most Likely Diagnosis

  • Infectious Mononucleosis: This condition, caused by Epstein-Barr virus (EBV), can present with recurrent fevers, and the patient's lack of response to antibiotics like doxycycline and Bactrim (which are effective against bacterial infections) suggests a viral etiology. The daily fever pattern, although not specific, can be seen in the context of infectious mononucleosis.

• Other Likely Diagnoses

  • Tuberculosis (TB): TB can cause recurrent fevers and is not responsive to the antibiotics mentioned. It's a consideration, especially if the patient has been exposed to TB or has risk factors.
  • Brucellosis: This zoonotic infection can cause undulant fever, which is characterized by a daily spike in temperature. It's less common but should be considered, especially if the patient has a history of exposure to animals or their products.
  • Lymphoma: Certain types of lymphoma can present with recurrent fevers as part of the systemic symptoms. The lack of response to antibiotics and the presence of daily fevers could prompt further investigation into this possibility.

• Do Not Miss Diagnoses

  • Endocarditis: Infective endocarditis can cause recurrent fevers and is a life-threatening condition if not promptly treated. The fact that the patient has not responded to doxycycline and Bactrim does not rule out endocarditis, as it often requires specific antibiotic regimens.
  • Osteomyelitis or Abscess: A deep-seated infection such as osteomyelitis or an abscess can cause recurrent fevers. These conditions require drainage and appropriate antibiotic therapy, which might not have been adequately addressed by the previous treatments.
  • Malaria: If the patient has traveled to areas where malaria is endemic, this diagnosis must be considered, as it can cause recurrent fevers and is not responsive to doxycycline and Bactrim in the same way as bacterial infections.

• Rare Diagnoses

  • Relapsing Fever: Caused by Borrelia species, this condition is characterized by recurring episodes of fever. It's less common and typically associated with exposure to ticks or lice.
  • Cyclic Neutropenia: A rare condition where the patient experiences periodic drops in neutrophil count, leading to recurrent infections and fevers.
  • Familial Mediterranean Fever: An autosomal recessive disorder that causes recurrent episodes of fever, often accompanied by abdominal, chest, or joint pain. It's rare and more common in certain ethnic groups.

Each of these diagnoses requires careful consideration of the patient's full clinical picture, including travel history, exposure history, and additional symptoms, to guide further diagnostic testing and treatment.