Differential Diagnosis

The patient's presentation of lethargy, decreased appetite, splenomegaly, and negative results for various tests (normal LP, normal Resp PCR, normal tick titers, normal MRI) suggests an infection that is not commonly covered by cephalosporin or doxycycline. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Brucellosis: This infection can cause lethargy, decreased appetite, and splenomegaly. It is not typically covered by cephalosporin or doxycycline, and diagnosis often requires specific serologic tests or culture, which might not have been mentioned.

• Other Likely Diagnoses

  • Tuberculosis (TB): Although TB can cause a wide range of symptoms including lethargy and decreased appetite, splenomegaly is less common. TB is not covered by cephalosporin or doxycycline, and diagnosis requires specific tests like the Mantoux test or PCR for TB.
  • Histoplasmosis: This fungal infection can cause systemic symptoms including lethargy, decreased appetite, and splenomegaly. It is not covered by cephalosporin or doxycycline, and diagnosis often requires serologic tests or culture.
  • Toxoplasmosis: In immunocompromised patients, toxoplasmosis can cause a range of symptoms including lethargy and splenomegaly. It is not typically treated with cephalosporin or doxycycline, but rather with antiparasitic medications.

• Do Not Miss Diagnoses

  • Meningococcemia: Although the LP is normal, meningococcemia can cause rapid deterioration and is not covered by doxycycline. It requires immediate treatment with appropriate antibiotics like penicillin or ceftriaxone.
  • Endocarditis: Infective endocarditis can present with systemic symptoms including lethargy, decreased appetite, and splenomegaly. It is crucial to diagnose and treat promptly with appropriate antibiotics, which may not include cephalosporin or doxycycline depending on the causative organism.

• Rare Diagnoses

  • Leishmaniasis: This parasitic infection can cause systemic symptoms including lethargy, decreased appetite, and splenomegaly. It is rare in many parts of the world and requires specific diagnosis and treatment not covered by cephalosporin or doxycycline.
  • Babesiosis: A rare tick-borne disease that can cause systemic symptoms. Although tick titers are normal, babesiosis can be missed if not specifically tested for, and it requires different treatment than what is typically used for tick-borne illnesses covered by doxycycline.