Differential Diagnosis for a 55-year-old Man with Fever and Sudden Change of Behavior

Single Most Likely Diagnosis

• Typhoid Fever: The positive typhi dot test strongly suggests typhoid fever, which is caused by Salmonella Typhi. The sudden change in behavior could be indicative of typhoid state, a condition where patients with typhoid fever can present with altered mental status, including confusion, delirium, and even coma. Fever is a hallmark symptom.

Other Likely Diagnoses

• Viral Meningitis or Encephalitis: Although the laboratories are mostly normal, viral infections affecting the central nervous system can cause sudden changes in behavior and fever. Further testing, such as lumbar puncture, would be necessary to confirm.
• Urinary Tract Infection (UTI) or Sepsis: In older adults, UTIs can present with altered mental status and fever, even in the absence of typical urinary symptoms. Sepsis from any source can also cause changes in behavior.
• Intracranial Infections or Abscesses: These can cause fever and changes in mental status. Imaging studies would be required for diagnosis.

Do Not Miss Diagnoses

• Meningitis (Bacterial): This is a medical emergency that requires prompt treatment. The presentation can be subtle in older adults, with fewer typical symptoms such as neck stiffness and headache.
• Subarachnoid Hemorrhage or Stroke: While these conditions might not directly cause fever, they can lead to sudden changes in behavior. A stroke or hemorrhage should be considered, especially if there are focal neurological deficits.
• Sepsis from Any Source: Sepsis can cause altered mental status and fever. Identifying and treating the source promptly is crucial.

Rare Diagnoses

• Brain Parasitic Infections (e.g., Neurocysticercosis, Toxoplasmosis): These can cause a variety of neurological symptoms, including changes in behavior and fever, especially in immunocompromised patients.
• Whipple's Disease: A rare, systemic bacterial infection that can cause fever, weight loss, and changes in mental status, among other symptoms.
• CNS Vasculitis: Inflammation of the blood vessels in the brain can lead to a wide range of neurological symptoms, including changes in behavior and fever.

Each of these diagnoses requires careful consideration of the patient's full clinical picture, including history, physical examination, and further diagnostic testing as indicated. The positive typhi dot test, however, makes typhoid fever a compelling initial diagnosis to investigate further.