In a 73-year-old male with a recent cat-scratch on the right leg, mild leg swelling, leukocytosis with neutrophilia, mild bilateral pneumonia with minimal effusion, altered mental status, normal cranial CT, normal electrolytes and liver enzymes, and mildly elevated serum creatinine, what is the most likely diagnosis and appropriate management?

Probable Diagnosis: Cat-Scratch Disease (Bartonella henselae) with Systemic Complications

This 73-year-old male most likely has disseminated cat-scratch disease (CSD) with pneumonia, altered mental status, and acute kidney injury, requiring immediate empiric antibiotic therapy with azithromycin or doxycycline plus rifampin while awaiting serologic confirmation.

Clinical Reasoning

The constellation of findings strongly suggests systemic Bartonella henselae infection:

• Cat scratch 3 weeks ago with minimal leg swelling (likely regional lymphadenopathy)
• Bilateral pneumonia with effusion - atypical presentation of CSD 1, 2
• Altered sensorium with normal CT - neurological CSD can present with encephalopathy, delirium, or seizures 1, 3, 4
• Leukocytosis with neutrophilia (WBC 14,000, neutrophils 93%) - consistent with bacterial infection
• Elevated creatinine (132 μmol/L) - suggests systemic involvement with acute kidney injury

While CSD typically presents as self-limited lymphadenopathy in immunocompetent hosts 5, disseminated disease with hepatosplenic involvement, pneumonia, and neurological manifestations occurs in approximately 5-10% of cases 3, 4. The neuropsychiatric symptoms (altered sensorium) with normal cranial CT is particularly characteristic of neurological CSD, which can manifest as encephalopathy without focal findings 1, 4.

Immediate Management Algorithm

1. Diagnostic Workup (Urgent)

• Blood cultures - obtain immediately before antibiotics
• Bartonella henselae serology - IgG titers >1:256 are diagnostic 5
• Metagenomic NGS of blood if available - can detect B. henselae 1, 2
• Lumbar puncture - perform immediately given altered mental status with normal CT (no contraindications per meningitis guidelines) 6
  • CSF for cell count, protein, glucose, Gram stain, culture
  • Consider CSF PCR for Bartonella if available
• Repeat chest imaging to assess for complications (empyema, abscess) 7
• Renal function monitoring - trend creatinine, check urinalysis

2. Empiric Antibiotic Therapy (Start Immediately)

Primary regimen for severe disseminated CSD:

• Azithromycin 500 mg IV/PO day 1, then 250 mg daily 3, 5
  • OR Doxycycline 100 mg IV/PO twice daily 3, 4
• PLUS Rifampin 300 mg PO twice daily 3

Rationale: Azithromycin has the best evidence for CSD (one controlled trial showing faster lymph node resolution) 3, 5. Rifampin shows greatest clinical efficacy in severe cases 3. Doxycycline is equally effective and appropriate for atypical pneumonia coverage 4, 8.

Duration: 4-6 weeks for disseminated disease with neurological involvement 3, 4

3. Concurrent Pneumonia Coverage

Given bilateral pneumonia and inability to exclude other pathogens initially:

• Add Ceftriaxone 2g IV daily to cover typical community-acquired pneumonia pathogens 7
• This provides coverage for Streptococcus pneumoniae, Legionella, and other bacterial causes while awaiting cultures
• Can narrow once Bartonella confirmed and other pathogens excluded

4. Supportive Care

• Fluid resuscitation - cautious 500 mL crystalloid boluses given elevated creatinine 6
• Monitor for sepsis - check lactate, follow vital signs closely 6
• Neurological monitoring - serial GCS assessments given altered sensorium 6

5. Specialist Consultation

• Infectious disease consultation - improves outcomes in severe infections 6
• Consider ICU transfer if GCS drops ≤12 or develops respiratory compromise 6

Important Caveats

Do NOT delay antibiotics for diagnostic testing. In a patient with altered mental status, leukocytosis, pneumonia, and recent cat exposure, empiric therapy must begin immediately after blood cultures are drawn 6, 3.

Lumbar puncture is safe and indicated here. The patient has altered sensorium but normal CT and no focal neurological signs, papilledema, or seizures - therefore LP should be performed urgently 6. Neurological CSD can cause aseptic meningitis or encephalitis that requires CSF analysis for diagnosis 1, 4.

Watch for treatment failure. If no improvement within 72 hours, consider bronchoscopy to exclude resistant pathogens, fungal infection, or alternative diagnoses 7. The elevated creatinine and bilateral effusions raise concern for potential complications requiring drainage.

Alternative diagnoses to exclude:

• Pasteurella multocida (cat bite/scratch) - but typically causes local wound infection 9
• Atypical pneumonia (Legionella, Mycoplasma) - covered by azithromycin/doxycycline 8
• Bacterial meningitis - LP will clarify 6
• Septic emboli from endocarditis - blood cultures critical

The combination of cat exposure, systemic symptoms, pneumonia, and neurological changes makes disseminated CSD the unifying diagnosis requiring immediate targeted therapy 1, 2, 3, 5, 4.