Which infections can cause fever and new‑onset confusion in an adult?

Infections Causing Fever and Confusion in Adults

The most critical infections to consider in adults presenting with fever and new-onset confusion are bacterial meningitis (particularly pneumococcal and meningococcal), viral encephalitis (especially HSV), and in specific contexts, cerebral malaria and other CNS infections—all require urgent hospital evaluation and empiric treatment before diagnostic confirmation.

Key Clinical Recognition

The combination of fever and altered mental status demands immediate action, as these patients can deteriorate rapidly. The "classic triad" of fever, neck stiffness, and altered consciousness appears in less than 50% of bacterial meningitis cases 1. Do not wait for complete symptom presentation before acting.

Priority Infections by Frequency and Severity

Central Nervous System Infections:

• Bacterial Meningitis - Most urgent, highest mortality

  • Streptococcus pneumoniae: Most common cause in adults; patients more likely to have seizures, focal neurologic signs, and reduced consciousness 1
  • Neisseria meningitidis: Presents with rapidly evolving rash (petechial in 89% when present), but 37% have no rash 1
  • Listeria monocytogenes: More common in elderly patients 1

• Viral Encephalitis 2, 3

  • HSV encephalitis: Requires immediate acyclovir
  • West Nile virus: Consider during summer/autumn with mosquito exposure history 4
  • Other viral causes: VZV, EBV, enteroviruses

• Meningoencephalitis: Combined features of both conditions 2, 5

Systemic Infections with CNS Manifestations:

• Pneumonia: 26% of febrile patients with altered consciousness in one tertiary center study 5
• Urinary tract infection: 22% in the same cohort, though the direct causative link between UTI and confusion remains poorly established 5, 6
• Typhoid fever: 14% of cases 5
• Cerebral malaria (Plasmodium falciparum): In endemic areas or with travel history 2

Parasitic/Protozoal CNS Infections 2:

• Toxoplasma gondii
• Trypanosoma species (African trypanosomiasis)
• Naegleria fowleri (primary amebic meningoencephalitis)

Other Considerations:

• Tuberculosis and tuberculoma 5
• Cytomegalovirus: Can cause hyperinflammatory state mimicking other conditions in immunocompetent adults 7

Critical Clinical Assessment Points

Document these specific findings 1:

• Fever presence and pattern
• Altered mental status severity (use GCS; score ≤12 requires ICU consideration)
• Headache characteristics
• Neck stiffness (though absence does not exclude meningitis)
• Rash type and evolution (petechial, purpuric, maculopapular)
• Seizure activity
• Focal neurologic deficits
• Signs of shock: hypotension, prolonged capillary refill, cold extremities

Age-specific presentations 1:

• Elderly patients: More likely to have altered consciousness, LESS likely to have neck stiffness or fever
• Young adults/adolescents: Higher risk for meningococcal disease; may maintain blood pressure despite severe shock

Essential history elements 1, 4:

• Recent mosquito bites (West Nile, other arboviruses)
• Travel history (malaria, trypanosomiasis)
• Source of infection: otitis media, sinusitis
• Contact with meningitis/sepsis cases
• Immunization status

Immediate Management Algorithm

All patients with suspected meningitis/encephalitis require 1:

1. Urgent hospital referral via emergency ambulance (goal: arrival within 1 hour)
2. Do NOT delay treatment for diagnostic procedures

Critical care involvement indicated for 1:

• Rapidly evolving rash
• GCS ≤12 or drop >2 points
• Cardiovascular instability
• Frequent/uncontrolled seizures
• Respiratory compromise
• Lactate >4 mmol/L (indicates cryptic shock even without hypotension) 1

Common Pitfalls to Avoid

1. Do not rely on Kernig's or Brudzinski's signs - sensitivity as low as 5%, though specificity up to 95% 1

2. Do not assume UTI is the cause of confusion in elderly patients - this association is poorly established and leads to overdiagnosis and inappropriate antibiotic use 6

3. Do not wait for complete triad - fever, neck stiffness, and altered consciousness together occur in <50% of cases 1

4. Young patients with meningococcal sepsis may appear deceptively stable - they maintain blood pressure until late, then deteriorate rapidly 1

5. Consider non-bacterial causes when initial workup is negative - viral encephalitis, parasitic infections, and even cytomegalovirus in immunocompetent hosts 4, 7, 3

Empiric Treatment Priority

For suspected bacterial meningitis 1:

• Dexamethasone 10 mg IV every 6 hours should be started on admission, either shortly before or simultaneously with antibiotics
• Continue for 4 days if pneumococcal meningitis confirmed
• Stop if another cause identified

For suspected viral encephalitis 2, 3:

• Acyclovir is strictly recommended for HSV encephalitis and should be started empirically while awaiting confirmation

The key principle: treat empirically based on clinical suspicion while pursuing diagnostic confirmation through lumbar puncture, neuroimaging, and appropriate microbiologic/serologic testing 2, 1.