In a 78-year-old farmer presenting in late August with meningoencephalitis, CSF reveals lymphocytic pleocytosis, normal CSF glucose relative to serum, and mildly elevated protein; which pathogen is most likely responsible for the meningitis: Streptococcus pneumoniae, Neisseria meningitidis, West Nile virus, Dengue virus, or none of the above?

West Nile Virus (Answer: C)

The most likely pathogen is West Nile virus (WNV), given the combination of late summer presentation, elderly farmer (outdoor occupation with mosquito exposure), meningoencephalitis, lymphocytic CSF pleocytosis with normal glucose ratio, and mildly elevated protein.

Clinical Reasoning Algorithm

The CSF profile and epidemiological context systematically exclude bacterial causes and point toward arboviral infection:

Why NOT Pneumococcus or Meningococcus (A & B):

• Bacterial meningitis typically shows:
  • Neutrophilic predominance (not lymphocytic)
  • Low CSF glucose (<2/3 serum glucose ratio)
  • Very high protein (often >100 mg/dL)
  • This patient has CSF glucose 75 mg/dL with serum 95 mg/dL = 79% ratio (normal)
  • Lymphocytic predominance rules out typical bacterial infection 1

Why West Nile Virus (C) is CORRECT:

Epidemiological factors:

• Late August timing = peak WNV season (summer/fall arbovirus transmission) 2
• Farmer occupation = outdoor mosquito exposure risk
• Age 78 years = high-risk demographic for neuroinvasive WNV disease

CSF profile matches WNV perfectly:

• Lymphocytic pleocytosis (300 cells) is characteristic; mean WNV cases show 226-227 cells/mm³ 3
• Normal CSF:serum glucose ratio (79%) is typical of viral CNS infections 1
• Mildly elevated protein (50 mg/dL) fits WNV pattern; 47% of WNV encephalitis cases have protein ≥100 mg/dL, but many have lower values 3
• Lymphocytic predominance occurs in 94% of WNV cases, though 37-45% may show neutrophilic predominance early 3, 4

Guidelines explicitly recommend: Summer/fall presentations with meningoencephalitis warrant arbovirus testing, particularly WNV 2

Why NOT Dengue (D):

While dengue can cause meningitis with lymphocytic pleocytosis 5, 6, it is:

• Geographically implausible for most North American/European settings where this clinical scenario typically occurs
• Dengue meningitis is rare as initial presentation 5
• Thrombocytopenia is a key diagnostic clue for dengue that is not mentioned here 5
• The question stem provides no travel history to dengue-endemic regions

Critical Diagnostic Pitfalls

Common WNV diagnostic errors:

• Only 25% of providers follow guidelines correctly; 73% inappropriately order only CSF RT-PCR instead of CSF IgM antibody testing 7
• Correct test: CSF WNV-specific IgM (sensitivity/specificity >95%) 1
• Avoid: Sole reliance on CSF RT-PCR in immunocompetent patients (low sensitivity) 7

Don't be misled by:

• Neutrophilic predominance early in WNV (can occur in 37-45% of cases) 3, 4
• Normal CSF in 5% of WNV encephalitis cases 3
• The Gram stain being "pending" - this is a red herring; bacterial causes are already excluded by the glucose ratio

Immediate Management Implications

While awaiting WNV serology:

• No specific antiviral therapy exists for WNV
• Supportive care focused on preventing complications in this elderly patient
• Monitor for acute flaccid paralysis (WNV-associated complication) 2
• Age is an independent predictor of poor outcome beyond CSF parameters 3