You open a question stem: fever, headache, altered mental status after a mosquito-heavy trip. CSF shows a viral pattern, and the CT is clean. You know it’s “viral encephalitis,” but the real score boost comes from doing one more step: using the vignette details to choose the right virus—and to eliminate every tempting distractor.
Tag: Microbiology > Virology
The Clinical Vignette (Classic Q-bank Style)
A 62-year-old man presents in late summer with 3 days of fever, severe headache, and progressive confusion. His family reports he has been spending evenings outdoors near a marsh. Exam shows disorientation and a mild tremor. No vesicular rash.
Lumbar puncture shows:
- Opening pressure: mildly elevated
- WBC: 110/µL (lymphocyte predominant)
- Protein: elevated
- Glucose: normal
MRI brain shows no focal temporal lobe abnormalities. EEG is nonspecific.
Most likely causative agent?
The Correct Answer: West Nile Virus (Arboviral Encephalitis)
Why West Nile fits best
This is arboviral encephalitis until proven otherwise because the stem screams:
- Seasonality: late summer/early fall
- Vector exposure: mosquitoes (marsh, evenings outdoors)
- Host risk: older age → higher risk of neuroinvasive disease
- Neuro clues: tremor (and often parkinsonian features), confusion
- CSF viral pattern: lymphocytic pleocytosis, ↑ protein, normal glucose
- No HSV temporal lobe MRI findings, no vesicular rash
High-yield West Nile facts (USMLE-friendly)
- Virus family: Flaviviridae (enveloped, +ssRNA)
- Transmission cycle: mosquito (often Culex) ↔ birds; humans are incidental hosts
- Clinical spectrum:
- Asymptomatic or mild febrile illness in most
- Neuroinvasive disease: meningitis, encephalitis, acute flaccid paralysis
- Neuroinvasive pearls:
- Tremor, myoclonus, parkinsonism
- Acute flaccid paralysis (anterior horn cell involvement) can mimic polio/GBS
- Diagnosis: West Nile IgM in CSF (or serum) is a common testable move
- Treatment: supportive (no specific antiviral routinely used)
Step-wise Approach to Arboviral Encephalitis Questions
When a stem hints mosquito/tick + neuro symptoms, rapidly sort by:
- Vector (mosquito vs tick)
- Geography (Northeast, Midwest, Southeast, etc.)
- Season (summer/fall = big clue)
- Signature feature (tremor/parkinsonism, hemorrhagic fever, biphasic illness, etc.)
- Host (older adults, immunocompromised)
Why Every Answer Choice Matters: Systematically Killing the Distractors
Below are common distractors that show up against West Nile/arboviral encephalitis. Learn the single detail that should make you cross each one out confidently.
Quick Table: Arboviruses & Common Viral Encephalitis Differentials
| Answer choice | Vector/setting | Hallmark clues | Why it’s wrong here |
|---|---|---|---|
| West Nile virus | Mosquito; late summer; older adults | Tremor/parkinsonism; acute flaccid paralysis; CSF lymphocytes | Best fit |
| HSV-1 | No vector; sporadic | Temporal lobe encephalitis, personality changes, seizures; RBCs in CSF | MRI lacks temporal lobe findings; mosquito exposure/seasonality points away |
| Eastern equine encephalitis (EEE) | Mosquito; Atlantic/Gulf coasts, Great Lakes | Severe encephalitis, high mortality | Could fit season/vector, but vignette pushes classic West Nile (older + tremor); EEE often more fulminant |
| St. Louis encephalitis | Mosquito; Midwest/South | Similar to West Nile | Possible, but less commonly tested vs West Nile; no distinguishing epidemiology provided |
| La Crosse encephalitis | Mosquito; Midwest/Appalachia | More common in children | Patient is 62 |
| Powassan virus | Tick; Northeast/Great Lakes | Encephalitis after tick bite | Stem is mosquito/marsh/evenings, not tick exposure |
| Rabies | Animal bite; bats | Hydrophobia, aerophobia, autonomic instability | No bite/exposure pattern; course doesn’t match |
| Enteroviruses (coxsackie/echo/polio) | Fecal-oral; summer | Viral meningitis; polio can cause paralysis | Exposure pattern doesn’t fit; tremor/parkinsonism points arboviral |
| VZV | Reactivation | Dermatomal rash; can cause encephalitis/vasculopathy | No rash; epidemiology less supportive |
Distractor Deep Dive (What Q-banks Want You to Say)
1) HSV-1 encephalitis
Why they want you to pick it: It’s the most famous encephalitis cause, and it’s deadly but treatable.
How to rule it out:
- Temporal lobe involvement is the giveaway:
- Aphasia, personality change, focal seizures
- MRI: hyperintensity in temporal lobes
- CSF can show RBCs due to hemorrhagic necrosis (not required but classic)
- No vector/seasonality needed
Key test/treatment nugget:
- Dx: CSF PCR
- Tx: IV acyclovir immediately if suspected
Why wrong here: No temporal lobe findings; clear mosquito-season story.
2) Eastern equine encephalitis (EEE)
Why it’s tempting: It’s also mosquito-borne and late-summer.
When to pick EEE:
- Geography: Atlantic/Gulf Coast + freshwater swamps
- Presentation can be rapidly progressive and severe
- High mortality; survivors often have significant neurologic deficits
Why wrong here: The vignette is more “typical arboviral encephalitis in an older adult” with tremor, a classic West Nile association. Q-banks usually make EEE feel more catastrophic.
3) St. Louis encephalitis
Why it’s tempting: It’s a flavivirus with a similar presentation.
When to pick it:
- Mosquito-borne in the Midwest/South
- Encephalitis in older adults
Why wrong here: With limited geographic specifics, USMLE-style questions tend to reward recognition of West Nile as the go-to mosquito encephalitis in the US, especially with movement findings (tremor/parkinsonism).
4) La Crosse encephalitis
Classic association: children with mosquito exposure, typically in the Midwest/Appalachia.
Why wrong here: Age mismatch is the point. La Crosse is “peds arboviral encephalitis” in many review resources and question banks.
5) Powassan virus
How it shows up: Encephalitis after a tick bite in the Northeast/Great Lakes.
Why wrong here: The vignette is mosquito-coded (marsh, evening outdoors, late summer). If they wanted Powassan, you’d get hiking/wooded exposure, tick attachment, or regional cues.
6) Rabies
High-yield rabies triad-ish clues:
- Animal bite (often bat exposure in US; “woke up with a bat in the room”)
- Prodrome → neurologic phase
- Hydrophobia, aerophobia, hypersalivation
Why wrong here: There’s no exposure history, and rabies isn’t a “viral CSF lymphocytic encephalitis after mosquitoes” story.
7) Enteroviruses (coxsackie/echovirus) and polio
When they’re right:
- Summer outbreaks, kids/young adults, viral meningitis
- Polio: asymmetric flaccid paralysis (anterior horn cells), typically unimmunized
Why wrong here: The stem emphasizes mosquito exposure and encephalitis with tremor. Also, West Nile can cause anterior horn cell disease too—but the vector/season/age steer you there.
8) VZV encephalitis
When to suspect:
- Immunocompromised or elderly with encephalitis + shingles
- Can cause vasculopathy → strokes
Why wrong here: No rash, and the vignette is built around mosquito transmission and seasonality.
High-Yield CSF Patterns (You’ll Use This Weekly)
| Condition | CSF WBC | Protein | Glucose | Extra clue |
|---|---|---|---|---|
| Viral meningitis/encephalitis (incl. arboviruses) | ↑ (lymphocytes) | ↑ | Normal | PCR/serology helps |
| Bacterial meningitis | ↑↑ (neutrophils) | ↑↑ | Low | Very ill, high opening pressure |
| TB/fungal | ↑ (lymphocytes) | ↑ | Low | Chronic course |
| HSV encephalitis | ↑ (lymphocytes) | ↑ | Normal | Temporal lobe, RBCs possible |
Exam-Day Takeaways (What to Memorize)
- West Nile = mosquito + late summer + older adult + encephalitis ± tremor/parkinsonism ± acute flaccid paralysis.
- HSV-1 = temporal lobe until proven otherwise; treat early with acyclovir.
- EEE = swampy mosquito encephalitis, very severe (high mortality).
- La Crosse = kids.
- Powassan = tick.
- For arboviruses: think supportive care + CSF IgM testing.