West Nile Virus

West Nile virus (WNV) is a mosquito-borne flavivirus that can cause a self-limited fever or, less commonly, neuroinvasive disease such as meningitis, encephalitis, or acute flaccid paralysis. Illness often appears in late summer and early fall, but exposures vary by region and travel. Because symptoms overlap with dengue, Zika, St. Louis encephalitis, influenza, and bacterial meningitis, laboratory testing is essential to confirm or rule out WNV.

A proactive testing plan starts with antibody testing (serum and, when indicated, CSF IgM) and adds confirmatory neutralization when cross-reactivity is possible. NAAT/PCR can help early in illness or in immunocompromisedpatients. These tests support screening, diagnostic work-ups, and monitoring, but they do not replace a clinician’s exam, neurologic assessment, or urgent care for severe symptoms.

Signs, Symptoms & Related Situations

• Uncomplicated febrile illness: fever, headache, fatigue, muscle aches, back pain, loss of appetite; sometimes a morbilliform rash

• Neuroinvasive clues: severe headache, stiff neck, photophobia, confusion, seizures, focal weakness, flaccid paralysis, tremor

• Systemic/context: nausea or vomiting, mild liver-enzyme elevation; older age and immune suppression increase risk for severe disease

• Exposure & season: mosquito bites (outdoors, dusk/dawn), regional outbreaks, recent travel to endemic areas, transfusion or transplant history (rare transmission)

• When to seek urgent care: rapidly worsening headache/fever with neck stiffness, confusion, trouble speaking, one-sided weakness, seizures, breathing difficulty, or dehydration
  Symptoms always require clinician evaluation.

Why These Tests Matter

What testing can do

• Confirm recent infection with serum or CSF IgM, and differentiate WNV from related flaviviruses using neutralization assays

• Support triage of neuroinvasive disease when CSF IgM is positive and routine CSF indices suggest viral meningitis/encephalitis

• Provide early evidence with PCR/NAAT in selected settings (very early illness or immunocompromise)

What testing cannot do

• Exclude WNV from a single early negative result—antibodies take time to appear; repeat testing may be needed

• Prove immunity or “cure”; IgG can persist for years and does not indicate active infection

• Replace clinician-directed CSF studies, imaging, or public-health reporting when required

What These Tests Measure (at a glance)

• WNV IgM (serum): appears about 3–8 days after onset; supports recent infection. May persist for months—interpret with timing and symptoms.

• WNV IgM (CSF): indicates intrathecal antibody and strongly supports neuroinvasive disease; rarely positive from passive transfer.

• WNV IgG (serum): indicates past exposure; not specific for timing or activity.

• Neutralizing Antibody (PRNT): confirmatory test to resolve cross-reactivity with other flaviviruses (e.g., St. Louis encephalitis, dengue, Zika, Japanese encephalitis).

• WNV NAAT/PCR (serum/CSF/whole blood): detects viral RNA early; sensitivity is higher in immunocompromised patients and early viremia; often negative after the first week.

• Context labs: CBC (leukopenia, lymphocytosis), CMP (AST/ALT), CSF cell count/protein/glucose for meningitis/encephalitis patterns; add arbovirus differentials when travel or clinical clues suggest.

Quick Build Guide

How the Testing Process Works

1. Select your pathway: start with serum WNV IgM for febrile illness; add CSF IgM when neuroinvasive disease is suspected.

2. Timing matters: very early illness may have negative serology—plan a repeat draw in 7–10 days if suspicion remains.

3. Provide samples: blood draw for serum tests; lumbar puncture and CSF testing are clinician-directed.

4. Confirm when needed: request neutralization (PRNT) if there is potential flavivirus cross-reactivity or results are equivocal.

5. Review & report: discuss results with your clinician; some positives require public-health notification.

Interpreting Results (General Guidance)

• Serum IgM positive ± CSF IgM positive: consistent with recent WNV infection (neuroinvasive if CSF is positive); consider PRNT if epidemiology suggests other flaviviruses.

• IgG positive, IgM negative: indicates past exposure; not evidence of current infection.

• All antibodies negative, early illness: does not rule out WNV—repeat serology in 7–10 days or consider NAAT/PCR if very early or immunocompromised.

• PCR positive: supports acute infection, especially early or in immunocompromised patients.

• Equivocal/cross-reactive serology: use PRNT to distinguish WNV from other flaviviruses.
  Always interpret results with a qualified healthcare professional; patterns, timing, and exposure history matter most.

Choosing Panels vs. Individual Tests

• Most febrile cases: Serum WNV IgM with reflex to IgG; repeat if early.

• Neuroinvasive suspicion: CSF WNV IgM + serum IgM; add PCR early and PRNT when cross-reactivity is possible.

• Travel/arbovirus differential: combine WNV testing with dengue/chikungunya/Zika panels as indicated by travel history and symptoms.

FAQs

How soon after symptoms should I test for WNV?
Serum IgM is usually detectable 3–8 days after onset. If you test earlier and it’s negative, repeat in 7–10 days.

Do I need a spinal tap?
Only if your clinician suspects neuroinvasive disease. CSF IgM can clarify the diagnosis.

Can a past dengue or yellow-fever vaccine affect results?
Yes—flaviviruses can cross-react on antibody tests. PRNT helps tell them apart.

Is PCR better than antibody tests?
PCR is useful very early or in immunocompromised patients. After the first week, IgM is usually more informative.

Does a positive IgG mean I’m protected?
IgG shows past exposure; it does not guarantee protection or rule out current illness.

Are these tests reportable?
Positive or confirmed cases may require public-health reporting. Your clinician will advise.

Do I need to fast?
No. WNV tests are standard blood (and sometimes CSF) tests; no fasting required.

Related Categories & Key Tests

• Infectious Disease Tests Hub

• Travelers’ Diseases Tests • Dengue/Chikungunya/Zika Tests • Encephalitis & Meningitis Evaluation • Respiratory Tests

• Key Tests: West Nile Virus IgM (serum) • West Nile Virus IgM (CSF) • West Nile Virus IgG • Plaque Reduction Neutralization Test (PRNT) • WNV NAAT/PCR (serum/CSF/whole blood) • CBC • CMP • CSF cell count/protein/glucose

References

• Centers for Disease Control and Prevention — West Nile Virus: Clinical Testing and Diagnosis.
• Infectious Diseases Society of America — Encephalitis clinical practice guidelines.
• Association of Public Health Laboratories — Serologic and PRNT guidance for flaviviruses.
• World Health Organization — West Nile virus fact sheet and laboratory guidance.
• Clinical microbiology reviews on WNV IgM/IgG kinetics, PCR performance, and cross-reactivity with other flaviviruses.

Available Tests & Panels

Your West Nile Virus Tests menu is pre-populated in the Ulta Lab Tests system. Start with serum WNV IgM (add IgGfor context), use CSF IgM when neuroinvasive disease is suspected, consider PCR in early or immunocompromised cases, and request PRNT when cross-reactivity is a concern. Review timing and results with your clinician to plan next steps and any required public-health follow-up.