What are the symptoms of West Nile virus?

West Nile Virus Symptoms

Most West Nile virus infections (70-80%) are asymptomatic, but symptomatic patients present with either mild West Nile fever (approximately 20% of infections) or severe neuroinvasive disease (less than 1% of infections) characterized by meningitis, encephalitis, or acute flaccid paralysis. 1, 2

Mild Disease: West Nile Fever

When symptomatic infection occurs without neurologic involvement, patients typically experience:

• Sudden onset of high fever (up to 104°F/40°C) 3
• Severe headache 3, 4
• Generalized myalgias and lumbosacral pain 3
• Nausea and vomiting 5
• Erythematous rash (may be present) 4
• Fatigue and weakness 6

This febrile illness is typically self-limited and resolves without specific intervention 2.

Severe Disease: Neuroinvasive Manifestations

Less than 1% of infected individuals develop central nervous system involvement, which carries significantly higher morbidity and mortality 1, 2. The neuroinvasive presentations include:

Encephalitis

• Fever with altered mental status 5
• Meningism (neck stiffness, photophobia) 5
• Seizures (indicating CNS inflammation) 5
• Cognitive impairment 6

Meningitis

• Fever, severe headache, and neck stiffness without altered consciousness 1
• Photophobia and nausea 5

Acute Flaccid Paralysis

• Sudden onset of asymmetric limb weakness resembling poliomyelitis 4, 1
• Muscle weakness that may be profound 4

Clinical Timing and Seasonality

• Incubation period: Symptoms typically develop 3-8 days after mosquito bite exposure 3
• Peak season: 89% of cases occur during July-September in the United States, though year-round transmission occurs in warm climates 4, 7

High-Risk Populations for Severe Disease

Advanced age is the single greatest risk factor for severe neurologic disease and death. 4 Specific high-risk groups include:

• Patients ≥70 years: Highest neuroinvasive disease incidence (1.22 per 100,000) and mortality (20%) 1, 7
• Immunocompromised patients: 30-40% mortality in those with hematologic malignancies, solid organ transplants, or receiving B-cell-depleting monoclonal antibodies 1
• Hospitalization rates exceed 85% in all age groups with neuroinvasive disease, reaching 98% in patients ≥70 years 7

Long-Term Sequelae

More than 50% of hospitalized patients experience persistent symptoms including:

• Chronic fatigue and weakness 6, 1
• Memory loss and cognitive problems 6, 1
• Depression 1
• Myalgia 1
• 30-40% require discharge to long-term care facilities 1

Laboratory Findings

While not symptoms per se, laboratory abnormalities may accompany clinical disease:

• Leukopenia (first week) or leukocytosis (second week) 3
• Anemia, lymphopenia 4
• Elevated prothrombin/partial thromboplastin times, decreased platelets 3

Common Clinical Pitfalls

• Do not dismiss fever with neurologic symptoms during summer/fall months as simple viral illness without considering West Nile virus, especially in older adults 4
• Recognize that respiratory symptoms are not typical of West Nile virus; their presence suggests alternative or concurrent diagnoses 4
• Remember that most infections are asymptomatic, so lack of symptoms in exposed individuals does not rule out infection 1, 2