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West Nile Virus Encephalitis

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West Nile Virus Encephalitis, West Nile Encephalitis, West Nile Fever, West Nile Virus

  • Epidemiology
  1. Most common vector-borne virus in United States (as of 2016)
  2. Most severe illness occurs in older patients
    1. Over age 50: Twenty fold risk of severe illness
  3. Timing
    1. Temperate climate: Late summer and early fall
      1. Range of infections: Mid-July to early December
      2. Peaks in late August to early September
    2. Southern climate: Year round transmission
    3. Most favorable weather conditions
      1. Hot,dry summer with brief unseasonably cool weather
  4. Distribution
    1. Africa
    2. West Asia
    3. Eastern Europe
    4. Middle East
    5. United States (as of 1999)
  • Pathophysiology
  1. Flavivirus Mosquito-borne infection
    1. Transmitted by Culex, Ochlerotatus, Culiseta and Aedes Mosquitos
  2. Humans are incidental hosts
    1. Typically cycles between birds, Mosquitos and then back to birds
  3. Incubation: 3-14 days
  4. Animals affected
    1. Wild birds are usual hosts
      1. Dead birds may suggest endemic area
      2. Corvids (Crows and Jays) are most often affected
    2. Horses are most affected domesticated animals
    3. Cats
    4. Bats
    5. Chipmunks
    6. Skunks
    7. Squirrels
    8. Rabbits
  • Symptoms
  1. Spectrum of disease
    1. Asymptomatic in 80% of cases
    2. Mild febrile self-limited infection in 20% of cases
    3. Severe disease (e.g. Meningitis, Encephalitis) in 1-2%
      1. More severe cases in older adults
  2. Symptoms (abrupt onset without prodrome): Flu-like illness
    1. Fever up to 40 Celsius
    2. Malaise
    3. Muscle Weakness to Flaccid Paralysis
    4. Profound Fatigue (may persist for weeks)
    5. Chills and myalgias (e.g. back pain)
    6. Drowsiness or lethargy
  3. Variable symptoms
    1. Severe frontal or retro-orbital Headache
    2. Nausea or Vomiting
    3. Eye Pain
    4. Cough
  4. Rare Symptoms
    1. Abdominal Pain (if hepatitis or Pancreatitis)
  5. Classic presentation
    1. Late summer, early fall onset of Prolonged Fever and neurologic symptoms
  • Signs
  1. Non-tender Generalized Lymphadenopathy
    1. Occipital Lymphadenopathy
    2. Axillary Lymphadenopathy
    3. Inguinal Lymphadenopathy
    4. Lymphadenopathy may persist for months
  2. Facial Flushing
  3. Conjunctival injection
  4. Coating of Tongue
  5. Pale maculopapular rash (Roseola-like)
    1. Affects trunk and upper arms
    2. Onset on days 2 to 5 (typically as fever subsides)
  6. Neurologic signs
    1. Severe Muscle Weakness to Flaccid Paralysis (related to anterior horn cell involvement)
    2. No sensory deficits
    3. Ataxia
    4. Extrapyramidal signs
    5. Cranial Nerve abnormalities
    6. Myelitis
    7. Optic Neuritis
    8. Polyradiculitis
    9. Seizures
  • Differential Diagnosis
  1. Metabolic panel
    1. Hyponatremia may occur
  2. Complete Blood Count (CBC)
    1. Leukopenia (Leukocytes <4000/mm3)
  3. Lumbar Puncture
    1. Initial Neutrophilia, then Lymphocytosis
    2. Normal CSF Glucose
    3. Mild increases in CSF Protein concentration
  • Labs
  • Diagnosis
  1. West Nile Virus serum or CSF IgM by MAC-ELISA (preferred)
    1. Best lab test for diagnosis (95% sensitive)
    2. Collect 8-21 days after onset of symptoms (False Negative in first 7 days)
    3. Positive CSF IgM confirms CNS Infection
    4. False Positives due to cross reactivity
      1. St. Louis Encephalitis virus
      2. FlavivirusVaccine (e.g. Yellow Fever, Dengue)
  2. Other testing (not used routinely)
    1. Blood isolation of virus
      1. Sensitivity on Day 1: 75% of cases positive
      2. Sensitivity decreases over first 5 infection days
    2. Virus culture of CSF or PCR testing
  • Complications
  1. Neurologic disease (one in 30-70 cases, some studies report 1 in 150 cases)
    1. Meningitis
    2. Encephalitis
    3. West Nile Poliomyelitis-like syndrome
      1. Asymmetric Flaccid Paralysis (may include respiratory muscles)
    4. Guillain-Barre Syndrome
    5. Extrapyramidal symptoms
    6. Long-term neuropsychiatric sequelae
      1. Fatigue
      2. Memory loss
      3. Difficulty walking
      4. Muscle Weakness
      5. Major Depression
  2. Other complications (rare)
    1. Myocarditis
    2. Pancreatitis
    3. Hepatitis
  • Management
  1. Supportive care in most cases
    1. Hydration
    2. Analgesia
  2. Investigational agents to consider in severe cases
    1. Ribavirin
    2. Interferon alfa-2b
  • Prognosis
  1. Most Mosquitos in endemic areas are not infected
  2. If infection occurs, 99% of cases are self-limited
  3. Severe cases (meningoencephalitis) occur in less than 1-2% of infections
    1. Mortality in severe cases is 5-15%
    2. Elderly account for majority of fatal cases
  • Course
  1. Incubation up to 6 days
  2. Duration for 3 to 5 days in 80% of cases
  3. Fatigue may take weeks to resolve
  • Prevention
  1. See Prevention of Vector-borne Infection
  2. Eliminate areas of standing water (and other Mosquito control)
  3. No available Vaccination
  4. Blood donor screening
  • Resources