West Nile Virus Encephalitis

1. See Also
   1. Vector-Borne Infection
2. Epidemiology
   1. Most severe illness occurs in older patients
      1. Over age 50: Twenty fold risk of severe illness
   2. 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
   3. Distribution
      1. Africa
      2. West Asia
      3. Eastern Europe
      4. Middle East
      5. United States
3. Pathophysiology
   1. FlavivirusMosquito-borne infection
   2. Humans are incidental hosts
   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
4. 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. Symptoms (abrupt onset without prodrome)
      1. Fever up to 40 Celsius
      2. Malaise and weakness
      3. Profound Fatigue (may persist for weeks)
      4. Chills and myalgias (e.g. back pain)
      5. Drowsiness or lethargy
   3. Variable symptoms
      1. Severe frontal Headache
      2. Nausea or Vomiting
      3. Eye Pain
      4. Cough
   4. Rare Symptoms
      1. Abdominal Pain (if hepatitis or Pancreatitis)
5. 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
   6. Neurologic signs
      1. Severe Muscle Weakness
      2. Ataxia
      3. Extrapyramidal signs
      4. Cranial Nerve abnormalities
      5. Myelitis
      6. Optic Neuritis
      7. Polyradiculitis
      8. Seizures
6. Differential Diagnosis
   1. See Viral Encephalitis
   2. St. Louis Encephalitis
   3. Dengue
7. Labs
   1. Complete Blood Count (CBC)
      1. Leukopenia (Leukocytes <4000/mm3)
   2. Lumbar Puncture
      1. Lymphocytosis
      2. Normal CSF Glucose
      3. Mild increases in protein concentration
   3. West Nile Virus serum or CSF IgM by MAC-ELISA
      1. Best lab test for diagnosis (95% sensitive)
      2. Collect 8-21 days after onset of symptoms
      3. Positive CSF IgM confirms CNS Infection
      4. False positives due to cross reactivity
         1. St. Louis Encephalitis virus
         2. Flavivirus vaccine (e.g. Yellow Fever, Dengue)
   4. 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
8. Complications
   1. Neurologic disease (one in 150 cases)
      1. Meningitis
      2. Encephalitis
      3. West Nile poliomyelitis-like syndrome
      4. 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
9. 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
10. Prognosis
    1. Most Mosquitos in endemic areas are not infected
    2. If infection occurs, 99% of cases are self-limited
    3. Severe cases occur in less than 1% of infections
       1. Mortality in severe cases is 5-15%
       2. Elderly account for majority of fatal cases
11. 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
12. Prevention
    1. See Prevention of Vector-borne Infection
    2. Eliminate areas of standing water
13. Resources
    1. CDC West Nile Virus
       1. http://www.cdc.gov/ncidod/dvbid/westnile/
14. References
    1. Douglas in Goldman (2000) Cecil Medicine, p. 1851
    2. Huhn (2003) Am Fam Physician 68(4):653-72
    3. Nash (2001) N Engl J Med 3441:1807-1814
    4. Petersen (2002) Ann Intern Med 137:173-9