II. Epidemiology

  1. Most common vector-borne Arbovirus in United States (as of 2016)
    1. Reported cases: ~50,000 between 1999 and 2018
  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
      1. First identified in Uganda's West Nile Province (1937)
    2. West Asia
    3. Eastern Europe
    4. Middle East
    5. United States (as of 1999)
      1. Central States
      2. Great Plains States

III. 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. Transmission
    1. Most commonly by Mosquito Bite
    2. Organ transplant or Blood Transfusion (screened in U.S. before transplant or transfusion)
    3. Transplacental transmission
  4. Incubation: 3-14 days
  5. 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

IV. Types

  1. Asymptomatic Infection (80%)
  2. West Nile Fever (WNF, 20%)
  3. West Nile Neuroinvasive Disease (WNND, 1%)
    1. Meningitis or Encephalitis
    2. More severe cases occur in older adults
    3. Risk Factors
      1. Age over 50 years old
      2. Diabetes Mellitus
      3. Alcohol Abuse
      4. Male gender
      5. Organ transplant or Immunosuppression
      6. Autoimmune Disease

V. Symptoms: West Nile Fever

  1. Spectrum of disease
    1. See Types above
  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

VI. Signs: West Nile Fever

  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)

VII. Signs: Neurologic signs (West Nile Neuroinvasive Disease or WNND)

  1. Severe Muscle Weakness to Acute Flaccid Paralysis or Myelitis (related to anterior horn cell involvement)
  2. No sensory deficits
  3. Ataxia
  4. Decreased or absent Deep Tendon Reflexes
  5. Extrapyramidal signs
  6. Myoclonus
  7. Tremor
  8. Cranial Nerve abnormalities
  9. Myelitis
  10. Optic Neuritis
  11. Polyradiculitis
  12. Seizures

VIII. Differential Diagnosis

IX. Labs: General

  1. Metabolic panel
    1. Hyponatremia may occur with Encephalitis
  2. Complete Blood Count (CBC)
    1. Leukopenia (Leukocytes <4000/mm3)
    2. Mild Leukocytosis may also occur
  3. Lumbar Puncture
    1. Initial Neutrophilia, then Lymphocytosis
    2. Normal CSF Glucose
    3. Mild increases in CSF Protein concentration

X. 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)
      3. Prior West Nile Virus (CSF IgM may persist for a longer period from prior infection)
  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

XI. Imaging

  1. MRI Brain
    1. Abnormal in 25-35% of cases
    2. Nonspecific findings (esp. in Thalamus, Basal Ganglia)

XII. 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. Chorioretinitis
    3. Cardiac Arrhythmias
    4. Pancreatitis
    5. Hepatitis
    6. Rhabdomyolysis

XIII. 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

XIV. 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

XV. Course

  1. Incubation up to 6 days
  2. Duration for 3 to 5 days in 80% of cases
  3. Prolonged recovery may take up to one year to return to full functional and cognitive capacity
    1. Fatigue may take weeks to months or longer for resolution
    2. Up to 50% of patients have persistent symptoms and functional Impairment at >1.5 years
    3. More prolonged course with West Nile Neuroinvasive Disease (WNND)

XVI. 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

XVII. Resources

XVIII. References

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Related Studies

Ontology: West Nile Fever (C0043124)

Definition (MSH) A mosquito-borne viral illness caused by the WEST NILE VIRUS, a FLAVIVIRUS and endemic to regions of Africa, Asia, and Europe. Common clinical features include HEADACHE; FEVER; maculopapular rash; gastrointestinal symptoms; and lymphadenopathy. MENINGITIS; ENCEPHALITIS; and MYELITIS may also occur. The disease may occasionally be fatal or leave survivors with residual neurologic deficits. (From Joynt, Clinical Neurology, 1996, Ch26, p13; Lancet 1998 Sep 5;352(9130):767-71)
Concepts Disease or Syndrome (T047)
MSH D014901
ICD9 066.40, 066.4
ICD10 A92.3 , A92.30
SnomedCT 12381007, 417093003
LNC LA10519-9
English West Nile Fever, West Nile fever, WN - West Nile fever, West Nile fever (diagnosis), West Nile virus infection, West Nile fever NOS, West Nile Fever [Disease/Finding], west nile fever, West Nile fever (disorder), West nile virus infection, West Nile; fever, fever; West Nile, West Nile Fever NOS, West Nile Fever, unspecified, Disease due to West Nile virus (disorder), Disease due to West Nile virus
Swedish West Nile-feber
Finnish Länsi-Niilin kuume
Russian ZAPADNOGO NILA LIKHORADKA, ЗАПАДНОГО НИЛА ЛИХОРАДКА
Korean 서나일 바이러스성 감염
Polish Gorączka Zachodniego Nilu
Japanese 西ナイル熱, ウェストナイル熱
Norwegian Vestnilfeber, Vest-Nilen-feber, Vest-Nilen-virusinfeksjon
Czech horečka západního Nilu, západonilská horečka
Spanish fiebre del oeste de Nilo, fiebre del oeste de Nilo (trastorno), enfermedad por virus del Oeste del Nilo (trastorno), enfermedad por virus del Oeste del Nilo, Fiebre del Nilo Occidental
Dutch West Nile; koorts, koorts; West Nile, Koorts, West-Nijl-, West-Nijlkoorts
French Fièvre West Nile, Fièvre du Nil Occidental, Fièvre à virus West Nile
German West-Nil-Fieber
Italian Febbre del Nilo occidentale
Portuguese Febre do Nilo Ocidental

Ontology: West Nile virus (C0043125)

Definition (NCI_CDISC) Any viral organism that can be assigned to the species West Nile virus.
Definition (NCI) A species of flavivirus, one of the Japanese encephalitis virus group. It can infect birds and mammals and is spread especially from birds to humans by mosquitoes. Human infection causes an illness marked by fever, headache, muscle ache, skin rash, and sometimes encephalitis or meningitis.
Definition (CSP) transmitted by Culex spp mosquitos; agent of emerging disease in North America.
Definition (MSH) A species of FLAVIVIRUS, one of the Japanese encephalitis virus group (ENCEPHALITIS VIRUSES, JAPANESE). It can infect birds and mammals. In humans, it is seen most frequently in Africa, Asia, and Europe presenting as a silent infection or undifferentiated fever (WEST NILE FEVER). The virus appeared in North America for the first time in 1999. It is transmitted mainly by CULEX spp mosquitoes which feed primarily on birds, but it can also be carried by the Asian Tiger mosquito, AEDES albopictus, which feeds mainly on mammals.
Concepts Virus (T005)
MSH D014902
SnomedCT 57311007
LNC LP28423-9
English West Nile virus, EGYPT VIRUS 101, West Nile Virus, west nile virus organism, Egypt 101 virus, WEST NILE VIRUS, WNV, West Nile virus (organism)
Swedish West Nile-virus
Czech virus západního Nilu
Finnish West Nile -virus
Russian LIKHORADKI ZAPADNOGO NILA VIRUS, ZAPADNOGO NILA LIKHORADKI VIRUS, EGIPET 101 VIRUS, ENTSEFALITA ZAPADNOGO NILA VIRUS, ЕГИПЕТ 101 ВИРУС, ЗАПАДНОГО НИЛА ЛИХОРАДКИ ВИРУС, ЛИХОРАДКИ ЗАПАДНОГО НИЛА ВИРУС, ЭНЦЕФАЛИТА ЗАПАДНОГО НИЛА ВИРУС
Japanese ウェストナイル熱ウイルス, エジプト101ウイルス, 西ナイルウイルス, クンジンウイルス, Kunjinウイルス
German ÄGYPTISCHES VIRUS 101, West-Nil-Fieber-Virus, Ägyptisches-101-Virus
Polish Wirusy gorączki Zachodniego Nilu
French Virus du Nil occidental, Egypt 101 virus, West Nile virus, WNV (West Nile Virus)
Portuguese Vírus West Nile, Vírus do Oeste do Nilo, Vírus do Nilo Ocidental, Vírus Egito 101
Spanish virus West Nile (organismo), virus West Nile, Virus del Nilo Occidental, Virus Egipto 101
Italian Virus della febbre del Nilo occidentale
Dutch Virus, West-Nijlkoorts-, West-Nijlkoortsvirus, Egypte 101-virus