Aetiology

West Nile virus is an enveloped, spherical, single-stranded RNA arbovirus of the family Flaviviridae, which belongs to the Japanese encephalitis complex. Other flaviviruses include yellow fever, dengue, Zika, and St. Louis encephalitis viruses.[14] Kunjin virus is a sub-type of West Nile virus endemic to Oceania.

The virus primarily circulates between infected birds (>130 different species) and the insects that bite them, usually female mosquitoes. The infected mosquitoes transmit the virus when they bite other animals (e.g., horses) or people.[13]

Rarely, the virus can be transmitted by transfusion of infected blood, transplantation of infected organs, or transplacentally from mother to fetus. Other uncommon routes of transmission are needle-stick injuries involving infected blood, exposure of the conjunctiva to infected blood, dialysis, or breast-feeding.[15][16][17][18][19][20]

Pathophysiology

The typical incubation period is between 2 and 6 days and may last up to 14 days.[21] The incubation period can last even longer (up to 21 days) in immunocompromised patients.[17] After the mosquito injects virus-laden saliva into the patient, the virus probably replicates in dendritic cells at the bite site and then spreads to lymph nodes and the bloodstream. Alternatively, the virus may enter the body through exposure to infected blood or organ transplants. The pathogenesis of severe infection is not well understood. In neuroinvasive disease, the virus crosses the blood-brain barrier into the central nervous system and directly infects neurons, especially in the deep nuclei and grey matter of the brain, brain stem, and spinal cord, causing neuronal inflammation, destruction, and death. The immune response to the virus may also contribute to neuronal damage. Demyelination and gliosis may occur in prolonged disease.[22][23][24]

Classification

Clinical classification

West Nile virus infection is usually classified based on its presentation, although there is no formal classification system.

  • West Nile fever (WNF): occurs in approximately 20% of patients and is characterised by an influenza-like illness that usually lasts 3 to 6 days but can persist for weeks or months.

  • West Nile neuroinvasive disease (WNND): occurs in <1% of patients when the virus infects the central nervous system.[1] Subtypes include:

    • West Nile virus encephalitis

    • West Nile virus meningitis

    • West Nile virus poliomyelitis (or acute flaccid paralysis).

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