West nile virus overview

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

West Nile virus (WNV) is a virus of the family Flaviviridae; part of the Japanese encephalitis (JE) antigenic complex of viruses, it is found in both tropical and temperate regions. It mainly infects birds, but is known to infect humans, horses, dogs, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits. The main route of human infection is through the bite of an infected mosquito. Image reconstructions and cryoelectron microscopy reveal a 45-50 nm virion covered with a relatively smooth protein surface. This structure is similar to the dengue fever virus; both belong to the genus flavivirus within the family Flaviviridae. WNV is a positive-sense, single strand of RNA, it is between 11,000 and 12,000 nucleotides long which encode seven non-structural proteins and three structural proteins. The RNA strand is held within a nucleocapsid formed from 12 kDa protein blocks; the capsid is contained within a host-derived membrane altered by two viral glycoproteins.

Historical Perspective

WNV was first isolated in 1937 in Uganda from a hospitalized patient who presented with isolated fever. Between 1950 and 1960, small villages in the Mediterranean basin had repeated outbreaks, especially in Israel and Egypt. These epidemics allowed researchers to study the molecular and clinical features of the disease and further understand its mode of transmission and natural history. Several WNV outbreaks were recorded in the second half of the 20th century in Europe, Middle East, Far East, and Africa. It was not until 1999 when the first WNV outbreak was documented in USA, making WNV a worldwide infection. Perhaps the most severe outbreak documented was in 2002 in USA, recording the highest number of meningoencephalitis from a single WNV outbreak. The first description of a person-to-person transmission was reported in 2002 among patients with blood transfusions and tissue transplantation.

Risk Factors

Certain factors may increase the risk of infection with WNV by a mosquito bite, such as warm temperatures, extensive outdoor exposure, homelessness, and absence of window screens. Occupational risk factors include in-field occupations, such as agriculture. Severe clinical disease is often associated with advanced age, immunosuppression, malignancy, diabetes mellitus, hypertension, and renal disease. An increased risk of death is observed among immunosuppressed patients and those presenting with altered level of consciousness. Certain conditions such as encephalitis, advanced cardiovascular disease, and hepatitis C virus may also carry an increased risk of death among patients infected with WNV.

Screening

Universal screening for West Nile virus is not recommended. As bloodand transplant-related transmission of the virus has been reported, nucleic acid tests (NAT) may be used to screening for WNV among potential blood and solid organ donors. In blood donation, individual screening is not recommended. Instead, a "minipool" nucleic acid testing program (MP NAT) is implemented. Positive pools warrant further investigation for individuals. Patients with positive NAT may not donate blood or solid organs for at least 120 days. Re-testing after 120 days is indicated.

References

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