Sources for this review were identified by searches of Medline, and citations from relevant articles and book chapters. Medline search terms were “arthropod-borne virus”, “arbovirus”, “flavivirus”, and “West Nile virus”. English and French language papers were reviewed. Unpublished data from the national arbovirus surveillance system (ArboNET) of the CDC were also used.
ReviewWest Nile virus
Section snippets
Causative agent
WN virus is taxonomically placed within the family Flaviviridae, genus Flavivirus. Within the genus Flavivirus, WN virus has been serologically classified within the JE virus antigenic complex, which includes the human pathogens JE, Murray Valley encephalitis, SLE, and Kunjin viruses. The spherical WN virus particle is approximately 50 nm in diameter and consists of a host-derived lipid bilayer membrane surrounding a nucleocapsid core containing a single-stranded positive-sense RNA genome of
Geographical distribution and epidemiology
WN virus sensu lato is indigenous to Africa, Asia, Europe, and Australia,22, 23 and was recently introduced to North America (figure 2), where it was first detected in New York City.1 The likely origin of the introduced strain was the Middle East,24 but the mode of introduction is unknown. During 1999–2002, WN virus extended its range throughout much of the eastern parts of the USA, and has now been detected from Maine to the Florida Keys, and from the Atlantic coast to eastern North Dakota
Transmission cycle and host range
WN virus is maintained in nature in a mosquito-bird-mosquito transmission cycle primarily involving Culex sp mosquitoes (figure 6).22, 39 The virus, however, has been isolated from 29 mosquito species belonging to ten genera in the USA alone (unpublished data).4, 36, 40 The vector status and epidemiological importance of many of these species are unknown. Although Culex pipiens (the northern house mosquito), a highly ornithophilic species that is often abundant in urban areas, was a major
Clinical features
Most WN viral infections are symptomless.7, 27 The incubation period is approximately 2–14 days for symptomatic infections overall, but 2–6 days is typical in WN fever cases.53, 54 The associated clinical syndromes are non-specific and a diagnosis cannot reliably be made on clinical grounds alone.
Uncomplicated WN fever typically begins with sudden onset of fever (usually >39°C), headache, and myalgia, often accompanied by gastrointestinal symptoms. The acute illness usually lasts less than a
Pathogenesis and pathology
The exact mechanisms and sites of WN virus replication following the bite of an infected mosquito are unknown but initial replication is thought to occur in the skin and regional lymph nodes and to produce a primary viraemia that seeds the reticuloendothelial system (RES).20 Depending on the level of secondary viraemia that results from replication in the RES, virus may then seed the CNS. In healthy infected persons, virus can generally be isolated from blood during peak viraemia that occurs
Laboratory diagnosis
Serology continues to have a dominant role in the laboratory diagnosis of WN viral infections (and most other arboviral infections) in human beings.77 The development of WN virus-specific neutralising antibody between the acute and convalescent phases of illness (as shown by a >four-fold rise in titre, typically by plaque-reduction neutralisation assay) remains the most convincing serological evidence of infection, and is associated with long-term immunity. A battery of other flaviviruses
Clinical management
Although the treatment of uncomplicated WN viral infections is symptomatic, all patients with suspected WN meningoencephalitis should be hospitalised for observation and supportive care, and to rule-out treatable CNS infections or conditions (eg, herpesvirus infection, Guillain-Barrè syndrome, and bacterial meningoencephalitis). The most frequent cause of death in WN encephalitis cases is neuronal dysfunction, respiratory failure, and cerebral oedema (following neuronal injury and death). No
Prevention
No human vaccine for WN virus is available, although several laboratories are currently conducting vaccine research. Given the low incidence of WN viral disease in human beings in most areas, however, it is unlikely that such a vaccine would be cost-effective for public health use. Both inactivated and DNA-based vaccines have been developed for use in equines,87 but their efficacy has yet to be demonstrated.
Effective prevention of human WN viral infections depends on the development of locally
Predicting the future
WN virus will almost certainly continue to spread into the contiguous western parts of the USA over the next several years, primarily via the movement of viraemic birds. Similarly, it is likely that this virus will be introduced into Central and South America and the Caribbean, if this has not already occurred. After many years or even decades, WN virus in the western hemisphere will likely achieve an ecological/epidemiological equilibrium resembling that of SLE virus. In the USA, this would
Search strategy and selection criteria
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Accelerating targeted mosquito control efforts through mobile West Nile virus detection
2024, Parasites and Vectors
In this review, the term “meningoencephalitis” is used to encompass encephalitis, meningitis, myelitis, and cases with overlapping features of these syndromes. Although some authors use “WN fever” to describe any illness caused by WN viral infection, including neuroinvasive illness, in this review “WN fever” refers only to the uncomplicated febrile illness.