West Nile virus: review of the literature

Abstract

Importance: Since its introduction in North America in 1999, West Nile virus has produced the 3 largest arboviral neuroinvasive disease outbreaks ever recorded in the United States.

Objective: To review the ecology, virology, epidemiology, clinical characteristics, diagnosis, prevention, and control of West Nile virus, with an emphasis on North America.

Evidence review: PubMed electronic database was searched through February 5, 2013. United States national surveillance data were gathered from the Centers for Disease Control and Prevention.

Findings: West Nile virus is now endemic throughout the contiguous United States, with 16,196 human neuroinvasive disease cases and 1549 deaths reported since 1999. More than 780,000 illnesses have likely occurred. To date, incidence is highest in the Midwest from mid-July to early September. West Nile fever develops in approximately 25% of those infected, varies greatly in clinical severity, and symptoms may be prolonged. Neuroinvasive disease (meningitis, encephalitis, acute flaccid paralysis) develops in less than 1% but carries a fatality rate of approximately 10%. Encephalitis has a highly variable clinical course but often is associated with considerable long-term morbidity. Approximately two-thirds of those with paralysis remain with significant weakness in affected limbs. Diagnosis usually rests on detection of IgM antibody in serum or cerebrospinal fluid. Treatment is supportive; no licensed human vaccine exists. Prevention uses an integrated pest management approach, which focuses on surveillance, elimination of mosquito breeding sites, and larval and adult mosquito management using pesticides to keep mosquito populations low. During outbreaks or impending outbreaks, emphasis shifts to aggressive adult mosquito control to reduce the abundance of infected, biting mosquitoes. Pesticide exposure and adverse human health events following adult mosquito control operations for West Nile virus appear negligible.

Conclusions and relevance: In North America, West Nile virus has and will remain a formidable clinical and public health problem for years to come.

Figure 1. Schematic of Pathogenesis of West Nile Virus Infection

Following the bite of an infected mosquito, infection begins in keratinocytes and immune cells in the epidermis and spreads to draining lymph nodes. Viremia originating in the lymph nodes disseminates infection to peripheral organs.

Figure 2. West Nile Virus Neuroinvasive Mechanisms

Potential mechanisms for neuroinvasion of West Nile virus include (1) direct infection of the vascular endothelium and subsequent entry to the central nervous system, (2) viral passage through the vascular endothelium due to disruption of the blood-brain barrier integrity by vasoactive cytokines, (3) a Trojan horse mechanism through which infected monocytes are trafficked into the central nervous system, or (4) retrograde axonal transport to the central nervous system following infection of peripheral neurons.

Figure 3

Map of Average Annual Human Neuroinvasive Disease Incidence in the United States, 1999–2012

Figure 4

Cumulative Number of Human West Nile Virus Neuroinvasive Disease Cases by Week of Onset, 1999–2012