West Nile Encephalitis

Updated: Mar 03, 2017
  • Author: Burke A Cunha, MD; Chief Editor: Michael Stuart Bronze, MD  more...
  • Print
Overview

Background

West Nile encephalitis (WNE), or neuroinvasive West Nile virus (WNV) infection, is distinguished from other arthropod-borne causes of viral encephalitis (eg, western equine encephalitis, eastern equine encephalitis [EEE], Japanese encephalitis, Venezuelan encephalitis) based on its geographic distribution, clinical features, and laboratory findings. (See Etiology, Differentials, and Workup.)

WNE is endemic in the Middle East, Africa, and Asia. In North America, WNE first occurred in the northeast United States along the eastern seaboard and now extends nationwide. WNE may have existed in antiquity in the Middle East. (Some have suggested that Alexander the Great may have died from WNE.)(See Epidemiology.) [1, 2]

In 1999, a late summer outbreak of West Nile encephalitis (WNE), not found previously in the United States, was implicated in several deaths in New York. By late summer 2002, West Nile virus has been identified throughout the eastern and southeastern United States. Following bird migration, the virus is presently extending westward, and by April 2003, virus activity had been detected in 46 states and the District of Columbia. The image below depicts the Culex mosquito, common in the eastern United States.

The Culex mosquito, common in the eastern United S The Culex mosquito, common in the eastern United States, is the primary vector responsible for infecting humans with West Nile virus. Prevention of West Nile virus is primarily directed at reducing the mosquito population from May to October and by taking precautions to limit human exposure during these months of high mosquito activity. Image courtesy of the Centers for Disease Control and Prevention.

See 7 Bug Bites You Need to Know This Summer, a Critical Images slideshow, for helpful images and information on various bug bites.

Throughout the world, outbreaks of WNE have been associated with severe neurologic disease, [3] although in general, only 1 in 150 affected patients develops symptomatic WNE. (See History.)

For more information, see the Centers for Disease Control and Prevention (CDC) fact sheet on West Nile virus, links to state and local government web sites on West Nile virus, and the Environmental Protection Agency (EPA) article on mosquito control.

For clinical information on the Internet, see West Nile Virus: A Primer for the Clinician from the August 6, 2002, issue of the Annals of Internal Medicine, which is available online in Adobe PDF format. [4] The Canadian equivalent, West Nile Virus: Primer for Family Physicians, was published June 10, 2005 in Canadian Family Physician. [3]

Next:

Prognosis

The prognosis is excellent in individuals infected with West Nile virus. However, those with symptomatic WNE may have a worse prognosis. In a large study reported by Bode et al in 2006, the medical records of 221 patients were reviewed, of which 168 patients had either West Nile meningitis or encephalitis. Mortality reached 18% in the encephalitis group, and many of the survivors were left with severe neurologic deficits. [5]

Nonetheless, neuropsychological testing shows evidence of cognitive impairments in both those with and those without neuroinvasive disease. Such dysfunction was seen more often 2-4 years postinfection. [6] Such a high rate of cognitive difficulties, even in noninvasive disease, explains the variable course of rehabilitation in these patients. [7]

Most children with WNE in Asia and Africa have a benign course and only rarely die of the disease. In the United States, most fatal cases have occurred in elderly patients. More severe disease is also seen in those with diabetes and alcoholism.

Previous
Next:

Etiology and Pathophysiology

Mosquito bites, which are particularly likely during feeding times (dawn and dusk) in the summer months, transmit West Nile virus. Prolonged contact or multiple mosquito bites enhance the risk.

Seventeen species of wild birds transmit West Nile encephalitis (WNE) to humans via the Culex, Aedes, and Anopheles mosquitoes. (See the image below.) WNE first causes symptomatic or asymptomatic illness in wild migratory birds that act as viral replication factories. Wild birds infected with WNE contain high titers of the virus and remain viremic for 1-2 weeks, making them ideal hosts to perpetuate the disease. Mosquitoes transmit WNE from birds to humans. Horses, dogs, and other small animals may harbor WNE after being bitten; however, they are inefficient transmitters because viral titers are relatively low, and WNE viremia is short-lived in these animals.

The Culex mosquito, common in the eastern United S The Culex mosquito, common in the eastern United States, is the primary vector responsible for infecting humans with West Nile virus. Prevention of West Nile virus is primarily directed at reducing the mosquito population from May to October and by taking precautions to limit human exposure during these months of high mosquito activity. Image courtesy of the Centers for Disease Control and Prevention.

WNE, like other arthropod-borne viral encephalitides, traverses the blood-brain barrier and infects the brain parenchyma, clinically manifesting as viral encephalitis. WNE may also affect the leptomeninges, resulting in a clinical presentation of aseptic meningitis (viral meningitis). Patients with WNE may present with features of encephalitis and aseptic meningitis (meningoencephalitis). [8]

West Nile virus may also be transmitted in organ transplants and has been found in breast milk. [9]

Previous
Next:

Epidemiology

In North America, as previously stated, WNE first occurred in the northeast United States along the eastern seaboard and now extends nationwide.

Approximately 50% of children in Egypt have West Nile virus seropositivity. West Nile encephalitis (WNE) is the most common cause of viral aseptic meningitis or encephalitis in patients presenting to emergency departments in Cairo. WNE is common in the Middle East, Asia, and Africa.

WNE usually occurs in the summer, when mosquitoes, wild migratory birds, and humans are in close proximity outdoors. Most US cases occur in elderly patients. Worldwide, most cases occur in young children or young adults; however, elderly patients are affected more severely.

WNE is more common in persons of advanced age and in persons with underlying malignancy (especially hematological) or who have undergone organ transplantation. There is clearly an age-dependent dysregulation of innate immune response to West Nile virus. [10]

The incidence of WNE declined between 2008 and 2011, but, in 2012, there was a major outbreak in the Dallas/Fort Worth area of Texas. This outbreak was also associated with an increased proportion of cases characterized by serious neuroinvasive illness. [11] Aside from this outbreak, it appears that the rate of neuroinvasive disease associated with West Nile virus infection remained stable over the past decade. [12]

Previous
Next:

Patient Education

Avoid exposure, particularly from dawn to dusk, in areas where mosquitoes and West Nile encephalitis (WNE) are present. Mosquito netting and mosquito repellents may also be used.

Avoid wearing bright colors and highly aromatic perfumes, deodorants, and hair products that attract mosquitoes. Avoid handling dead or diseased wild birds without proper aseptic precautions.

For patient education information, see the Brain and Nervous System Center and the Infections Center, as well as West Nile Virus and Encephalitis.

Previous