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The Increasing Burden of West Nile Virus

By Amesh A. Adalja, MD, FACP, August 24, 2012

The mosquito-borne West Nile virus first appeared in the western hemisphere in 1999 when it sparked an outbreak of thousands of cases and then spread throughout North America. As a result, West Nile encephalitis became the most common cause of epidemic viral encephalitis in the United States. Since 1999, West Nile has established itself in US bird and mosquito populations and has caused approximately 1,000 neuroinvasive cases annually.1 This relatively stable state has been disrupted this year by the more than 500 cases reported in Dallas that prompted the Texas city to declare a state of emergency on August 15.2

Rare but Serious Neuroinvasive Disease

Although the majority of West Nile virus cases are asymptomatic, 20% experience West Nile fever, a self-limited influenza like illness. The neuroinvasive West Nile encephalitis occurs in up to only 1% of cases but is responsible for most West Nile virus–related hospitalizations. The case fatality rate can be as high as 14%, and up to 50% of patients have ongoing neurologic impairment on discharge.1 Diagnosis relies on demonstration of West Nile–specific IgM antibody in cerebral spinal fluid,1 and treatment is most often supportive care. There are some reports of successful treatment with interferon and IVIG under experimental conditions, but neither is used routinely.1

Risk factors for neuroinvasive disease (encephalitis, meningitis, and a poliolike acute flaccid paralysis) include advanced age, diabetes, hypertension, immunocompromised status, and several specific genetic deficiencies (CCR5 ∆32, gene polymorphisms).3

Vector Control for Prevention

Because no human vaccine exists, protective measures are largely centered on reducing the numbers of the Culex mosquitoes responsible for spreading West Nile virus and minimizing exposure to mosquito bites through personal protection. A commercially available vaccine exists for horses.1

West Nile in 2012

The cause of the increase of cases in Texas is not clear, but it seems likely that either the population of or the rate of infection among Culex mosquitoes has increased. At the same time, however, media attention appears to have resulted in increased testing for West Nile virus, fever, and neuroinvasive disease. Separating the effects of increased testing from any change in the vector or viral virulence will be crucial to understanding the dynamics of this once emerging, but now endemic disease, and the increased case count offers the opportunity to conduct clinical trials of interferon and/or IVIG for treatment of severe cases.


  1. Vaughn DW, Barrett A, Solomon T. Flaviviruses (yellow fever, dengue, dengue hemorrhagic fever, Japanese encephalitis, West Nile encephalitis, St. Louis encephalitis, tick-borne encephalitis). In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Churchill Livingstone; 2010:2133-2156.

  2. West Nile—August 17, 2012. Accessed August 20, 2012.

  3. Beckham JD, Tyler KL. Encephalitis. In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases class. 7th ed. Philadelphia, PA: Churchill Livingstone; 2010:1243-1263.