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Timothy C. Hain, MD 
Return to Index. Page last modified:
March 20, 2008
This web page is largely based on a recent review article (Davis, DeBiasi et al. 2006). West Nile Virus (WNV) is an RNA virus belonging to the Japanese encephalitis family. It is transmitted by mosquitos. WNV first appeared in North America in 1999. It has subsequently spread throughout the continental United States, where it has flourished. The 2003 US epidemic was the largest WNV outbreak ever reported. In the US, less than 3% of the population has antibodies, suggesting that WNV could continue to cause large epidemics.
WNV is harbored in both mosquito's and birds. A sudden die-off in bird populations, such as crows, sometimes precede a subsequent human epidemic. Such a die-off occurred in Chicago. WNV is not common in animals -- but horses can also become infected. WNV can be also be transmitted from breast milk.
About 80% of WNV virus infections are asymtomatic. 20% develop West Nile Fever (WNF), and less than 1% develop West Nile Neuroinvasive Disease (WNND).
In WNF, persons develop fever, headache, fatigue and a generalized rash. headache typically lasts for 10 days, weakness for 1 month, and fever for about 1 week.
In WNND, patients may develop meningitis, encephalitis, and a polio like syndrome. About 40% have meningitis, and 60% encephalitis.
Cranial nerve palsies are common in WNND. The 7th cranial nerve (causing Bell's palsy) is the most commonly affected. Next most common is the 8th cranial nerve, which may account for dizziness, vertigo and nystagmus.
Video of opsoclonus in young woman, developed after the West Nile outbreak in Chicago. See the site DVD page for a list of more movies like this one.
Opsoclonus-myoclonus is thought to be unusual in WNND (Sayao, Suchowersky et al. 2004; Khosla, Edelman et al. 2005; Alshekhlee, Sultan et al. 2006; Joyner, Kelly et al. 2006; Prasad, Brown et al. 2006; Wong 2007), but the author has seen a huge surge in cases of ocular flutter and opsoclonus, after WNV became prevalent in Chicago. In 2006, about 20% of the mosquito's on the Chicago South Side carried WNV. All of these patients developed flutter or opsclonus accompanied by headache and ataxia. It is the author's opinion that West Nile may be a common cause of opsoclonus, at least in parts of the country where WNV is common.
General laboratory testing simply indicates that the patient is ill, but is not at all specific for WNV.
Spinal fluid, which must be abnormal to make a diagnosis of WNND anyway, is generally very abnormal in WNND in a nonspecific way with increased inflammatory cells and protein. The specific identification of WNNV is generally basied on detection of specific antibodies to WNV in serum or CSF. The cumulative percentage of patients who are seropositive increases by about 10% per day during the first week.
Because the diagnosis of WNV is based on antibody testing, and a "gold standard" is lacking, false-negatives could be common.
Neuroimaging studies are often normal in WNV, with DWI (diffusion weighted MRI) being the most sensitive modality. DWI doesn't diagnose WNV, but it may show where the worst brain damage is located.
There is currently (2007) no treatment proven effective in WNV.
Patients with WNND generally take months or even years to recover. In the NY outbreak of 1999, only 37% of infected patients felt that they were fully recovered at 1 year post onset. WNV persists in the brains of infected monkeys for 5-6 months after resolution of their clinical illness, and the prolonged course may be in part related to persistent virus.
Prevention of WNV requires prevention of mosquito bites, and eradication of mosquitos. In spite of considerable publicity about WNV, and even a 20% infection rate of mosquito's in certain parts of Chicago, utilization of mosquito repellant and mosquito abatement has not been commonly adopted.
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