The history and background of west nile

Human West Nile meningitis and encephalitis cases in , by location and time of illness onset.

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As of April 15, , there were 4, reported cases. Unpublished data compiled by ArboNET. West Nile meningitis and encephalitis are notifiable diseases and should be reported to local health departments.

West Nile Virus: Biology, Transmission, and Human Infection

Local health departments and cooperating state and federal agencies collect and test specimens from ill animals, dead birds, captive sentinel animals mostly chickens , mosquitoes, and live birds trapped in the wild. Data generated locally are used to direct public health resources for research and control activities.

Humans typically acquire West Nile virus through a bite from an infected adult mosquito. In the United States, the virus is maintained in an enzootic mosquito-bird-mosquito cycle that primarily involves Culex mosquito species. Infected adult mosquitoes also may overwinter in culverts and catch basins.

Natural History of West Nile Virus Infection - Full Text View -

In the United States, West Nile virus appears to be particularly virulent in the corvids e. When weather or climate promotes significant viral amplification in the enzootic cycle, bridge vector mosquitoes i. Louis encephalitis virus have occurred during similar conditions. Humans and horses usually are incidental hosts because they may not develop sufficient viremia to propagate virus amplification after a mosquito bite. During the epidemic, human viremia was implicated for the first time in person-to-person transmission through blood, blood products, and organ transplantation, and possibly through breast milk and intrauterine infection.

West Nile virus infection appears to be equally distributed among all age groups and in both sexes. However, the incidence of encephalitis and death increases fold among persons 50 years of age and older. In recent urban epidemics, risk factors associated with West Nile virus infection included length of time spent outdoors, failure to apply mosquito repellent regularly, living in a building with a flooded basement, and presence of mosquitoes in the home.

West Nile virus has been reported in laboratory workers exposed to infected animals. Standard droplet and contact precautions, including safe disposal of sharp instruments and minimizing the generation of aerosols, should be used by laboratory workers who handle potentially infected animals or tissue specimens. In December , the U. The risk of blood-product contamination was thought to be low but may have varied regionally. In May , the FDA revised guidelines on deferring blood donations from persons with a medical diagnosis of West Nile virus infection and from persons who report postdonation illness with symptoms suggestive of infection with the virus.

As recently introduced assays for blood-donor screening are evaluated, donor suitability guidelines may be revised. Most persons with West Nile virus infection are asymptomatic. The majority of symptomatic patients have a self-limited, febrile illness, occasionally with headache, nausea, and vomiting.


Chills, myalgias, and overwhelming fatigue are common complaints. Although the acute illness usually lasts less than seven days, fatigue may persist for several weeks. Some patients may have lymphadenopathy and conjunctivitis. Occasionally, patients develop a brief, nonpruritic, papular rash, often on the arms, legs, or trunk.

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Rarely, hepatitis, pancreatitis, and myocarditis have been associated with West Nile virus infection. A study 1 of 59 hospitalized patients in New York state found that the most common symptoms were fever 90 percent , weakness 56 percent , nausea and vomiting 51 to 53 percent , headache 47 percent , and changes in mental status 46 percent. Rash, cough, stiff neck, myalgia, or arthralgia was found in less than 20 percent of these patients; only 2 percent had lymphadenopathy. Approximately 5 percent of patients with symptomatic West Nile virus infection less than 1 percent of all infected persons develop more severe neurologic illness, including meningitis, encephalitis, and acute flaccid paralysis characteristic of a poliomyelitis-like syndrome.

Patients with West Nile encephalitis have altered mental status and may have signs of meningeal irritation; those with the most severe encephalitis may progress to coma.

West Nile Virus in the United States: An Update on an Emerging Infectious Disease

Patients with West Nile poliomyelitis—like syndrome have acute onset of asymmetric weakness and diminished or absent deep tendon reflexes in the absence of pain or other sensory symptoms. Electrodiagnostic studies generally show lower motor neuron involvement with preserved sensory potentials. The poliomyelitis-like syndrome may be present in the absence of fever, meningismus, or other symptoms that raise suspicion for West Nile virus infection. Patients with nervous system involvement may have tremor, which is generally static or kinetic in nature, and myoclonus, which often affects the upper extremities and face.

Patients also may have signs of parkinsonism, including cogwheel rigidity, bradykinesia, and postural instability. A prospective study 26 conducted during the West Nile virus epidemic in Louisiana found that the median age was 70 years in patients with encephalitis, 56 years in those with acute flaccid paralysis, and 36 years in those with meningitis. Age-related underlying risk factors for invasive neurologic disease likely account for this difference. Family physicians should consider West Nile virus infection in febrile patients who have unexplained neurologic symptoms, muscle weakness, erythematous rash, or headache during late spring through early fall, or at any time of the year in areas with warmer climates, such as the Mississippi Delta.

Initial laboratory tests should include the following: a complete blood count; serologic testing for West Nile virus; CSF analysis for cell count, glucose level, and protein level; and, if indicated, neuroimaging and electrodiagnostic studies. Laboratory results from patients during recent outbreaks of West Nile virus disease have shown normal or elevated total white blood cell counts and, sometimes, anemia.

Computed tomography of the brain usually does not demonstrate acute abnormalities, but magnetic resonance imaging may show leptomeningeal or parenchymal changes.


This simple, sensitive 95 percent diagnostic test is available commercially and through local or state health departments. Testing of serum in the acute and convalescent phase i. Since IgM antibody usually does not cross an intact blood-brain barrier, a single West Nile virus—specific IgM positive antibody titer in CSF can confirm central nervous system infection. Viral culture of CSF and polymerase chain reaction PCR testing of serum should not be used as routine screening tools because of their low sensitivity for West Nile virus diagnosis.

Isolation of West Nile virus from tissue, blood, CSF, or other body fluid, or demonstration of West Nile viral antigen or genomic sequences. At least a fourfold serial change in plaque-reduction neutralizing antibody titer to West Nile virus in paired, appropriately timed i. Information from reference Four points about serum serology further underscore the importance of following the testing of sera from the acute phase with testing of sera from the convalescent phase: Approximately 10 percent of West Nile virus—infected patients in New York City and tested negative for serum IgM antibody when the acute specimen was collected within eight days of symptom onset.

Because IgM antibodies may persist for one year or longer after infection, the presence of a single IgM antibody titer may be unrelated to a patient's current illness. Antibodies to St. Louis encephalitis virus, which is enzootic in many states, may cross-react with West Nile virus antigen in serologic tests.

When interpreting serologic tests, physicians should ask patients if they recently have been vaccinated against or infected with related flaviviruses e. Treatment of West Nile virus infection is supportive. Patients with volume depletion because of nausea and vomiting may need oral or intravenous fluids.

Patients with meningitis or encephalitis may need pain control for headache. Patients with profound encephalopathy or acute flaccid paralysis may require mechanical ventilation.

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  • In vitro studies have suggested that ribavirin and interferon alfa-2b may be useful in the treatment of West Nile virus disease, but the efficacy of these agents has not been demonstrated in controlled clinical trials. No vaccine is licensed for the prevention of West Nile virus disease in humans, although laboratory development is currently underway.

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    Thus, comprehensive mosquito-control programs and personal protective measures are paramount in preventing West Nile virus infection in endemic areas. State and local agencies should identify vector species and breeding sites, and implement aggressive surveillance and targeted control early in the transmission season to restrict enzootic viral amplification. In southern areas such as the Mississippi Delta, the predominant mosquito vector for West Nile virus is Culex quinquefasciatus , which breeds in drainage ditches and septic ponds.

    Municipal mosquito-abatement programs should emphasize preventive strategies such as larvicide treatment. In northern regions, the principal mosquito vector is Culex pipiens , which develops in standing water. Eliminating household stagnant-water sources, such as flowerpots, pet bowls, clogged rain gutters, discarded tires, swimming pool covers, and birdbaths, can reduce breeding sites. Protective measures to limit exposure to potentially infected mosquitoes include avoiding outdoor activity during peak mosquito-biting periods usually dusk to dawn , wearing long-sleeved shirts and pants when outdoors during the early-evening and night hours, and applying mosquito repellent containing N,N -diethyl- m -toluamide DEET to clothing and exposed skin.

    Some mosquito-repellent preparations e. West Nile virus likely will continue to advance throughout the western United States within the next few years and is expected to follow local and regional flyway patterns of viremic birds. Physicians provide a vital link in recognizing new and unexpected disease activity.

    Few data exist regarding long-term sequelae from West Nile virus infection, the magnitude of risk of transmission associated with organ transplantation and blood transfusion, potential therapeutic interventions, and vaccine options. Physicians engaged in diagnosing, treating, and reporting West Nile virus infection may greatly enhance knowledge of this emerging infectious disease. Already a member or subscriber? Log in. Address correspondence to Gregory D. Huhn, M. LaSalle St. Reprints are not available from the authors. The authors thank Lisa Pealer, Ph.

    The authors indicate that they do not have any conflicts of interest. Sources of funding: none reported. N Engl J Med. Hayes CG. West Nile fever. In: Monath TP, ed.