Disease Control Bulletin: August 2000


disease control bulletin

West Nile Virus

The following summary of the 1999 outbreak of West Nile Virus in New York City is an abridged version from the New York City Department of Health1. Information regarding West Nile virus surveillance in Vermont is included at the end of this article.

On August 23, 1999, an infectious disease physician in Queens reported two cases of presumptive viral encephalitis to the New York City Department of Health; both were associated with the unusual manifestation of diffuse muscle weakness. The initial epidemiologic investigation over the ensuing week identifieda cluster of eight patients with the following common characteristics: (a) all were relatively healthy, active, older adults between the ages of 58 and 85 years who resided ina four-square-mile area of Northern Queens; (b) all reported a prodromal illness with fever and gastrointestinal symptoms, followed by the acute onset of altered mental status; (c) seven of eight patients developed diffuse muscle weakness, characterized in several persons by flaccid paralysis and/or elec-tromyographic evidence of an axonal neuropathy; (d) cere-brospinal fluid (CSF) and peripheral blood parameters suggested a viral etiology; and (e) the only common risk factor was that all eight had spent time outdoors in the evening hours during the two weeks prior to illness onset. Results of serologic testing for the common North American arboviruses were initially reported as consistent with St. Louis encephalitis.

Concurrent with the human outbreak, wildlife enthusiasts and veterinary specialists had begun to notice an increase in bird fatalities, primarily among crows. This avian die-off was initially thought to be unrelated to the human outbreak because, while birds are the primary hosts for arboviruses such as St. Louis encephalitis and West Nile, infected birds are usually asymptomatic. When it was recognized, however, that many of the birds had pathologic evidence of viral encephalitis, avian tissue samples were sent to theU.S. Department of Agriculture National Veterinary Services Laboratories in Ames, Iowa, where a flavivirus was isolated that was subsequently identified by the CDC as West Nile Virus.

Since West Nile virus had never been identified in the Western Hemisphere, understanding of its epidemiological and clinical characteristics was based entirely on published reports from overseas. Simultaneously occurring human epidemics and avian epizootics had never been reported. This remains an unexpected and unique aspect of the New York outbreak.

Active surveillance in the New York City area confirmed 62 human cases of West Nile viral disease during the 1999 outbreak. Illness onsets ranged from August 2 to September 22; most patients became ill during the third and fourth weeks of August. Most cases occurred among persons aged 50 and older (median age, 68 years); only three cases occurred among children aged 16 years or younger.

The most commonly reported signs and symptoms among the 59 case patients who were hospitalized included fever (90%), muscle weakness (54%), headache (46%), altered mental status (14%), rash (22%), stiff neck (19%), arthral-gia (17%), photophobia (15%), and myalgia (14%). Clinical presentations included encephalitis with muscle weakness (39%; median age, 75 years); aseptic meningitis (32%; median age 60 years) and milder illness characterized by fever and headache only (7%; median age, 64.5 years). The case fatality rate among the 59 hospitalized patients was 12 percent; the seven deaths occurred among persons aged 68 to 87 years. One patient was infected with HIV but had not been diagnosed with AIDS; three patients were receiving immuno-suppressive therapy for cancer.

The predominant clinical finding of severe, diffuse muscle weakness had not been described in previous West Nile viral outbreaks overseas, and is another unique characteristic of the 1999 New York epidemic. At its most severe, the weakness was characterized by flaccid paralysis, often requiring ventilatory support, with evidence of an axonal neuropathy on electromyographic (EMG) testing. Radiologic studies, including computed tomography (CT) scans of the head and magnetic resonance imaging (MRI), were non-diagnostic. Neurohistopathology and immunohistochemical staining of autopsy tissues revealed primary involvement of the brainstem and some cranial nerve roots; there were minimal pathologic findings in the cortex and cerebellum.

To better assess the overall infection rate and the spectrum of illness associated with the outbreak, the New York City Department of Health and the CDC conducted a household-based seroprevalence survey last October in a foursquare-mile area around the epicenter of the outbreak in northern Queens. Of the representative sample of 1,069 households that were invited to participate, 470 (44%) agreed; 677 persons aged 5 and older anonymously completed interviews and gave blood samples. Based on 19 individuals who tested positive for West Nile viral infection, seroprevalence in the surveyed area (population 46,220), was estimated to be 2.6 percent (95% confidence intervals were 1.2 to 4.1%), an infection rate comparable to that found ina recent outbreak in a non-endemic area overseas. This rate cannot be extrapolated to other areas of New York City because the survey area had the most West Nile encephalitis cases; rates elsewhere in the city are likely to be substantially lower. Approximately 20 percent of seropositive individuals reporteda febrile illness during the late summer or fall that could be attributed to West Nile viral infection. Among those persons, predominant symptoms were fever (100%), myalgia (100%), headache (89%), fatigue (87%), and arthralgia (76%).

The New York City and State departments of Health tested mosquitoes and birds for West Nile virus as part of the 1999 outbreak investigation. The virus was detected in mosquitoes collected through early October and in avian samples collected through mid-November. The geographic distribution of West-Nile positive mosquito and avian specimens correlated strongly with the human cases.

Physicians should report any hospitalized patient with clinical evidence of viral encephalitis, especially when associated with diffuse muscle weakness, which was a unique characteristic of West Nile viral infection during the 1999 New York City outbreak. (In Vermont, reports should made be to the Epidemiology Field Unit, Vermont Department of Health, 1-800-420-4374 or 863-7240, including weekends.)

Physicians should also consider West Nile virus in the differential diagnosis of aseptic meningitis, especially in older patients presenting during the summer months. At the peak of the outbreak in August 1999, only 10 percent of meningitis cases reported to the New York City Department of Health tested positive for evidence of infection with West Nile virus; compared with 40 percent of all encephalitis cases and 60 percent of encephalitis cases associated with diffuse muscle weakness. Testing should be considered for older patients presenting with aseptic meningitis, given that neurological disease is more common in older adults infected with West Nile virus.

Physicians should also consider West Nile viral infection in the differential diagnosis of Guillain-Barré syndrome. In the 1999 outbreak, several West Nile case-patients with severe muscle weakness were initially misdiagnosed with Guillain-Barré syndrome and treated with plasmapheresis.

What to report.

Hospitalized patients with any of the following clinical syndromes—

  1. Viral encephalitis,a clinical diagnosis characterized by:

    1. Fever greater than 38°C or 100°F, AND
    2. CNS involvement, including altered mental status (altered level of consciousness, confusion, agitation, or lethargy) or other signs (cranial nerve palsies, paresis or paralysis, or convulsions), AND
    3. An abnormal CSF profile suggesting a viral etiology (a negative bacterial stain and culture with pleocytosis and/or an elevated protein level).
  2. Viral meningitis among persons aged 17 years and older, without other probable cause of infection.
  3. Guillain-Barré syndrome, especially with atypical features (fever, altered mental status, and/or pleocytosis).

How to arrange testing.

It is critical that you first call the Vermont Department of Health Laboratory (800-660-9997 ext.7553) to arrange for testing through the CDC Arbovirus Laboratory. A CDC History Form must accompany any specimens forwarded to CDC through the laboratory. If you are usinga private laboratory for arbovirus serology, please contact the Vermont Department of Health Laboratory for testing recommendations.

Specimens to obtain.

CSF for virus isolation and serology: Collect as soon as possible after onset of illness. Collect two tubes without preservatives. Each tube should contain at least 1cc of CSF. The specimens must be frozen at -70ºC or below until shipping arrangements are made.

Sera: Acute phase serum should be collected during the first week of illness. Convalescent phase serum should be collected two to three weeks after the acute phase serum. Collect5-10ml of blood ina red-top (or tiger-top) tube. Centrifuge, separate from clots, dispense into two sterile tubes (at least 2cc each) for transport, and refrigerate (do not freeze). Transport with cold packs to reach the laboratory within 24 hours of shipment.

West Nile Virus Surveillance in Vermont

As of August 22, 2000 West Nile virus had been confirmed in Massachusetts, Connecticut, and Rhode Island, and in many New York counties, including counties bordering Vermont. In Vermont, surveillance is being conducted. Dead birds submitted from several parts of the state have been tested. As of August 22, over 20 birds were tested, all with negative results. Mosquito surveillance is being conducted, which includes locating mosquito pools, speciating them and testing appropriate species for the virus. Any positive findings in Vermont will be reported through the media, and will be announced on the Health Department’s website at www.state.vt.us/health. Mosquito control measures will be implemented as needed in consultation with local officials and after assessment of viral activity ina region, and potential risk to people.

Even without confirmation of West Nile virus in Vermont, the distribution detected in New York State indicates that it is likely present here. As the first line of efforts to decrease the risk of acquiring West Nile virus, the Health Department is recommending that individuals practice personal risk reduction measures as described in the enclosed fact sheet and at www.state.vt.us/health.


1. New York City Department of Health. West Nile Virus: A Briefing. City Health Information, Vol 19, no.1. May 2000.


West Nile Virus Fact Sheet

What is West Nile virus encephalitis?

West Nile virus encephalitis is an infection of the brain caused by the West Nile virus. West Nile virus first appeared in the United States in 1999, when at least 62 people in the New York City area got sick and seven people died.

How do people get West Nile virus encephalitis?

People get it from the bite of an infected mosquito. Mosquitos are infected when they feed on an infected bird. When an infected mosquito bites a person, the virus is injected into the person and may cause illness. West Nile virus encephalitis is NOT spread from person to person.

Can people get West Nile virus encephalitis from birds?

There is no evidence that a person can get the virus from handling live or dead birds. However, wear gloves whenever handling a dead animal, including birds.

What are the symptoms of West Nile virus encephalitis?

Most cases are mild. Symptoms may include fever, headache, bodyaches, skinrash, and swollen lymph glands. More severe cases can cause headache, highfever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and rarely, death.

Who is at greatest risk of becoming ill from West Nile virus?

People over age 50 have the highest risk of getting severely ill if bitten by a mosquito infected with West Nile virus. Infants may also be at increased risk.

How is West Nile virus encephalitis treated?

There is no specific treatment for West Nile virus encephalitis, but the symptoms can be treated. In severe cases, hospitalization and treatment in an intensive care unit maybe required.

How common is West Nile virus in Vermont?

There have been no documented cases of human illness caused by West Nile virus in Vermont. However, a dead bird infected with the virus was found in Vermont in 2000. West Nile virus has been detected in mosquitos, horses, and/or birds in several surrounding states including New York, New Hampshire, Connecticut, and Massachusetts.

If a mosquito bites me, will I get sick?

Most mosquitos are NOT infected with West Nile virus. Even if an infected mosquito bites you, your chances of getting sick are low. However, you should see a doctor immediately if you develop symptoms such as high fever, confusion, muscle weakness, or severe headaches.


Preventing West Nile Virus

What can I do to protect myself from West Nile Virus encephalitis?

You can begin by taking steps to reduce mosquito breeding areas near your home and bypreventing mosquito bites.

What can I do to keep mosquitos from biting?

What can I do to reduce the number of mosquitos around my home?

Mosquitos need water to reproduce. They can breed in any puddle or standing water that lasts more than four days. By removing areas of standing water around your house, you will eliminate their breeding grounds and reduce the number of mosquitos.



Vermont: Selected Reportable Diseases
January 1, 2000 - August 5, 2000



Pneumococcal Conjugate Vaccine

On February 17, 2000, the Food and Drug Administration announced the licensure of the first pneumococcal conjugate vaccine for the prevention of invasive pneumococcal disease in infants and young children, Prevnar‚ manufactured by Wyeth-Lederle. In June, the Advisory Committee on Immunization Practices (ACIP) finalized recommendations for the use of this vaccine. These recommendations are currently scheduled to be published in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report in early October. Once those recommendations have been published, the Vermont Department of Health will mail additional information to health care providers. If you have any questions, call the Immunization Program at 800-464-4343, x 7638 (in VT) or 802-863-7631.


Delay in Influenza Vaccine and ACIP Influenza Vaccine Recommendations

For the 2000-2001 influenza season in the U.S., lower than anticipated vaccine production and other manufacturing problems are expected to lead toa substantial delay in the distribution of influenza vaccine and possibly substantially fewer total doses of vaccine for distribution than last year.A more precise estimate of the vaccine supply will be available as production progresses during the summer and fall. Because many vaccine providers currently are planning their fall vaccination activities, CDC and the Advisory Committee on Immunization Practices (ACIP) are issuing adjunct influenza vaccination recommendations beyond those made by ACIP on April 14, 2000. The adjunct recommendations given below are specific to the 2000-2001 influenza season.

Adjunct Influenza Vaccine Use Recommendations for the 2000-01 Influenza Season:

from the July 14, 2000 Morbidity and Mortality Weekly Review (MMWR)

  1. Beginning organized influenza vaccination campaigns should be delayed. Health-care providers, health organizations, commercial companies, and other organizations planning organized influenza vaccination campaigns for the 2000-01 influenza season should delay vaccination campaigns until early to mid-November. The purpose of this recommendation is to minimize cancellations of vaccine campaigns and wastage of vaccine doses resulting from delays in vaccine delivery.
  2. Influenza vaccination of persons at high risk for complications from influenza and their close contacts should proceed routinely during regular health-care visits. Routine influenza vaccination activities in clinics, offices, hospitals, nursing homes, and other health-care settings (especially vaccination of persons at high risk for complications from influenza, health-care staff, and other persons in close contact with persons at high risk for complications from influenza) should proceed as normal with available vaccine.
  3. Provider-specific contingency plans for an influenza vaccine shortage should be developed. All influenza vaccine providers, including health-care systems and organizers of vaccination campaigns, should develop a provider-specific contingency plan to maximize vaccination of high-risk persons and health-care workers. These plans should be available for implementation ifa vaccine shortage develops.

The Vermont Department of Health will work with local hospitals, doctors and nurses to implement these recommendations if they become necessary. For information, call the Immunization Program at 800-464-4343, x7638 (in VT) or 802-863-7638, or check the website: www.state.vt.us/health.

1-800-640-4374 OR 1-802-863-7240

Vermont Department of Health
Division of Health Surveillance P.O. Box 70 Burlington, VT 05402-0070
Agency of Human Services
Jan K. Carney, MD, MPH


Peter Galbraith
State Epidemiologist

Susan Schoenfeld
Epidemiology Field Unit Chief