DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

WEST NILE VIRUS
 West Nile Virus

West Nile virus was first recognized in the

United States in 1999 as the cause of severe

and fatal human illness in metropolitan New York

City. West Nile virus is commonly found in Africa,

West and Central Asia, and the Middle East. It is

not known how the virus was first introduced into

the United States, but since the initial

appearance it has spread rapidly, and by 2001

was found throughout the eastern half of the

country.

How is West Nile virus transmitted?

The principal transmission cycle of West Nile

virus involves several species of mosquitoes

and various species of birds. Mosquitoes

become infected with West Nile virus when they

feed on a bird carrying the virus in its blood.

After 10 to 14 days, the virus can be transmitted

to another bird, person, or other animal that the

mosquito bites. During blood feeding the

mosquito injects the virus, contained in its

saliva, into the bird, animal, or person – where

the virus replicates and may cause illness.

See the CDC West Nile virus Web site for

updated informaton:

http://www.cdc.gov/ncidod/dvbid/westnile/

West Nile Virus in the United States, 1999-2001

1999-2001

Pattern indicates human cases 2001

DC

2000-2001* * Vermont

reported WNV

infection in

2000, but not in

2001
Although humans and some other animals may

get sick when infected with West Nile virus, it is

thought that they usually do not develop enough

virus in the bloodstream to infect mosquitoes.

For this reason, humans and animals such as

horses are referred to as incidental hosts. West

Nile virus is not spread from person to person.

The main role that people play in contributing to

the continuing cycle of West Nile virus is by

maintaining environments (especially standing

water) in which mosquitoes can lay eggs.

What are the symptoms of

West Nile virus infection?

Most people who are infected with West Nile

virus will not develop symptoms. Some people

may become ill 3 to 15 days after the bite of an

infected mosquito. Evidence suggests that a

minority of infected persons will develop a mild

illness with fever, headache, body aches, and

sometimes skin rash and swollen glands. There

are no known long-term effects due to mild

illness.

West Nile virus infection infrequently results in a

severe illness, however, known as West Nile

encephalitis. Encephalitis is an inflammation of

the brain that may be marked by headache, high

fever, stiff neck, stupor, disorientation, coma,

tremors, convulsions, muscle weakness, and

paralysis. A small number of cases have been

fatal.
Who is at risk for West Nile virus

infection?

People are only at risk for becoming infected

with West Nile virus when in areas where the

virus is circulating in nature. In northern states,

this is during the summer months, but in

southern states transmission may occur yearround. The risk of West Nile encephalitis (the

severe form of disease) is higher for persons 50

years of age and older.

Even in areas where West Nile virus

transmission is occurring, infection of humans is

still relatively rare. A study carried out in 1999

among residents in the most affected areas of

New York City showed that 2.6% had been

infected with West Nile virus. Studies elsewhere

have shown a lower infection rate. As noted

earlier, most infected persons have no or only

mild symptoms.

There is no documented evidence that a

pregnancy is at risk due to a mother’s infection

with West Nile virus.

How is West Nile virus infection

treated?

No specific therapy is available for infection with

West Nile virus. In the case of West Nile

encephalitis, intensive supportive therapy may

be required–such as hospitalization, intravenous

fluids, respiratory support (ventilator), prevention

of secondary infections (such as pneumonia,

urinary tract infections, etc.) and good nursing

care.

Persons with severe or unusual headaches

should seek medical attention as soon as

possible.
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http://www.cdc.gov/w
For more information on DEET and other

pesticides, see the Web site of the Environmental

Protection Agency (http://www.epa.gov/

pesticides/) or the National Pesticide

Information Center (NPIC) (http://npic.orst.edu/),

phone: 1-800-858-7378, 6:30 a.m. to 4:30 p.m.

Pacific time, 7 days/week, except holidays.

Avoid breeding mosquitoes

Mosquitoes lay their

eggs in standing

water. To avoid

helping mosquitoes

breed in your

environment, drain

standing water.

Routinely empty

water from flower

pots, pet bowls, clogged rain gutters, swimming

pool covers, discarded tires, buckets, barrels,

cans, and other items that collect water in which

mosquitoes can lay eggs.

Community Prevention

Watch for mosquito-breeding areas

Alert local health authorities to potential mosquito

breeding sites in your area, such as storm

sewers, ditches, and abandoned properties with

standing water.

Support disease-tracking activities

Since 1999, federal, state and local governments

have been tracking the occurrence of West Nile

virus infections in birds, mosquitoes, horses, and

people. This tracking relies in part on people

reporting dead and dying birds and horses that

are suspected of being infected with West Nile

virus. Each state or territory collects and reports

this information to CDC. CDC, in collaboration

with the U.S. Geological Survey, maintains
up-to-date maps detailing the distribution and

spread of West Nile virus. These can be seen at:

http://cindi.usgs.gov/hazard/event/west_nile/

west_nile.html

Report dead or dying birds to your state

health department. Avoid touching

dead birds, or any dead

animal, with your bare

hands. Use gloves or an

inverted plastic bag to

place the carcass in a

plastic bag. As policies

may vary, ask your state or local health

department about their requirements for

submitting specimens for testing.

Local governments may make use of other

resources, such as “larvicides” (chemicals that

kill immature mosquitoes) applied to standing

water where mosquitoes breed, or “adulticides”

(chemicals that kill adult mosquitoes) to control

mosquito populations in an area, especially if

people have been infected. Check with your

local health department or local government for

more information about programs to control

disease-carrying mosquitoes in your area.

For more information:

Centers for Disease Control and Prevention (CDC)

National Center for Infectious Diseases

Division of Vector-Borne Infectious Diseases

PO Box 2087

Fort Collins, CO 80522

Fax: 970-221-6476

Email: dvbid@cdc.gov

Web site: http://www.cdc.gov/ncidod/dvbid/

CDC Voice and Fax Information Service:

1-888-232-3228
Fort Collins, CO 80522

P.O. Box 2087, Foothills CampusDivision of Vector-Borne Infectious DiseasesNational Center for Infectious DiseasesCenters for Disease Control and Prevention