West Nile virus (WNV) is a flavivirus, a virus of the family Flaviviridae;
these viruses are usually transmitted by mosquitoes and ticks and
cause other serious diseases in addition to WNV, including yellow
fever, dengue fever, and hepatitis C. WNV was first isolated
in 1937 and has been commonly found in humans, birds, and other
vertebrates in Africa, Eastern Europe, West Asia, and the Middle
East, but it had not been documented in the Western Hemisphere
until cases were found in the New York City area in the fall of
1999. The virus can cause encephalitis, which is an inflammation
of the brain. WNV interferes with normal central-nervous-system
functioning. It is closely related to the St. Louis encephalitis
virus which is also found in the United States. In 2000,
the disease occurred in twenty-one people in the New York City
area and northern New Jersey, with only two fatalities. By
2005, the virus was found in all the conterminous United States
and that year it caused a total of 116 deaths.
MosquitoWest Nile Virus
transmitted to humans through the bite of a mosquito. Mosquitoes
become infected when they feed on infected birds, which may have
high virus levels in their blood for a few days. Infected mosquitoes
can then transmit WNV when they feed on humans and animals. The
virus has been found in at least 48 species of mosquitoes, over
250 species of birds, and at least 18 mammalian species. WNV
is not transmitted from person to person and there is no evidence
that a person can get the virus from handling infected birds
or other animals, either live or dead.
Most human infections are mild and symptoms include fever, headache,
and body aches, often with skin rash and swollen lymph glands. More
severe infection may be marked by headache, high fever, neck stiffness,
stupor, disorientation, coma, tremors, convulsions, muscle weakness,
paralysis, and rarely, death.
There is no specific treatment for WNV and, while a vaccine for
horses is available, for people there is no vaccine against the
disease. In more severe cases, treatment may include hospitalization,
use of intravenous fluids and nutrition, airway management, ventilator
support, prevention of secondary infections, and good nursing care. Anyone
who thinks they have WNV should seek medical care as soon as possible. Fatality
rates in humans range from 3 percent to 15 percent and are highest
in the elderly.
The best way to avoid WNV is to prevent mosquito bites.
- When you are outdoors, use insect repellent containing an EPA-registered
active ingredient. Follow the directions on the package.
- Many mosquitoes are most active at dusk and dawn. Be sure to
use insect repellent and wear long sleeves and pants at these
times or consider staying indoors during these hours.
- Make sure you have good screens on your windows and doors to
keep mosquitoes out.
- Get rid of mosquito breeding sites by emptying standing water
from flower pots, buckets and barrels. Change the water in pet
dishes and replace the water in bird baths weekly. Drill holes
in tire swings so water drains out. Keep children's wading pools
empty and on their sides when they aren't being used.
The Centers for
Disease Control and Prevention (CDC), Division
of Vector-Borne Infectious Diseases collects information
on the occurrence of WNV and works to develop improved methods
for disease diagnosis, surveillance, prevention, and control. In
the year 2000, organizations in only 19 States and localities,
along the Atlantic and Gulf coasts, actively participated in
this program. By 2005, 49 states, 5 cities, and the District
of Columbia were participating in surveillance activities. Data
are collected weekly and include information on wild birds, sentinel
chicken flocks, human cases, veterinary cases, and mosquito surveillance. WNV
activity is reported to the CDC through ArboNet, a web-based
Surveillance activities are intended to:
- detect WNV activity
- monitor trends
- assess the impact of the disease on the human population
- identify both geographic areas and populations that are at
- provide information for the allocation of resources
- assess the need for and timing of interventions
- identify larval habitats for targeted control
- improve prevention and control measures
- develop a better understanding of transmission cycles and potential
- provide national and regional information to public health
officials, elected government officials, and the public
Wild Bird Surveillance: Both live and dead birds may be tested
for evidence of the disease. Surveillance for dead crows,
in particular, is a sensitive means to detect the presence of WNV
in an area.
Sentinel Chicken Flock Surveillance: Uninfected chicken flocks
are placed in areas where WNV transmission is expected and are
then monitored for infection. Chickens readily survive WNV
Human Surveillance: Physicians observe patients for clinical signs
of WNV and conduct laboratory testing as appropriate. In
fatal cases, additional laboratory tests may be conducted on tissue
and blood specimens.
Veterinary Surveillance: Veterinarians monitor horses, dogs, cats,
and other species for clinical signs of disease and conduct laboratory
testing as appropriate.
Mosquito surveillance: Adult mosquitoes are trapped and
tested for the presence of WNV. Information on the
affected species and the density of infection is used to identify
potential vector species (those mosquito species which are capable
of spreading the disease) in a particular area, and to predict
the subsequent risk to humans, wild birds, and other animals.