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West Nile Virus Facts
What is EPA's involvement with the West Nile Virus issue?
EPA reviews and approves pesticides and their labeling to ensure that the pesticides used to protect public health are applied by methods which minimize the risk of human exposure and adverse health and environmental effects.
In relation to mosquito control, the Agency also serves as a source of information about pesticide and non-pesticide controls to address the concerns of the general public, news media, and the state and local agencies dealing with outbreaks of infectious diseases or heavy infestations of mosquitoes.
- What is it?
- What are the symptoms of infection?
- Where did it come from?
- Why all the concern?
- WNV is spreading rapidly
- What do dead crows have to do with it?
- Prevention and control
- Ways to reduce your risk of being infected
- Eliminate mosquito breeding sites
- What is EPA's involvement with the West Nile Virus issue?
- What is EPA Region 5 doing on WNV?
More information
Fact sheets, state contacts, online resources
What is it?
The West Nile Virus (WNV) is a mosquito-borne virus that can cause a mild fever to encephalitis (swelling of the brain) or meningitis (swelling of the membranes surrounding the brain and spinal cord) in humans and other mammals.
The virus is transmitted to humans, horses and other mammals and birds by the bite of an infected mosquito, most commonly stagnant water species (Culex pipiens).
The virus cycle is maintained in nature between mosquitoes and birds, the latter serving as reservoir hosts. The mosquito becomes infected by biting a bird that carries the virus. An infected mosquito can also spread the virus to a healthy bird. Overwintering adult Culex mosquitoes can harbor the virus and thereby serve as one way of sustaining the disease year to year.
In rare cases, WNV may be transmitted through blood transfusions, organ transplants, transplacentally (mother-to-embryo), and even through breast milk. Otherwise, WNV is not normally transmitted person-to-person in the absence of any exchange of blood. Normally, WNV is not contracted from contact with infected birds, but rare cases have been documented of lab workers being infected through breaks in the skin from infected bird blood. As a precaution, everyone who handles dead birds should wear gloves.
More information (fact sheets, state contacts, online resources)
What are the symptoms of infection?
Most people infected with the virus have no symptoms but, for the majority of those who do become ill, the infection is mild.
Flu-like symptoms, which may appear 5-15 days after the bite of an infected mosquito, include:
- fever
- body aches
- skin rash
- swollen lymph glands.
The elderly and people with compromised immune systems are at a slightly greater risk of a severe infection (encephalitis) which is marked by:
- a high fever
- headaches
- muscle weakness
- convulsions
- coma
- in rare cases, death.
There is no human vaccine or treatment for the disease, but a horse vaccine is available.
More information (fact sheets, state contacts, online resources)
Where did it come from?
The West Nile virus made its first documented appearance in the western hemisphere with the 1999 outbreak in New York City. Previously, the disease was recognized in Africa, Southeast Asia, the Middle East, and Europe.
Insect-borne viruses, referred to as arboviruses, are not new to North America. mosquitoes in the U.S. are known disease carriers of St. Louis encephalitis, LaCrosse encephalitis, Eastern Equine encephalitis, Western Equine encephalitis, and dog heartworm.
More information (fact sheets, state contacts, online resources)
Why all the concern?
During the initial outbreak in 1999, 62 people were hospitalized and there were 7 deaths in the New York City area.
In 2002, there were 4156 cases of acute illness reported, including 284 deaths in the U.S. In 2002, Illinois led the nation in the greatest number of human cases (800 cases, 64 deaths) and Michigan was second (644 cases, 51 deaths). 2002 saw the largest arbovirus epidemic ever reported in the Western Hemisphere and the largest WNV epidemic ever reported worldwide.
A study conducted in 2000 by the CDC and New York City Department of Health found that between 1% to 4% of the people in the surveyed area (north Queens) were infected with the virus, or as many as 1,900 people in a population of 46,220. Further extrapolation by Cornell researchers suggests that from 1999-2002, an estimated 750,000 people may have been exposed to WNV.
These findings suggest that WNV has a greater impact on public health than realized, with milder forms of the illness going unreported.
More information (fact sheets, state contacts, online resources)
WNV is spreading rapidly
Migratory birds spread WNV rapidly.
In 1999, WNV was identified in four states (Connecticut, Maryland, New Jersey, and New York).
In 2000 it had spread to 8 other states (Delaware, Massachusetts, New Hampshire, North Carolina, Pennsylvania, Rhode Island, Vermont, and Virginia) and Washington D.C.
In 2002, the virus was detected in 39 states and the District of Columbia. (Current case count from the Centers for Disease Control (CDC))
Cases of WNV infection were detected earlier in the 2002 mosquito season than in the previous 3 years. For 2002, CDC reported 4156 confirmed human cases of WNV and 284 deaths nationwide.
More Information (fact sheets, state contacts, online resources)
What do dead crows have to do with it?
The West Nile Virus is transferred back and forth between a mosquito (vector) and a bird reservoir (host). Although humans and other mammals can be infected with the virus they are not part of the transmission cycle in nature. Although many bird species can carry the virus, it is particularly virulent in crows and jays in the U.S. As a result, crow deaths serve as a potential indicator of West Nile virus activity in a region.
More information (fact sheets, state contacts, online resources)
Prevention and control
Minimizing mosquito attacks on people is currently the only effective prevention of WNV. This may be achieved by using personal insect repellants when out of doors (see below) and the eliminating potential mosquito breeding sites on your property and in the neighborhood.
Area-wide mosquito control is best handled by local area municipalities or locally taxed mosquito agencies (mosquito abatement districts). Agencies responsible for mosquito control usually employ an Integrated Pest Management (IPM) approach, which includes surveillance, source reduction, larviciding (mosquito larval control) and adulticiding (adult mosquito control) as a last option.
Mosquito, bird, horse, sentinel animal, and human populations may also be monitored for the presence of West Nile and other arboviruses by state agencies. Surveillance is intensified if the area is already affected or is adjacent to an infected area.
mosquitoes must have water to breed. Eliminating mosquito breeding sites (source reduction) is the most effective way to reduce mosquito populations and lessen the threat of disease. This may be as simple as turning over containers that hold water, or it may involve larger-scale engineering projects such as improving a roadway drainage system or managing areas prone to flooding.
Effective control of mosquito larvae will prevent the development of adult mosquitoes. Larviciding (controlling mosquito larvae) can reduce the overall pesticide use in a mosquito control program by decreasing or eliminating the need for ground or aerial spraying to kill adult mosquitoes. The regular sampling of potential mosquito breeding sites and larviciding are the bulk of the work done by local mosquito control agencies. In the Chicago vicinity, for example, there are 11 mosquito abatement districts (MADs).
Adult mosquito control is undertaken to combat the outbreak of mosquito-borne diseases or a heavy infestation of nuisance mosquitoes. Adulticides are U.S. EPA registered pesticides used for this purpose. Adulticides are usually applied as ultra-low volume (ULV) sprays in small amounts (approximately three ounces per acre) to minimize unnecessary environmental exposure while still killing adult mosquitoes . The ULV sprayer disperses fine aerosol droplets that stay air-borne and kill flying mosquitoes on contact.
More information (fact sheets, state contacts, online resources)
Ways to reduce your risk of being infected
- Avoid outdoor activities between dusk and dawn when mosquitoes are likely to be biting.
- If you must be outdoors when mosquitoes are active, cover up by wearing shoes, socks, long pants and long-sleeved shirts.
- Apply mosquito repellent to exposed skin or on clothing. An effective repellant will contain DEET (N, N-diethyl-m-tolumide) as an active ingredient. Follow directions on the label for use, and take special precautions for children. Other repellents are less effective as studies have proved. Ultrasonic devices, "bug zappers," and purple martins are not effective in preventing mosquito bites.
- Maintain screening on windows and doors to keep mosquitoes out of buildings.
More information (fact sheets, state contacts, online resources)
Eliminate mosquito breeding sites
- Remove all discarded tires from your property.
- Empty standing water from buckets, tin cans, plastic containers, flower pots, or similar water-holding containers.
- Make sure roof gutters drain properly, clean clogged gutters in the spring and fall.
- Clean and chlorinate swimming pools and hot tubs. If not in use, keep empty and covered.
- Change the water in bird baths at least once a week.
- Turn over plastic wading pools, and wheelbarrows, etc. when not in use.
- Eliminate any standing water that collects on your property.
- Encourage neighbors to eliminate breeding sites on their properties.
- Alert health authorities to potential breeding sites in your area.
More information (fact sheets, state contacts, online resources)
What is EPA's involvement with the West Nile Virus issue?
EPA reviews and approves pesticides and their labeling to ensure that the pesticides used to protect public health are applied by methods which minimize the risk of human exposure and adverse health and environmental effects.
In relation to mosquito control, the Agency also serves as a source of information about pesticide and non-pesticide controls to address the concerns of the general public, news media, and the state and local agencies dealing with outbreaks of infectious diseases or heavy infestations of mosquitoes.
More information (fact sheets, state contacts, online resources)
What is EPA Region 5 doing on WNV?
Region 5 Pesticides Program Section (PPS) has established a network of public health and other State and local agencies in order to share information among States on West Nile and other mosquito-borne diseases.
PPS continues to monitor WNV activity both in Region 5 and nationwide. PPS provides label interpretations to state regulators and the public regarding mosquito control products. The Region 5 Pesticide Enforcement staff may investigate mosquito control products which are unregistered or improperly labeled.
PPS promotes increased oversight/inspections of mosquito applicators by state regulators.
More information (fact sheets, state contacts, online resources)