BASIN Home
BASIN Text Image
Current Theme: Waterworks
West Nile Virus Information

West Nile Virus in the Boulder Creek Watershed

With 2003 Colorado deaths from West Nile Virus in the double digits and several hundred ill this new mosquito spread virus has become a significant public health concern. To help the public understand the risks of West Nile Virus and to provide easy access to more detail information, the BASIN team has assembled this summary of West Nile Virus information from local and national sources. We welcome all comments, suggestions, and corrections though the BASIN comment form ).



What is the West Nile Virus ?

West Nile Virus is a blood borne viral infection beleived to have orginated in Central Africa. While relatively common in Africa and Europe thoughout the last half of the 20th century it was unknown in the America's until 1999, when several cases surfaced in the NYC area. In the last 4 years it has been tracked migrating across the United States, first surfacing in Colorado in 2002. Currently it is believed that mosquitos biting infected birds or mammals are the primary vector for the spread of West Nile.

The CDC lists the viral source of the infection as Family "Flaviviridae" and Genus "Flavivirus Japanese Encephalitis Antigenic Complex" which includes the Alfuy, Cacipacore, Japanese encephalitis, Koutango, Kunjin, Murray Valley encephalitis, St. Louis encephalitis, Rocio, Stratford, Usutu, West Nile, and Yaounde viruses. See the CDC Virology: Classification of West Nile Virus page for more information and photos.


WNV Global history

The USGS reports West Nile was first isolated in Uganda in 1937 and it is supected that WNV has caused asymptomatic (undetected) infections over Europe, Africa, the Mid-East and West Asia thoughout the 20th century; however WNV was undetected in the Western Hemisphere until 1999.


WNV migration in the United States

In 1999 the first recorded case of the West African Nile Virus on the North American continent appeared in the New York city area. In 3 short years the virus was tracked crossing the United States and in 2002 the first cases were detected in Colorado; first in birds, then horses and finally humans. To track the spread of this virus the CDC and supporting agencies are currently monitoring 5 populations: mosquitos, wild bird poulations (Avian), "sentinal" domestic poultry populations, horse (Vetinary) and humans.

USGS maps of West Nile Detection Nationwide:

1999Detection limited to New York City Area
2000 Detection limited to Mid Alantic States, human infections limited to NYC and Northern New Jersey Mosquito Avian Horse Human
2001 Detected in several the eastern and upper Mid-West States Mosquito Avian Horse Human
2002 detected from Alantic seaboard through the Rocky Mountain States Mosquito Avian Horse Human
2003 West Nile detected from Alantic through the Rocky Mountain States Mosquito Avian Horse Human

West Nile detection in Colorado ?

In the 3 years WNV has been found in Colorado several local and federal agencies have tracked its spread closly.

USGS Maps of Colorado WNV testing and detection

The following USGS maps indicate the counties within Colorado that confirmed cases of WNV in 2001 and 2002, as well as the counties that had implemented a WNV monitoring program.
2001 14 counties tested birds and 5 counties tested for human cases, none detected Mosquito Avian Horse Human
2002 Numerous horse cases confirmed; 14 tested for human cases with 9 confirmed Mosquito Avian Horse Human

Local Monitoring of NVM incidence

Who is affected by West Nile?

West Nile has been found to infect a variety of mammal and bird species.

The Corvid family of bird species (crows, magpies, ravens, and jays) appear to most susceptible to fatality. Early in the WNV season (May - July) local health departments may be interested in Corvid fatalities as they track the increases in incidence and distribution of the virus. As annual distributions of the virus become well established collection of bird carcasses by local agencies may be suspended. Currently the Boulder County Health www site indicates interests only in recent (<48 hours ie: without odor or insect infestation) carcasses of Corvid species.

West Nile Virus has also been confirmed in many other Colorado bird species, including the rock dove, sandhill crane, fish crow, blue jay, bald eagle, laughing gull, black-crowned night heron, mallard, Canada goose, American robin, sparrow, racing pigeon, red-tailed hawk, broad-winged hawk, ring-necked pheasants, owls as well as domestic and pet birds (source: CO Mosquito Control). The CDC lists 138 bird species. in which WNV has been detected nationwide.

Horses and Humans are the best documented cases of mammal infections however several cases have been confirmed in house pets and wild mammals. WNV can make horses very ill but a horse vacccination has been developed so horse owners are advised to consult a vetinarian. ( CDC FAQ on WNV in Horses)

WNV has also been detected in dogs, cats, bats, chipmunks, raccoons, skunks, squirrels, and domestic rabbits (source: Colorado Mosquito Control, see also CDC FAQ on Pets, and Squirrels.)

While WNV has been detected in several game species hunters should probably be more concerned about exposure to mosquito bites then the safety of game (see the CDC FAQ on Wild Game). One should remember there is currently no evidence of animals transmiting WNV though direct contact, bites or meat consumption.


How is WNV transmitted ?

While the best documented vector for the spread of West Nile is through the mosquito, the CDC has not yet ruled out other transfer mechanisms.

The CDC reports that while WNV has been detected in ticks in Africa, no cases of tick transmission have been documented in North America.

The CDC has now documented several cases of transmission through blood transfusion and organ transplant from an infected sources. (CDC FAQ on transfusions)

The CDC has also confirmed a Michigan case in which an new born child tested positive for WNV following breast feeding and confirmed the presence of the virus in that mother's breast milk. CDC FAQ on breast feeding A single case of intra-placental transmission from a pregnant mother to her fetus. has also documented by the CDC.

Otherwise the CDC currently has no evidence of direct transmission from infected birds, mammals or humans though physical contact. WNV has not been detected in animal saliva so little risk is expected from animal bites. Further there is no evidence of WNV transmission resulting from the consumption of infected birds or mammals.


What are the symptoms West Nile Virus ?

The CDC reports most WNV infected humans have few symptoms. A small proportion may develop mild symptoms that include fever, headache, body aches, skin rash and swollen lymph glands. Less than 1% of infected people develop more severe illness including meningitis (inflammation of the spinal cord) or encephalitis (inflammation of the brain). The symptoms of these illnesses can include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. The CDC reports symptoms typically appear 1-4 days following infection.

The most serious symptoms of West Nile Virus result from the rapid mutiplication of the virus in the blood stream, leading to transmission across the brain or nerve membranes and causing inflammation of brain or spinal tissues (encephalytis or meningitis). Of the few cases that develop encephalitis or meningitis a small proportion die but, overall, the CDC estimates such cases occur in less than 1 out of 1000 infections. CDC on West Nile Symptoms


What treatment is recommended ?

There is no specific treatment for WNV infection or vaccine to prevent it. Treatment of severe illnesses includes hospitalization, use of intravenous fluids and nutrition, respiratory support, prevention of secondary infections, and good nursing care. Medical care should be sought as soon as possible for persons who have symptoms suggesting severe illness; hospitalization can reduce the impact of serious ccases and mimimize the risk of secondary infections.

The CDC reports that individuals do develop an effective immunity upon recovery from WNV and this immunity is anticipated to persist for many years. WNV antibodies have also detected in birds and other mammals.


How does WNV spread ?

While the source of the original NY area infection has not been identified vector control specialists speculate that the virus may have arrived on North American shores in mosquito larvea unintentionally transported in a water filled container, such as a used tire.

Once local mosquito are infected it is suspected the virus spreads via native bird populations, as bird species range far farther than mosquitos (3 - 10 miles) and survive longer than mosquitos (female ~30 days). Birds play an additional critical role in the spread of the virus as an "reservior host", sustaining the virus long (1-4 days) enough and in high enough concentrations to infect a sufficient number of mosquito vectors and pass the virus onto new bird or mammal hosts. While corvid bird species are at createst risk the CDC lists 138 bird species in which WNV has been reported.

Mosquitoes become infected when they feed on infected hosts that have high levels of WNV in their blood. After some internal incubation and multiplication WNV migrates to the mosquito's saliva glands. Infected female mosquitoes then transmit the virus when they feed on humans or other animals. Female mosquitos require mammal blood for the development of eggs, while male mosquitos do not bite mammals.

Of the approximately 65 different species of mosquitoes identified in Colorado only about 10 species are known vectors of WNV, however the CDC lists 36 mosquito species in the US in which WNV or its RNA or antigens have been detected.

WNV is not transmitted from person to person contact and there is no evidence that a person can get infected by handling live or dead infected animals. However, to insure a further level of safety, a protective barrier (such as gloves or an inverted plastic bag) should be used when handling potentially infected birds or other animals (other contamination risks may also be present).


Who is at risk ?

Exposure to active and biting mosquitos appears to be the greatest predictor of risk. The CDC reports that generally risk levels are fairly uniform across all human populations, though weakened indivuals such as the elderly may be at greater risk from the more serious symptoms such as encephalitis or meningitis.


How can it be controlled ?

Public Health responses to West Nile Virus are concentrating on 2 major approaches; monitoring and control of mosquito populations and education of the public on the symptoms and risk of WNV.

Locally, individual communities in Boulder County are working closely with the Boulder County Mosquito Control District to control mosquito breeding and adult populations. Communities in Boulder County have also joined a state wide public education effort called "Fight The Bite" to provide educational materials.


What steps should individuals take ?

The primary risk of exposure to WNV is mosquito bites; accordingly the most recommended procedures to avoid exposure focus on reducing mosquito populations and minimizing the risk of mosquito bites. The "4Ds" catch phrase has been suggested to summarize the most effective individual steps which can help control WNV exposure:

CDC site on WNV Prevention


Mosquito Control Information


Where can one get more information ?

Federal government reources
State resources

Local government resources
Local and regional media resources


INVITATION BASIN is a community project actively seeking public participation. We appreciate all feedback and welcome comments, suggestions and contributions. To find out more about how you can be involved, click here. Help BASIN serve your needs, take our "10 questions in 10 seconds" survey.

BASIN is supported by the US EPA, the City of Boulder, the Keep it Clean Partnership, BCWI and BCN

Home | Site Map | Glossary | Bibliography | Contributors
About BASIN | Attribution | Feedback | Search
Last Page Update - [an error occurred while processing this directive]