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  West Nile Virus

For the latest up-to-date information on West Nile Virus
in Colorado and across the U.S. please visit the following links:

or Call:  CO HELP  toll free at 1 (877) 462-2911
CO HELP (Colorado Health Education Line for the Public)


1. Where did the West Nile Virus come from?
Answer:  Historically, this virus was found in many parts of Asia, Eastern Europe, Africa, and the Middle East.  In these places, it is mainly known as a disease of children.  In areas where the West Nile Virus originated, most people are believed to be exposed to the disease at an early age, and develop some immunity to it by the time they reached adulthood.  The virus was first detected in the United States during an outbreak in New York City in 1999, and has since spread westward throughout the continent.

2. How do people get the West Nile Virus?
Answer:  Viruses like the West Nile Virus are transmitted to people (and many other animals) when they are bit by an infected mosquito.  Several species of birds serve as a reservoir for the disease, and the mosquitoes can become infected by biting a bird that already has the virus.

3. Do all mosquitoes carry West Nile Virus?
Answer:  No, only certain species of mosquitoes can carry the virus and transmit it efficiently to other animals.  The disease is mainly transmitted between birds and mosquitoes.  People and horses are dead-end hosts and therefore cannot infect other mosquitoes.  In Colorado, the main mosquito vector species for the West Nile Virus is Culex tarsalis, a fairly common mosquito in most parts of the state below 8,000 feet in elevation.  Females of this species will bite birds and mammals (including people), and tend to seek out blood meals during the couple hours before and after sunset and occasionally around sunrise.  Although the species is a very competent vector of the virus, only a very few individual mosquitoes actually become infected, estimated at usually less than 3 out of 1000 in areas where the disease is established.  

4. What are the symptoms of West Nile Virus?
Answer:  Scientists and medical professionals believe that most people who become infected with the virus do not display any symptoms.  For those people who do display symptoms, these can begin anytime from 3 to 20 days after the actual bite occurred.  Two forms of the disease occur in humans – West Nile Fever and West Nile Virus Neuroinvasive Disease.  The “fever” form usually includes a high fever, headache, general weakness, and occasionally a rash.  These symptoms may last for just a few days or as long as several weeks.  The more serious but much less common “neuroinvasive disease” is a form of encephalitis or meningitis, an inflammation of the brain or surrounding tissues.  In these cases, what begins as a sudden high fever and headache quickly progresses to muscle stiffness in the neck, disorientation, muscle tremors, and even coma, and can cause permanent brain damage or may be fatal in the most extreme cases.  Death from the West Nile Virus is very rare, but if you believe you have symptoms including high fever, severe headaches, and a stiff neck, seek out professional medical care. 

5. What other animals get the West Nile Virus?
Answer:  West Nile Virus is mainly a disease of birds.  Although infected mosquitoes may bite any number of different kinds of animals to obtain a blood meal, most will not become ill.  Among the more than 70 bird species recorded to get infected, corvid species (crows, jays, magpies, and ravens) seem most susceptible to the disease and show the highest death rates.  Initially it was a very serious disease for horses, but confirmed cases are less common in recent years with the availability of vaccines for horses to protect them against West Nile.  The disease is extremely rarely reported in other domestic animals such as dogs and cats.

6. What can I do to keep from getting the West Nile Virus?
Answer:  There is a lot you can do!  The FIGHT THE BITE COLORADO campaign ( recommends that you folloow the four D's:

DRAIN standing water around your house weekly since that is where mosquitoes lay eggs, including tires, cans, flowerpots, clogged rain gutters, rain barrels, toys, and puddles.

DUSK AND DAWN are when mosquitoes that carry the virus are most active, so limit outdoor activities or take precautions to prevent mosquito bites.

DEET is an effective ingredient to look for in insect repellants.  Always follow label instructions carefully.

DRESS in long sleeves and pants during dawn and dusk or in areas where mosquitoes are active.