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West Nile Virus Disease

West Nile virus is a disease that can be transmitted to humans by mosquitoes. It has been common in Africa, west Asia and the Middle East for decades. It first appeared in the U.S. in 1999 in New York. It has since traveled westward across the U.S. and now is in Colorado.

Nearly 3,000 human cases were reported in Colorado in 2003; 63 Coloradans died from the disease and almost 900 were hospitalized. Most of those who became seriously ill recovered, but some suffered permanent disabilities.

It is very rare to catch this illness, and most infected people will not get sick or will have only mild symptoms. However, West Nile virus can be fatal (by tracey). We want you to have the facts, in order to ease your fears, and so that you can take appropriate prevention measures.

How it is spread:
The virus is carried long distances by infected birds and then spread locally by mosquitoes that bite these birds. Infected mosquitoes can then bite and pass the virus to humans and animals, primarily birds and horses. There is a vaccine for horses, but none for humans. House pets do not spread the illness.

Health departments across Colorado and around the country are closely monitoring human and horse illnesses, testing dead birds, and monitoring mosquito breeding areas.

Mosquito season in Colorado starts in the Spring and ends in mid-September.

Risks and symptoms:
Even in areas where the virus is circulating, very few mosquitoes are infected with the virus. Even if a mosquito is infected, less than 1% of people who get bitten and become infected will get severely ill. The chances a person will become severely ill from any one mosquito bite are extremely small. The risks are low, but why take chances?

All residents of areas where West Nile virus activity has been confirmed are at risk, but people over age 50 seem to be especially vulnerable to the severe forms of disease. In rare cases, it can be fatal. Symptoms generally appear 3 to 14 days after exposure.

West Nile virus can cause paralysis, encephalitis (inflammation of the brain) and/or meningitis (inflammation of the brain's lining). However, most infections are mild and symptoms include fever, headache, body aches, and occasionally include skin rashes and swollen lymph nodes.

More severe infections may include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, muscle weakness or convulsions. Persons with these symptoms need to seek medical attention immediately.

Q. How long does the West Nile virus remain in a person’s body after they are infected?
A. There is no scientific evidence indicating that people can be chronically infected with West Nile virus. What remain in a person’s body for long periods of time are antibodies and “memory” white blood cells (T-lymphocytes) that the body produces to the virus. These antibodies last for years, and may last for the rest of a person’s life. Antibodies are what many diagnostic tests look for when testing blood. Both antibodies and “memory” T-lymphocytes provide future protection from the virus.

Q. If a person contracts West Nile virus, does that person develop a natural immunity to future infection by the virus?
A. It is assumed that immunity will be lifelong; however, it may wane in later years. Continue to use insecrt repellent to prevent any further mosquito bites.

• Print a personal protection tip sheet
• Print a brochure

• Questions and Answers (CDC)
• West Nile Information Pages (Colo.)
• US surveillance maps
• New - Self Care Tips
• New - Support Groups

For more information call the toll-free
Colorado West Nile Virus HelpLine

1- 877- 462-2911
Open 7 a.m. to 11 p.m. daily


CDC West Nile Virus InfoLines
English: 1-888-246-2675
Español: 1-888-246-2857
Hearing-impaired TDD: 1-866-874-2646


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