Epidemiology Corner

June 21, 2002

West Nile Virus

Two birds found in Harris County tested positive for West Nile Virus this week. West Nile Virus is a mosquito-borne virus that can cause encephalitis, a swelling of the brain, or meningitis, swelling of the lining of the brain and spinal cord, in humans, horses and other mammals. One of a group of viruses called Flaviviruses, West Nile Virus is transmitted to people by the bite of a mosquito. Mosquitoes become vectors, or carriers of the virus, after feeding on an infected bird. Birds are highly susceptible to virus and become very sick and die in large numbers. Certain bird species such as crows and jays are infected more than others, but it is not clear which species will be most affected in the Houston area.

West Nile Virus was first found in the United States in New York in 1999 and birds spread it rapidly down the east coast. In 2001 the virus spread across the south coast and inland from the east coast. From 1999 to 2001, there have been 149 documented human cases of West Nile Virus encephalitis, including 18 deaths. Louisiana has had one human case.

Most people who become infected with the virus do not become ill. People usually experience a mild "West Nile Fever" and do not need medical attention. Encephalitis or meningitis is very rare, usually occurring in persons over 50 with other serious medical conditions. Horses experience more illness than humans, but meningitis and encephalitis are rare, even in horses.

West Nile Virus is closely related to the St. Louis encephalitis virus, another Flavivirus endemic or present in Houston. It also shares a common vector mosquito with St. Louis encephalitis. This mosquito, the Culex, has long been well established in Houston.

In July of 2000, the Centers for Disease Control and Prevention awarded a grant to HDHHS to conduct active physician and laboratory surveillance of human cases of West Nile Virus in Houston and Harris County. Houston is one of only five cities awarded grants for surveillance of this type by the CDC. All other surveillance grants went to states. Surveillance has not detected any human cases in Texas. No mosquitoes infected with the West Nile Virus have yet been found in Texas.

PREVENTION

A vaccine to protect against the virus is available for horses, and this vaccine has been modified to vaccinate rare birds at the Houston Zoo. No vaccine is yet available for humans.

The best prevention of the disease is to eliminate mosquitoes breeding places like standing water and prevent mosquito bites.

• Empty, remove, cover or turn upside down any container that holds water, such as saucers for flowerpots and buckets.
• Change water and scrub birdbaths and vases holding flowers or cuttings at least twice each week.
• Repair leaking pipes and outdoor faucets.
• Empty plastic wading pools weekly and store indoors when not in use.
• Move drain hoses of window or central air conditioning units frequently to reduce standing water.
• Get rid of old tires, cans, bottles, etc.
• Keep gutters free of debris and standing water.
• Check screens on all windows and doors and patch if necessary.
• Wear protective clothing, especially long sleeves and slacks if you have to be outside at dusk, night and dawn when the Culex mosquito is most active.
• Avoid woods, wetlands or marshy areas.
• Use an insect repellant containing DEET and apply as directed. Spray form is best since the product in concentration can be toxic, especially to children. Insect repellant can be applied to your pets and horses as well.

ADDITIONAL INFORMATION