West Nile Virus Factsheet

WNV has been commonly found in humans and birds and other vertebrates in Africa, Eastern Europe, West Asia, and the Middle East, but until 1999 had not been documented in the Western Hemisphere.

Scientists believe the virus has been in the eastern U.S. since the early summer of 1999, possibly longer.

Since the disease is spread by mosquitoes, it occurs during seasons when mosquitoes are active, typically late spring through early fall. In very warm climates, it is possible that WNV could be transmitted year round.

Most commonly, people become infected by the bite of a mosquito infected with WNV.

Mosquitoes become infected when they feed on infected birds, which circulate the virus in their blood. Infected mosquitoes can then transmit WNV to humans and animals while biting to take blood. During blood feeding, the virus may be injected into the animal or human, where it may multiply, possibly causing illness.

No. Even in areas where the virus is circulating, very few mosquitoes are infected with the virus. Even if the mosquito is infected, less than 1% of people who get bitten and become infected will get severely ill. The chances you will become severely ill from any single mosquito bite are extremely small.

West Nile encephalitis is NOT transmitted from person-to-person through casual contact. In 2002, WNV was transmitted to a small number of people through transfusions, transplants, and through breast milk.

Persons should avoid bare-handed contact when handling any dead animals. Use gloves and double plastic bags to handle any dead bird.

Following transmission by an infected mosquito, WNV can multiply in the person's blood system and cross the blood-brain barrier to reach the brain. The virus can interfere with normal central nervous system functioning and cause inflammation of brain tissue.

Less than 1% of persons infected with West Nile virus will develop severe illness. Among those who have developed a severe illness with WNV, between 3% and 15% have died. Most of the deaths have occurred among the elderly.

Everyone has a role to play in eliminating standing water in which mosquitoes may breed. Once virus activity is detected in the area, residents should increase their efforts to reduce contact with mosquitoes.

No, but several companies are working towards developing a vaccine. There is a vaccine available for horses, but its effectiveness is not fully known at the present time.

• Stay indoors at dawn, dusk, and in the early evening.
• Wear long-sleeved shirts and long pants whenever you are outdoors.
• Apply insect repellent sparingly to exposed skin. An effective repellent for adults will contain 35% DEET (N,N-diethyl-meta-toluamide), with concentrations of 10% or less for children aged 2-12. Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children.
• Whenever you use an insecticide or insect repellent, be sure to read and follow the manufacturer's DIRECTIONS FOR USE, as printed on the product.
• Install or repair window and door screens so that mosquitoes cannot get indoors.

All residents of areas where virus activity has been identified are at risk of getting West Nile encephalitis. Persons over 50 years of age and those with weakened immune systems have the highest risk of severe disease.

Most infections are mild, and symptoms include fever, headache, and body aches, occasionally with skin rash and swollen lymph glands. More severe infection may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and, rarely, death.

Usually 3 to 15 days.

In the 1999 New York area epidemic, there was a large die-off of American crows. WNV infection has been identified in dozens of species of birds in the United States.  Most of these birds were identified through reporting of dead birds by the public.

As part of their West Nile virus surveillance efforts, the Orange County Vector Control District is selectively testing birds that may have been dead for less than 24 hours. To contact a Vector Control representative, call (714) 971-2421. The State of California has also established a toll-free telephone number for the public to report birds that have been dead for less than 24 hours. That number is 1-877-WNV-BIRD.

• The Centers for Disease Control and Prevention WNV page
  www.cdc.gov/ncidod/dvbid/westnile/index.htm
• The State of California's WNV web site
  www.westnile.ca.gov/
• OC Animal Care WNV Questions and Answers for pets
• OC Animal Care WNV Questions and Answers for horses