West Nile encephalitis, a mosquito-transmitted disease, was first documented in North America during the summer of 1999, when an outbreak occurred in New York City.  Since that time, cases of West Nile Virus (WNV) infection have been confirmed in northeastern, north central, southern and western regions of the United States, and the virus is expected to spread throughout North America.  As of December 2002, there were 3,949 confirmed human cases of WNV, including 254 fatalities.

The strain on WNV circulating in the United States causes significant mortality in exotic and native bird species, especially in the American crow (Corvus brachyrhynchos). Dead birds serve as an early warning that the virus may be active in your area, and these deaths should be reported to your local health department.  (see Related Links for the Utah Health Department).

WNV was first isolated in 1937 in the West Nile province of Uganda, Africa, and can adversely affect humans, birds, and other animals in Africa, western Europe, western Asia, and the Middle East.  Epidemics have occurred in Africa, Europe, and the Middle East, most recently in Israel during 2000.

WNV is transmitted by infected mosquitoes, primarily Culex, Aedes, and Ochlerotatus spp. C.  pipiens, the northern house mosquito, is a common household mosquito and the primary vector of WNV.  Mosquitoes become infected after biting infected wild birds that serve as the primary host of the virus.  The virus undergoes a reproductive cycle inside the mosquito, in which it must pass though the midgut wall, multiply in many tissues, and accumulate in the salivary glands of the mosquito.  Mosquitoes salivate every time they bite, and they are capable of transmitting the virus 10 to 14 days after feeding on an infected bird, so bites after that time are infectious.

Most human infected with WNV have no symptoms.  A small percentage of individuals infected develop mild symptoms that include fever, headaches, body aches, swollen lymph glands, and a body rash.  Encephalitis develops in less than 1 percent of infected people, with severe symptoms that include headache, high fever, neck stiffness, disorientation, tremors, convulsions, paralysis, and coma.  If you experience these symptoms, contact a physician or hospital immediately.  Occasionally, death can occur.  The elderly are most at risk of death due to encephalitis.

The U.S. Geological Survey and the Centers for Disease Control and Prevention are tracking the distribution of the virus in birds, mosquitoes, humans and other animals.  State health departments and university extension personnel may have mosquito control and WNV detection programs for your state. 

For information about West Nile Virus from the MAD-D, click here for a copy of our brochure (pdf).