West Nile Virus and Zika Virus

The West Nile virus, like most mosquito-borne viruses, is found in wild birds. The disease caused by the virus was first described in Africa in 1937, with outbreaks occurring later in Asia, Europe, and the Middle East. The virus was first documented in North America in 1999.

Most people infected with West Nile virus do not experience any symptoms. In about 20% of infected people, symptoms include fever, headache, tiredness, and body aches. In rare instances the virus can infect the human nervous system, causing diseases such as meningitis or encephalitis.

West Nile virus is maintained in nature through a complex life cycle involving wild birds and mosquitoes. When a mosquito feeds on an infected bird, it can pick up the virus and transmit it to other, uninfected birds. Occasionally, infected mosquitoes will feed on mammals such as horses, dogs, cats, and humans, and transmit the virus to them. Mammals are dead-end hosts, however, and do not contribute to the transmission cycle.

*Bulletin for public and private health professionals:

The Centers for Disease Control and Prevention has expanded the case definition of diseases caused by insect-borne viruses (arboviruses). The new 2004 case definition includes the non-neuroinvasive form of arboviral infection (such as West Nile virus fever), and recognizes that arboviral infections involving the central nervous system may result in meningitis or encephalitis, with or without additional signs of brain dysfunction.

Reporting of arboviral infections under the “Encephalitis or Meningitis, Arboviral” category should be disease specific and can include the following: St. Louis encephalitis virus disease, West Nile virus disease, Powassan virus disease, Eastern equine encephalitis virus disease, Western equine encephalitis virus disease, and California serogroup virus disease (includes infections with the following viruses: La Crosse, Jamestown Canyon, snowshoe hare, trivittatus, Keystone, and California encephalitis viruses).

The addition of non-neuroinvasive arboviral infection will promote better tracking of WNV activity, demonstrate the need for public health intervention programs and federal resources, and provide data to allocate resources. In addition, this change will help public health agencies identify high-risk population groups or geographic areas to target interventions and guide analytic studies.