Mosquito- and tick-borne encephalitis

Viruses carried by mosquitoes and sometimes ticks (arboviruses) are some of the most common causes of encephalitis throughout the world. Most people who are bitten by infected mosquitoes do not develop any symptoms or develop only a mild flu-like illness. For those who do develop encephalitis, symptoms may develop within a few days or up to 2 weeks after the mosquito bite occurred, depending on which virus the mosquito was carrying.

Encephalitis caused by mosquito-borne viruses is spread by the bite of an infected mosquito. It also may be transmitted through blood transfusions and breast-feeding. There is no evidence that it can be passed from one person to another through casual contact or from an animal to a person.

The mosquito-borne viruses that cause encephalitis cannot be destroyed with antiviral or other medicines. Treatment for these types of encephalitis focuses on reducing symptoms and providing supportive care while the body heals.

Types of viral encephalitis carried by mosquitoes in North America include:

  • West Nile encephalitis. Until recently, West Nile encephalitis had occurred mostly in Africa, the Middle East, western Asia, and parts of Europe. It first appeared in North America in 1999, when a number of cases were reported in the northeastern U.S. (New York, Connecticut, New Jersey, and Maryland). In 2002, a wider outbreak occurred in the southeastern United States and moved west. For updated information, see the CDC figures online at
  • St. Louis encephalitis. St. Louis encephalitis is the most common mosquito-borne illness in the U.S. Epidemics of St. Louis encephalitis have occurred in the Midwest and southeastern states, but cases have been reported throughout the U.S. Severe illness most often affects older adults. The death rate is 5% to 15%.1
  • La Crosse encephalitis. This type of encephalitis occurs mainly in the upper Midwest (Wisconsin, Minnesota, Iowa, Illinois, Indiana, and Ohio), although there have been some cases reported in the mid-Atlantic and southeastern states. La Crosse encephalitis typically affects children under 16 who live on farms or in forested suburbs. Young children are most likely to have lasting problems after recovery, such as seizures or partial paralysis. Death occurs in less than 1% of cases.1
  • Eastern equine encephalitis. This rare and often fatal type of encephalitis occurs mainly in the Atlantic and Gulf Coast areas of the United States. It most often affects horses but also may cause illness in humans and other animals. About one-third of people who develop eastern equine encephalitis die from the illness.1 Many people who recover from this illness have permanent brain damage.1 Death and severe illness are most common in young children and older adults. A vaccine is available for horses.
  • Western equine encephalitis. This type of encephalitis occurs mainly in the western United States and Canada. Children, especially babies, are affected more severely than adults, and up to 30% may have lasting problems, such as seizures or delays in development. Death occurs in about 3% of cases.1 The virus also is common in horses, and a vaccine is available for horses.

Most cases of mosquito-borne encephalitis occur during the warmest months of the year, when mosquitoes are most active. In areas that are warm year-round, people may get encephalitis throughout the year.

Powassan encephalitis is a rare tick-borne arbovirus found in North America. Although it is one of the most rare causes of arbovirus encephalitis, Powassan encephalitis has one of the highest death rates among the arboviruses. This virus is spread mainly through tick bites, but it may also be transmitted by drinking raw milk from infected animals.


  1. Centers for Disease Control and Prevention (2005). Information on arboviral encephalitides. Available online:

Last Updated: August 5, 2008

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