Viruses and bacteria carried by mosquitoes and sometimes ticks 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 proof 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.
If you have signs of encephalitis caused by bacteria, your doctor will prescribe antibiotics. This type of encephalitis is more common during tick season.
Types of encephalitis include:
- West Nile encephalitis. West Nile encephalitis happens mostly in North and Central America, Africa, parts of Asia, the Middle East, and southern Europe. It is more common in older adults and people with an impaired immune system.
- St. Louis encephalitis. St. Louis encephalitis happens mostly in North America (western and eastern United States), Central America, and South America. It is more common in older adults.
- La Crosse encephalitis. La Crosse encephalitis happens mostly in the Midwestern and eastern United States. It is more common in school-aged children.
- California encephalitis. California encephalitis happens mostly in the Midwestern and northeastern United States. It is more common in children.
- Japanese encephalitis. Japanese encephalitis happens mostly in Japan, China, Korea, Taiwan, Southeast Asia, India, Nepal, and northern Australia. It is more common in children.
- Eastern equine encephalitis. Eastern equine encephalitis happens mostly in North America (especially in states in the Atlantic and Gulf Coast of the United States). It is more common in older adults and children.
- Venezuelan equine encephalitis. Venezuelan equine encephalitis happens mostly in Florida and the southwestern United States, Central America, and South America.
- Western equine encephalitis. Western equine encephalitis happens mostly in North America (especially in states west of the Mississippi in the United States and in the prairie provinces in Canada), Central America, and South America. It is more common in older adults and children.
- Powassan encephalitis. Powassan encephalitis happens mostly in the North America (especially the states in New England in the United States, and Canada) and Asia. This type of viral encephalitis is caused by a tick bite.
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.
Rocky Mountain spotted fever can cause encephalitis in some cases. It happens mostly in North, Central, and South America. In the United States, it is most common in the area between North Carolina and Oklahoma. This type of bacterial encephalitis is caused by a tick bite.