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Causes
By Mayo Clinic staff
The cause of encephalitis is most often a viral infection. Some examples include:
- Herpes viruses
- Arboviruses transmitted by mosquitoes, ticks and other insects
- Rabies transmitted through animal bites
Encephalitis takes two forms, categorized by the two ways that viruses can infect your brain:
- Primary encephalitis. This occurs when a virus directly invades your brain and spinal cord. It can happen to people at any time of the year (sporadic encephalitis), or it can be part of an outbreak (epidemic encephalitis).
- Secondary (postinfectious) encephalitis. This form occurs when a virus first infects another part of your body and secondarily enters your brain.
Also, bacterial infections, such as Lyme disease, can sometimes lead to encephalitis, as can parasitic infections, such as toxoplasmosis, in people with weakened immune systems.
Here are some of the more common causes of encephalitis:
Herpes viruses
Some herpes viruses that cause common infections may also cause encephalitis. These include:
- Herpes simplex virus. There are two types of herpes simplex virus (HSV) infections. HSV type 1 (HSV-1) more commonly causes cold sores or fever blisters around your mouth. HSV type 2 (HSV-2) more commonly causes genital herpes. HSV-1 is the most important cause of fatal sporadic encephalitis in the United States, but it's also rare.
- Varicella-zoster virus. This virus is responsible for chickenpox and shingles. It can cause encephalitis in adults and children, but tends to be mild.
- Epstein-Barr virus. This herpes virus causes infectious mononucleosis (mono). If encephalitis develops, it's usually mild, but can be fatal in a small number of cases.
Childhood infections
In rare instances, secondary encephalitis occurs after vaccine-preventable childhood viral infections, including:
- Measles (rubeola)
- Mumps
- German measles (rubella)
In such cases, encephalitis may be due to hypersensitivity — an overreaction of your immune system to a foreign substance.
Arboviruses
Viruses that are transmitted by mosquitoes and ticks (arboviruses) have, in recent years, produced well-publicized encephalitis epidemics. Organisms that transmit disease from one animal host to another are called vectors. Mosquitoes are vectors for the transmission of encephalitis from small creatures — usually birds and rodents — to humans. This type of encephalitis is fairly uncommon.
In the United States, the following types of mosquito-borne encephalitis occur:
- Eastern equine encephalitis. This infection generally afflicts horses and birds, especially birds that live near freshwater swamps. It can also affect humans, although fewer than 10 cases are reported in most years. Eastern equine encephalitis outbreaks occur most commonly in the eastern United States. Although some people experience it only as a mild illness, eastern equine encephalitis is fatal in about one-third of the cases. Symptoms of eastern equine encephalitis usually appear three to 10 days after a bite by an infected mosquito.
- Western equine encephalitis. Most reports of western equine encephalitis come from the central and western Plains of the United States. Like eastern equine encephalitis, this infection affects horses and, rarely, humans. It flourishes in birds that live near irrigated fields and farming areas. Symptoms appear between five and 10 days after a bite. Western equine encephalitis is less likely to be fatal than is its eastern cousin, but can result in brain damage and other major complications, particularly in infants.
- St. Louis encephalitis. This virus is transmitted to mosquitoes by birds. The mosquito vector of St. Louis encephalitis breeds in areas of standing water, including polluted pools, roadside ditches and containers such as birdbaths, flowerpots and discarded tires. Symptoms appear within a week to 10 days. The mortality rate can be as high as 30 percent for this type of encephalitis. Older adults have a higher mortality rate from St. Louis encephalitis.
- La Crosse encephalitis. This virus is named for La Crosse, Wis., where the virus was first recognized in 1963. It's most common in the hardwood forest areas of the Upper Midwest and in Appalachia. Unlike other forms of viral encephalitis, this virus is passed to mosquitoes from chipmunks and squirrels. La Crosse encephalitis usually affects children and is rarely fatal. Generally, fewer than 100 cases are reported annually. Symptoms appear five to 15 days after a bite by an infected mosquito.
- West Nile encephalitis. This virus first appeared in the United States in 1999 and spread across most of the country over the next several years. The virus is similar to other encephalitis viruses in that birds are its main animal hosts. However, in rare cases, it's possible for the disease to spread from person to person through organ transplant, blood transfusions or breast-feeding, or from mother to unborn child. Symptoms of West Nile encephalitis are generally mild, but the disease can be severe, especially in older adults and those with weakened immune systems. Symptoms appear within five to 15 days of being bitten by an infected mosquito.
- Encephalitis and meningitis fact sheet. National Institute of Neurological Disorders and Stroke. http://www.ninds.nih.gov/disorders/encephalitis_meningitis/detail_encephalitis_meningitis.htm. Accessed March 21, 2009.
- Tyler KL. West Nile virus infection in the United States. Archives of Neurology. 2004;61:1190.
- Johnson RP. Viral encephalitis in adults. http://www.uptodate.com/home/index.html. Accessed March 12, 2009.
- Willoughbly RE Jr, et al. Encephalitis, meningoencephalitis, acute disseminated encephalomyelitis, and acute necrotizing encephalopathy. In: Long SS, et al. Principles and Practice of Pediatric Infectious Diseases. 3rd ed. Philadelphia, Pa.: Churchill Livingstone; 2008. http://www.mdconsult.com/das/book/body/127224884-3/819357724/1679/50.html#4-u1.0-B978-0-443-06687-0..50049-7--cesec19_1078. Accessed March 21, 2009.
- Eastern equine encephalitis fact sheet. Centers for Disease Control and Prevention. http://www.cdc.gov/ncidod/dvbid/arbor/eeefact.htm. Accessed March 21, 2009.
- St. Louis encephalitis fact sheet. Centers for Disease Control and Prevention. http://www.cdc.gov/ncidod/dvbid/sle/Sle_FactSheet.html. Accessed March 21, 2009.
- LaCrosse encephalitis. Directors of Health Promotion and Education. http://www.dhpe.org/lacenceph.asp. Accessed March 21, 2009.
- Peters CJ. Infections caused by arthropod- and rodent-borne viruses. In: Fauci AS, et al. Harrison's Principles of Internal Medicine. 17th ed. New York, N.Y.: McGraw-Hill Medical; 2008. http://www.accessmedicine.com/content.aspx?aID=2895916. Accessed March 21, 2009.
- Encephalitis. The Merck Manuals: The Merck Manual for Healthcare Professionals. http://www.merck.com/mmpe/print/sec16/ch217/ch217c.html. Accessed March 21, 2009.
- Voelker R. Effects of West Nile virus may persist. Journal of the American Medical Association. 2008;299:2135.
- Insect repellant use and safety. Centers for Disease Control and Prevention. http://www.cdc.gov/ncidod/dvbid/westnile/qa/insect_repellent.htm. Accessed March 22, 2009.
- Follow safety precautions when using DEET on children. American Academy of Pediatrics. http://www.aap.org/family/wnv-jun03.htm. Accessed March 22, 2009.
