Encephalitis
Encephalitis Overview
Encephalitis is an acute infection and inflammation of the brain itself. This is in contrast to meningitis, which is an inflammation of the layers covering the brain.
Encephalitis is generally a viral illness. Viruses such as those responsible for causing cold sores, mumps, measles, and chickenpox can also cause encephalitis. A certain family of viruses, the Arboviruses are spread by insects such as mosquitoes and ticks. The equine (meaning horse), West Nile, Japanese, La Crosse, and St. Louis encephalitis viruses are all mosquito-borne. Although viruses are the most common source of infection, bacteria, fungi, and parasites can also be responsible.
The illness resembles the flu and usually lasts for 2-3 weeks. It can vary from mild to life-threatening, and even cause death. Most people with a mild case can recover fully. Those with a more severe case can recover although they may have damage to their nervous system. This damage can be permanent.
- Age, season, geographic location, regional climate conditions, and strength of the person's immune system play a role in development of the disease and severity of the illness.
- Herpes simplex (the virus causing cold sores) remains the most common virus involved in encephalitis in the United States and throughout the world.
- In the United States, there are 5 main viruses spread by mosquitoes: West Nile, Eastern equine encephalitis (EEE), Western equine encephalitis (WEE), La Crosse, and St. Louis encephalitis.
- The 1999 New York City outbreak of West Nile virus, spread by the Culex mosquitoes, has caused great concern as the virus continues to spread across the US.
- Venezuelan equine encephalitis is found in South America. It can be a rare cause of encephalitis in Southwestern United States, particularly Texas. The infection is very mild, and nervous system damage is rare.
- Japanese encephalitis virus is the most common arbovirus in the world (virus transmitted by blood-sucking mosquitoes or ticks) and is responsible for 50,000 cases and 15,000 deaths per year. Most of China, Southeast Asia, and the Indian subcontinent are affected.
Encephalitis Causes
- Herpes simplex: This virus causes cold sores and lesions of the genitals. It is transmitted directly through human contact. Newborns can also get the virus by passing through an infected birth canal. Once inside the body, the virus travels through nerve fibers and can cause an infection of the brain. The virus may also undergo a period of latency in which it is inactive. At a later time, emotional or physical stress can reactivate the virus to cause an infection of the brain.
- Arbovirus: Hosts are animals such as birds, pigs, chipmunks, and squirrels that carry the virus. Mosquitoes (known as vectors or ways of transmitting the virus) feed on these animals and become infected. The virus grows and cycles between the hosts and the vectors. Humans become infected through mosquito bites. Once inside the body, the virus replicates and travels in the bloodstream. If there is a large enough amount of the virus, the brain can become infected. The majority of cases occur between June and September when the mosquitoes are most active. In warmer climates, the disease can occur year-round.
- West Nile virus (WNV): This virus was first isolated from an adult woman with a fever in the West Nile District of Uganda in 1937. The nature of the virus was studied in Egypt in the 1950s. In 1957, as a result of an outbreak in Israel in the elderly, the WNV became recognized as a cause for severe inflammation of the spinal cord and brain in humans. In the early 1960s, it was first noted that horses were becoming ill in Egypt and France. This virus then emerged in North America in 1999, with encephalitis reported in humans and horses.
- In 1999, 62 cases of severe disease, including 7 deaths, occurred in the New York area. In 2001, 21 cases were reported, including 2 deaths in the New York area. In 2002, 24 deaths have been reported (as of August 28, 2002).
- The virus cycles between the Culex mosquito and hosts such as birds, horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits. The mosquito feeds on the infected hosts, carries the virus in its salivary glands, and then passes it on to humans or other animals during a blood meal. It usually takes 3-15 days from the time of infection to the onset of disease symptoms.
- West Nile encephalitis is NOT transmitted from person to person (such as through touching or kissing or from a health care worker caring for a sick person) nor can it be passed from animal to human.
- The chance that you will become severely ill from a mosquito bite is extremely small. According to the Centers for Disease Control and Prevention (CDC), even in areas where mosquitoes are reported to carry the virus, much less than 1% are infected. Furthermore, less than 1% of the people who get bitten and become infected will become severely ill. Therefore, the majority of cases are mild, and people can fully recover.
- Prognosis is usually guarded in the extremes of age (infants and elderly). Death as a result of West Nile encephalitis ranges from 3-15% and are highest among the elderly. At the present time, there is no documented evidence to suggest that a pregnancy is at risk due to WNV infection. It is assumed that if a person contracts WNV, he or she will develop a natural immunity that is lifelong. However, it may wane in later years.
- Since the virus first appeared in New York, researchers began looking for a vaccine. According to the Proceedings of the National Academy of Sciences, US government scientists have now developed a vaccine that protects mice from the West Nile Virus infection. Tests showed that mice injected with this vaccine were protected from subsequent exposures to the New York strain of West Nile virus. Researchers were expected to begin testing the vaccine in monkeys in March 2002, with testing in humans likely to take place in late 2002.
- La Crosse encephalitis: The first case occurred in La Crosse, Wisconsin, in 1963. Since then, the largest number of cases has been identified in woodland areas of the Midwestern and Mid-Atlantic United States. This virus is the most common cause of mosquito-borne encephalitis in children younger than 16 years. Each year, about 75 cases are reported to the Centers for Disease Control and Prevention (CDC). The virus cycles between the daytime-biting treehole mosquito (Aedes triseriatus) and hosts such as chipmunks and squirrels. The La Crosse encephalitis virus can cause adverse effects on IQ and school performance. About 1% of people with this infection die.
- St. Louis encephalitis: Since 1964, an average of 128 cases are reported per year. Outbreaks can occur throughout most of the United States, although large urban epidemics occurred in the Midwestern and Southeastern regions. The last major epidemic of St. Louis encephalitis occurred in the Midwest from 1974-1977. There were 2,500 cases in 35 states reported to the CDC. Most recently, there were 20 reported cases in New Orleans in 1999. The virus cycles between birds and the Culex mosquitoes breeding in stagnant water. It grows in both the mosquito and the bird but does not make either one sick. Only the infected mosquito can transmit the disease to humans during the blood meal. The virus cannot be transmitted from person-to-person through kissing or touching nor can it be transmitted from the infected bird. The disease tends to affect mostly adults and is generally milder in children.
- Eastern equine encephalitis (EEE): According to the CDC, there have been 153 confirmed cases in the United States since 1964. This virus is found along the East and Gulf coasts. The virus causes severe disease in horses, puppies, and birds such as pheasants, quails, and ostriches. In humans, flulike symptoms develop 4-10 days after the bite of an infected mosquito. Usually, human illnesses are preceded by those in horses. EEE can cause death in 50-75% of all cases. Those who recover may suffer severe permanent brain damage such as mental retardation, seizures, paralysis, and behavior abnormalities.
- Western equine encephalitis (WEE): This virus was isolated from the brain of a horse with encephalitis in California in 1930. Since 1964, there have been 639 confirmed cases. Today, it remains an important cause of encephalitis in the western part of the United States and Canada. In 1994, there were 2 confirmed and several suspected cases of WEE reported in Wyoming. In 1997, 35 strains of WEE virus were isolated from mosquitoes collected in Scotts Bluff County, Nebraska. The WEE virus cycles between certain types of birds (small, mostly songbirds) and the Culex tarsalis mosquitoes, a species associated with irrigated agriculture and stream drainage. The virus has also been found in several other mammals. Horses and humans become sick through mosquito bites. Infants are particularly affected and can have permanent problems such as seizure disorders and developmental delay.
- Venezuelan equine encephalitis (VEE): This virus is found in Central and South America and is a rare cause of encephalitis in the Southwestern part of the United States. It is an important cause of encephalitis in horses and humans. From 1969-1971 an outbreak from South America to Texas killed over 200,000 horses. In 1995, there were an estimated 90,000 human infections with VEE in Columbia and Venezuela. The virus cycles between forest-dwelling rodents and mosquito vectors, especially the species Culex. VEE infection in humans is much less severe than that of WEE and EEE. While adults tend to develop a flulike illness, children tend to develop overt encephalitis. Deaths are rare in humans but are common in horses. There is an effective vaccine for horses.
- Japanese encephalitis: This virus is responsible for 50,000 cases and 15,000 deaths per year. Most of China, Southeast Asia, and the Indian subcontinent are affected. The geographic distribution is expanding. Rarely, cases may appear in United States civilians and military personnel traveling to and living in Asia. Children and young adults are mostly affected. Older adults are affected when there are epidemics in new locations. The virus cycles between domestic pigs, wild birds, and the Culex tritaeniorhynchus mosquitoes, which breed in rice fields. The disease is not transmitted through human contact, pigs, or birds. Only the mosquitoes can transmit the disease during feedings.
| Disease |
Geographic Location |
Vector/ Hosts |
Comment |
| Herpes encephalitis |
United States/The world |
Human-to-human
contact |
Prompt treatment with acyclovir
increases survival to 90%
|
| West Nile encephalitis |
Africa, West Asia, Middle East, United States |
Mosquito/mostly birds |
Majority are mild cases. Less than 1% of those infected will become severely ill. Full recovery expected. Vaccine for humans tested late 2002. |
Eastern equine
encephalitis |
East Coast (from
Massachusetts to Florida),
Gulf Coast |
Mosquito/ Birds |
Often occurs in horses. High
mortality rate (50-75%);
frequent outcomes (seizures, slight paralysis), especially
in children |
Western equine
encephalitis |
Western United States and
Canada |
Mosquito/ Birds |
Often occurs in horses.
Particularly affects infants |
Venezuelan equine
encephalitis |
Western Hemisphere |
Mosquito/ Rodents |
Rare in United States; low
mortality rate, rare after-effects |
| La Crosse encephalitis |
Throughout the United States,
especially in Midwestern &
Southeastern regions
|
Mosquito/ Chipmunks,
Squirrels |
Most common cause of
encephalitis in children younger
than 16 years old
|
| St. Louis encephalitis |
Midwestern & Mid-Atlantic
United States
|
Mosquito/ Birds |
Mostly affects adults |
| Japanese encephalitis |
Temperate Asia, southern
and southeastern Asia
|
Mosquito/ Birds and pigs |
Vaccine available. See
Prevention section.
High morbidity/mortality rates
|
Summary of Viral Encephalitis. Sources: Tierney, et al.; CDC: cdc.gov
Encephalitis Symptoms
The signs and symptoms of encephalitis are the same for adults and children.
- Signs and symptoms may last for 2-3 weeks, are flu-like, and can include 1 or more of the following:
- More severe cases may involve these signs and symptoms:
- Seizures
- Muscle weakness
- Paralysis
- Memory loss
- Sudden impaired judgment
- Poor responsiveness
When to Seek Medical Care
Call your doctor for immediate advice if you develop signs and symptoms of encephalitis and you have any of these conditions:
- You have sores around the lips or genitals through contact with another person.
- You were in wooded or forest areas and suspect mosquito bites.
- You were visiting an area where these diseases are common, especially outside the United States.
- You were bitten by a tick.
If you develop signs and symptoms of encephalitis and your doctor is not available, go immediately to a hospital's
Emergency Department for further evaluation. Do not hesitate or decide on your own that you simply have the flu. Symptoms indicating severe illness require emergency treatment.
Exams and Tests
Geographic location and seasonal occurrence can help identify the specific cause of encephalitis. Depending on your unique situation, the doctor may perform 1 or more of the following tests:
- A picture of the brain such as a CT scan or magnetic resonance imagining (MRI) is often done. MRI is the procedure of choice if herpes encephalitis is suspected.
- A reading of the electrical activity of the brain with an EEG can detect irregularities. Herpes encephalitis produces a characteristic EEG pattern.
- A lumbar puncture, also known as a spinal tap, may be necessary to isolate the virus. During this procedure, the doctor applies local numbing medication and then inserts a needle into your lower back to collect fluid from the space around the spinal column for analysis.
- The virus may also be isolated from tissue or blood.
- Brain biopsy is an option although it is rarely done and usually only if the other tests do not give an answer.
Encephalitis Treatment
Self-Care at Home
Because encephalitis can cause death, seek treatment in a hospital's emergency department. Any home treatment to relieve the flulike symptoms should be carried out according to the doctor's advice and recommendation after diagnosis.
Medical Treatment
Encephalitis is usually a viral illness, which means that antibiotics are not used to treat it. The only available vaccine for prevention is for Japanese encephalitis.
- With the exception of herpes encephalitis, the mainstay of treatment is symptom relief. People with encephalitis are kept hydrated with IV fluids while monitoring for brain swelling. Anticonvulsants can be given for seizure control. Steroids have not been established as being effective.
- Herpes encephalitis can cause rapid death if not diagnosed and treated promptly. Therefore, medication is usually started when the doctor suspects herpes to be the diagnosis without waiting for the confirmatory results. The recommended treatment is acyclovir (Zovirax) given by IV for 2-3 weeks. Acyclovir-resistant herpes encephalitis can be treated with foscarnet (Foscavir). Liver and kidney functions are monitored through the course of medication.
- Currently, the use of ribavirin (Rebetol, Virazole) in the treatment of a child with La Crosse encephalitis is being studied.
Next Steps
Follow-up
It is important to follow up with your doctor after the initial treatment because certain nervous system problems can develop after what appears to be a successful initial treatment. Relapse can occur with herpes encephalitis.
Prevention
- Seek early treatment for any high fever or infections.
- Wear long pants and long-sleeved shirts to avoid ticks and mosquitoes when in forests or grassy areas.
- Use insect repellant in exposed areas of the body.
- Avoid spending a long time outdoors during dusk when insects tend to bite.
- National surveillance and control of mosquitoes through aerial spraying can keep insect populations under control.
- A Caesarian section (C-section) can be performed if the mother has active herpes lesions to protect the newborn.
- Vaccinate children against viruses that can cause encephalitis (measles, mumps).
- Japanese encephalitis can be prevented with 3 doses of the vaccine. Take precautions when traveling to areas where this strain is common.
- According to the Centers for Disease Control and Prevention, the vaccine is NOT recommended for all travelers to Asia. It should be offered to people spending a month or longer in areas where the disease-causing mosquitoes are known to be and during the transmission season. However, travelers spending fewer than 30 days should receive the vaccine if the area is experiencing an epidemic outbreak.
- The benefit of the vaccine should be weighed against the side effects and the risk of developing the disease by getting the shot. The risk of developing serious allergic reaction such as hives is low.
- Special consideration should be given to the elderly and pregnant women. The elderly have a higher chance of developing symptoms with infection. The Japanese encephalitis virus can infect the fetus and cause death. Therefore, these 2 groups should be cautious when traveling abroad.
Outlook
The outcome of the disease varies and depends on factors such as age, severity of the case, and strength of the immune system. For example, people who are HIV positive, have cancer, or other illnesses have a weaker immune system and are less able to withstand another disease. In general, those with mild cases will recover without any problems.
- The death rate for viral encephalitis can be high.
- The St. Louis encephalitis virus can cause death in up to 30% of the cases.
- Japanese encephalitis can cause death in up to 60% of the cases, usually within the first week of illness.
- In untreated cases of herpes encephalitis, 50-75% of people die within 18 months. Acyclovir (Zovirax) can increase survival up to 90%.
For More Information
Web Links
MedlinePlus, Encephalitis
Multimedia
Media file 1: Culex mosquito laying eggs. Courtesy of Centers for Disease Control, Division of Vector-Borne Infectious Diseases.
Media type: Photo
Media file 2: How to recognize the Culex mosquito. Courtesty of Centers for Disease Control, Division of Vector-Borne Infectious Diseases.
Media type: Photo
Media file 3: Horses die from viral encephalitis. Courtesty of Centers for Disease Control, Division of Vector-Borne Infectious Diseases.
Media type: Photo
Media file 4: Number of cases of St. Louis encephalitis by state. Courtesy of Centers for Disease Control, Division of Vector-Borne Infectious Diseases.
Media type: Photo
Media file 5: Transmission cycle of St. Louis encephalitis. Courtesy of Centers for Disease Control, Division of Vector-Borne Infectious Diseases.
Media type: Photo
Media file 6: Where Japanese encephalitis occurs. Courtesy of Centers for Disease Control, Division of Vector-Borne Infectious Diseases.
Media type: Photo
Synonyms and Keywords
encephalitis, brain inflammation, West Nile virus, WNV, West Nile encephalitis, Japanese encephalitis, La Crosse encephalitis, arboviruses, St. Louis encephalitis, herpes encephalitis, Venezuelan equine encephalitis, VEE, Eastern equine encephalitis, EEE, Western equine encephalitis, WEE, horse
Authors and Editors
Author: Todd Mundy, MD, Staff Physician, Department of Emergency Medicine, New York Methodist Hospital.
Coauthor(s):
Mai Kim Lai, MD, Staff Physician, Department of Emergency Medicine, Sparrow Hospital, Michigan State University College of Human Medicine;
Steven H Silber, DO, FACEP, Clinical Assistant Professor, Department of Emergency Medicine, Weill Medical College of Cornell University; Vice Chair, Department of Emergency Medicine, New York Methodist Hospital.
Editors: Michael D Burg, MD, Assistant Clinical Professor, Department of Emergency Medicine, University Medical Center, University of California at San Francisco-Fresno; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; James S Cohen, MD, Consulting Staff, James Cohen, PC.
Last Editorial Review: 10/6/2005
