What Is Encephalitis?

Encephalitis is an inflammation of the brain. It usually results from an infection, most often by a virus, but sometimes by bacteria, a fungus, or parasites. In rare cases, it is caused by brain injury, a drug or vaccine reaction, or poison.

A virus, or other germ in the blood stream, can be carried to the brain. Germs in the brain attract white blood cells, the body’s main line of defense against invaders, and this sets up an inflammatory reaction. The brain tissue then swells (called cerebral edema); bleeding may occur within the brain (called intracerebral hemorrhage), and brain damage may occur.

About 1 in 200,000 people develop encephalitis each year in the United States. While anyone can succumb, children, the elderly, and those with weakened immune systems are more vulnerable.

Facts About Encephalitis

  • The patients featured in the film “Awakenings” were victims of an epidemic in the 1920s of encephalitis lethargica, which left tens of thousands of people with permanent brain damage. Its cause was never definitively identified, and it was thought to have died out. However, a handful of potential new cases have cropped up in England.
  • Keeping track of encephalitis became more difficult in 1995 after it was taken off the list of illnesses that must be reported to the U.S. Centers for Disease Control and Prevention.


Encephalitis: What Are The Symptoms?

The symptoms of encephalitis may vary widely depending on the cause and severity. They range from no symptoms in very mild cases, to more severe symptoms, including:

  • Muscle weakness; unsteadiness standing up or trouble walking
  • Headache, fatigue, fever, neck pain
  • Stiff neck
  • Nausea and vomiting
  • A foul mood, irritability
  • Aversion to light; squinting
  • Seizures
  • Clumsiness
  • Mental confusion
  • Drowsiness
  • A coma (in severe cases)

Babies may not show symptoms as distinctly. Their bodies may become weak or limp and they may squint or turn away from a light source.

What Causes Encephalitis?

Doctors know surprisingly little about the causes of encephalitis. Outbreaks of encephalitis in a community are usually linked to insect-borne viruses, like the West Nile virus carried by mosquitoes that has sickened more than 50,000 people since it was first detected in the United States in 1999.

But many more encephalitis cases are isolated, and usually even the most diligent attempts to pinpoint a cause fail.

Viruses and bacteria are believed to account for most cases, but a specific germ is positively identified in fewer than half of all cases.

The virus gets into the body through insect bites, skin contact, or in food or drink. The viruses may be carried by mosquitoes or ticks (see arboviruses), especially in rural areas. In urban areas, other types of viruses, usually enteroviruses (see below), may be responsible.

Rabies, passed to people when they are bitten or scratched by an infected animal, causes a deadly form of encephalitis, but is exceedingly rare.

Better known viruses like measles, mumps, rubella, and herpes may also cause encephalitis.

In post-infectious encephalomyelitis, the body apparently overacts to more common infections, like measles (measles encephalitis) and the flu, attacking protective coatings around brain and nerve cells. It has been likened to an “allergic reaction” to an earlier infection. Fortunately it is rare. Measles encephalitis can be particularly severe.

While there are more than 100 germs known to cause encephalitis, some of the more common are:

  • Enteroviruses, which live in the gut and are and spread via feces. Most of these cases are mild with no lasting effects. However, death can occur in rare, very severe cases. They are most likely to appear in summer and fall. They include echovirus, coxsackievirus, and many other viruses.
  • Herpes Simplex Virus, type 1 and type 2. These viruses are responsible for more serious cases. HSV type 2 (genital herpes) is usually passed from a mother to her newborn during delivery. HSV type 1 (which causes cold sores and is carried by most adults) lies dormant in most people, but can become activated and travel to the brain by an infection or another trigger.
  • Arboviruses, which are viruses spread by insects, can cause St. Louis encephalitis, Western equine encephalitis and Eastern equine encephalitis. The most common is California encephalitis, particularly the strain called La Crosse, which is found mostly in the Midwest and eastern United States. Arboviruses are most likely to appear in summer and fall.

Bacterial causes of encephalitis include mycobacterium tuberculosis (TB) and Bartonella henselae, also known as cat scratch disease because that is how it is usually transmitted.

More rarely, encephalitis can be triggered by a brain injury, a brain tumor, drug reactions, or a poison (e.g., lead poisoning), or by as a reaction to a vaccine.

How Is Encephalitis Diagnosed?

Typically, a doctor will ask for a blood sample and order a lumbar puncture (sometimes called a spinal tap), in which a needle is inserted into the lower back and a small amount of fluid (called CSF or cerebro-spinal fluid) is taken from the spinal canal. This may be uncomfortable but is usually not painful when a local anesthetic is used. Doctors send the fluid for tests to see if it contains viruses or substances known to be associated with certain kinds of infections, and to test its chemistry for clues.

Some hospitals are also equipped to take a biopsy , where a tiny amount of tissue is taken from the brain while the patient is under general anesthesia. It is then tested to see if it contains viruses.

Doctors also frequently order a CT Scan or Magnetic Resonance Image (MRI), in which computerized images of the brain are obtained that show the extent of the swelling and damage to the brain.

Another test sometimes used to help confirm a diagnosis is anelectroencephalogram (EEG), which records electrical events in the brain.

How Is Encephalitis Treated?

During the “acute” phase

After the inflammation has subsided

During The “Acute” Phase (while the patient is actively ill with encephalitis)

This phase usually lasts a week or two, after which symptoms resolve gradually or sometimes suddenly.

Because it is often difficult to ascertain the cause of encephalitis, doctors generally prescribe an antibiotic immediately until a bacterial cause can be ruled out.

If herpes virus is suspected, doctors will put the patient on the anti-viral drug acyclovir, usually given intravenously. This is normally done right away because the medicine can help if given early enough. It can also be discontinued if another cause is found.

If the illness is caused by an enterovirus or arbovirus, patients are usually treated for their symptoms and made as comfortable as possible.

  • A quiet, dark environment and medication for headaches is most helpful.
  • Narcotics are sometimes given for pain, but used sparingly to avoid any drug-induced changes in brain function.
  • Drugs like dexamethasone (a steroid) injections or other steroids may be given to reduce the inflammation and swelling of the brain.
  • If seizures occur, they will require treatment with anti-convulsant medications.
  • In rare, severe cases where the patient is unconscious, he or she may need help with breathing. Patients are monitored closely to watch for any possible complications.

For post-infectious encephalomyelitis, doctors can do little more than make the patient comfortable. There have been some reports that corticosteroid treatments may help, although there is not much data available at this point.

After The Inflammation Has Subsided

As the patient recovers, the focus often shifts to rehabilitation if there appears to be any lingering neurological problems. Follow-up evaluations for psychiatric, intellectual, vision or hearing abnormalities are usually warranted. If problems are found, a rehabilitation specialist can prepare a course of treatment to correct them.

Many cases of encephalitis are linked to viruses. The expanding research in virology triggered by the AIDS epidemic may lead to better drugs to combat viral forms of the illness in the future.

How Serious Is Encephalitis?

All cases of encephalitis should be considered a medical emergency.

The severity of the illness varies a great deal. Some cases are mild, and full recovery follows in a week or two. Other cases may be severe with permanent effects or even death.

One of the most dangerous types is measles encephalitis, which can follow a bout with measles and has a death rate approaching 50%. Survivors are likely to have lasting effects like learning difficulties or a physical handicap. Luckily, it is preventable by the common measles vaccination.

Encephalitis due to herpes virus may be serious. When it is passed on to a newborn, it often leaves the baby with neurological damage and is fatal in many instances.

Of the illnesses caused by arboviruses, the most serious are Western equine encephalitis, found mostly in the western U.S. and Canada, and Eastern equine encephalitis, seen mostly in the Atlantic and Gulf Coast regions of the U.S. During outbreaks of Eastern equine encephalitis, as many as 70% of those affected die, and survivors often have permanent brain or nervous system damage.

Western equine encephalitis is most severe in young children and causes seizures in 90% of infants. Half of them will have permanent brain damage.

Other types of encephalitis, such as St. Louis encephalitis and those caused by enteroviruses, are less deadly but can still have serious lasting effects. They include seizures (which can be controlled with anti-convulsant medicines), weakness in a part of the body, partial hearing or vision loss, and a slight alteration of personality. Many problems improve over time but, in some cases, they can be permanent.

Can Encephalitis Be Prevented?

Childhood vaccination has already prevented many cases, but it only works against a narrow range of causes, such as measles encephalitis. If a person is bitten by a rabid animal, an emergency course of treatment is available if started in time.

Pregnant women who carry the genital herpes virus – even if they are not currently showing symptoms – may need to take anti-viral drugs or have a C-section to avoid exposing their newborn to the virus in the birth canal.

The same kinds of precautions used against infectious disease in general also apply to encephalitis prevention, including maintaining good hygiene and avoiding food or water that may be tainted.

Travelers headed overseas may want to get vaccinated or take drugs with them that are effective against infections common in the region they are visiting.

Encephalitis: Putting It All Together

Here is a summary of the important facts and information related to encephalitis.

  • Encephalitis is a serious medical emergency caused when the brain becomes inflamed, usually as the result of an infection.
  • Fever, headaches, fatigue, seizures, nausea and vomiting, and mental confusion are some of the common signs of encephalitis.
  • In most cases, doctors are not able to definitively identify what germ caused the illness. They often give antibiotics and anti-viral medications in the absence of a firm diagnosis so as to err on the side of caution.
  • Outbreaks of encephalitis are usually traced to insect-borne viruses called Arboviruses. However, the majority of cases are sporadic and isolated.
  • While there is a wide variation in outcome, patients with serious cases can die or be left with permanent brain damage. Sometimes the after-effects will improve with time. Many patients recover fully.

Encephalitis: Glossary

Here are definitions of medical terms related to encephalitis.

Arbovirus: A type of virus that lives in insects like mosquitoes and is passed onto humans through insect bites.

Biopsy: A procedure in which a piece of tissue is taken from the body to study under a microscope.

Cerebral spinal fluid: The fluid that surrounds and cushions the brain and spinal cord.

Electroencephalogram (EEG): A test that records electrical events in the brain.

Enterovirus: A type of virus that lives in the gut and is usually passed through feces.

Lumbar puncture: A procedure in which a very fine needle is inserted into the lower back to remove a small amount of cerebral spinal fluid for testing.

Encephalitis: Additional Sources of Information

Here are some reliable sources that can provide more information on encephalitis:

Centers for Disease Control and Prevention 

Mayo Clinic 

National Institute of Allergy and Infection Diseases 
Phone: (301) 496-5717

National Institutes of Health 

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