From New World Encyclopedia
Classification and external resources
ICD-10 A83-A86, B94.1, G05
ICD-9 323
DiseasesDB 22543
eMedicine emerg/163 
MeSH D004660

Encephalitis is an acute inflammation of the brain, commonly caused by a viral infection. An inflammation that includes both the brain and the spinal cord is called encephalomyelitis (Longe 2006). While the term encephalitis also includes inflammations resulting from a bacterial infection, sometimes an inflammation of the brain specifically caused by a bacterial infection is labelled as cerebritis (Krapp and Wilson 2005). Certain parasitic protozoal infestations, like by toxoplasma or Naegleria fowleri, also can cause encephalitis in people with compromised immune systems.

Encephalitis often is accompanied by an inflammation of the brain's covering (the meninges), a condition called meningitis. Although encephaltis and meningitis are distinctly different diseases, they often share signs and symptoms of inflammation of the meninges (Chamberlin and Narins 2005).

Encephalitis can be very serious. Brain damage occurs as the inflamed brain pushes against the skull, and can lead to death. While most cases of encephalitis are mild, with no residual neurological problems, about ten percent of those with encephalitis die from their infections or secondary complications (Chamberlin and Narins 2005). In the case of herpes encephalitis, the mortality rate is 70 to 80 percent without treatment and still 15 to 20 percent with treatment, and the mortality rate for eastern equine encephalitis is 30 percent (Chamberlin and Narins 2005).

Many cases of encephalitis are preventable, stressing the role of personal and social responsibility. For example, most infections are spread by hand-to-hand or hand-to-mouth contact and frequent hand washing can reduce such infections (Chamberlin and Narins 2005). One can avoid contact with those who have infectious diseases causing meningitis and insect repellents and mosquito nets can minimize danger from mosquito-borne viral infections. Herpes simplex virus (HSV) type two and HIV are sexually transmitted. Vaccines are also available for some viruses, such as polio, herpes B, equine encephalitis, and Japanese encephalitis, as well as rabies for animals (Chamberlin and Narins 2005).


Inflammation is a localized protective response of a body's living tissue to injury, infection, irritation, or allergy. Inflammation is characterized by the following quintet: redness (rubor), heat (calor), swelling (tumor), pain (dolor), and dysfunction of the organs involved (functio laesa). It is part of the innate immune system, that is, the immediate "first-line" of defense to illness or pathogens. Inflammation is usually indicated by using the English suffix "-itis," such as appendicitis, laryngitis, pancreatitis, hepatitis, and encephalitis for inflammation of the appendix, larynx, pancreas, liver, and brain, respectively.

Inflammation is neither “healthy” nor "unhealthy" on its own. Inflammation helps fight disease or injury, such as by removing pathogens and debris, and walling off infection from spreading. However, it comes at the cost of suspending the body's normal immune and catabolic processes. In the short term, this is often a valid trade-off, but in the long term it causes progressive damage.

Encephalitis results in the brain's tissues becoming swollen, which can lead to a headache or fever, or even more severe symptoms (Longe 2006). In the United States, there are about 2,000 cases of encephalitis reported each year to the Centers for Disease Control (Longe 2006).

There are two basic forms of encephalitis. Primary encephalitis occurs when the virus directly infects the brain. Secondary encephalitis occurs when the virus first infects another organ and only secondarily enters the brain, or when there is a post-infectious immune reaction to a viral infection somewhere else in the body (Longe 2006; Chamberlin and Narins 2005). Secondary infections can occur with chickenpox, measles, mumps, and rubella (Longe 2006). Encephalitis may also occur in response to vaccination against a viral disease or by infection by prions, a type of infectious agent made only of protein, lacking the nucleic acids that are part of viruses.

Primary encephalitics can have an epidemic or sporadic cause. Epidemic encephalitis is part of an outbreak, such as the polio virus (Longe 2006; Chamberlin and Narins 2005). Arthropod-borne viral encephalitis, where the viruses live in mosquitoes and animal hosts that transmit the disease, is responsible for most epidemic viral encephalitis (Longe 2006). In the United States, the risk of contracting a virus from a mosquito is greatest in mid to late summer, when they are most active (Longe 2006). Sporadic encephalitis or non-epidemic encephalitis can happen to people at any time of the year, with the most common form caused by herpes simplex virus type one (HSV-1) and another being mumps (Longe 2006; Chamberlin and Narins 2005).


3D reconstruction of herpes simplex virus 1 (HSV-1), based on EM map

Encephalitis is most commonly caused by viral infections, which may be viruses responsible for childhood diseases, or herpes viruses, or arboviruses (those transmitted by insects, such as mosquitoes and ticks, which cause diseases collectively known as arbovirus encephalitis) (Chamberlin and Narins 2005). Common viral diseases and viruses that may result in encephalitis include (Krapp and Wilson 2005; Chamberlin and Narins 2005):

  • herpes simplex (HSV)— the most common cause of sporadic (and fatal) encephalitis
    • HSV-1 (causes fever blisters or cold sores around mouth)
    • HSV-2 (causes genital herpes)
  • herpes B
  • Varicella-zoster virus (cause of chickenpox and shingles.
  • Epstein-Barr virus — causes infectious mononucleosis
  • measels
  • polio
  • rabies (spread by animal bites, such as from dogs and bats)
  • arboviruses — the most common causes of episodic encephalitis
    • St. Louis encephalitis
    • West Nile encephalitis
    • Powassan encephalitis
    • Eastern equine encephalitis
    • Western equine encephalitis
    • California serogroup viral encephalitis
    • Japanese encephalitis
    • Lyme disease (spread by ticks)
    • Colorado tick fever
  • mumps
  • HIV

Herpes simplex encephalitis, if untreated, has a mortality rate between 60 and 80 percent; if treated, that number drops to 15 to 20 percent. About ten percent of all encephalitis cases are caused by herpes simplex virus.

Equine encephalitis is carried by mosquitoes, but ones that normally transmit the disease to horses and birds, which may be picked up by other mosquitoes that do bite humans (Longe 2006). Japanese encephalitis and St. Louis encephalitis are also carried by mosquitoes, whereas Lyme disease and Colorado tick fever are carried by ticks. Japanese encephalitis is found in much of Asia (Longe 2006).

In addition to viral causes, bacterial pathogens can result in inflammation of the brain, and encephalitis can also result from parasites and fungi. As noted above, an infectious particle known as a prion can also result in encephalitis, and vaccination against some viral diseases listed above can also be a causal agent.


Patients with encephalitis suffer from fever, headache and photophobia, with weakness and seizures also common. Less commonly, stiffness of the neck can occur with rare cases of patients also suffering from stiffness of the limbs, lethargy or decreased consciousness (slowness in movement, decreased alertness, fatigue, sleepiness, confusion), and clumsiness depending on which specific part of the brain is involved. The symptoms of encephalitis are caused by the brain's defense mechanisms activating to get rid of the infection. Another symptom of encephalitis is hallucination.


Adult patients with encephalitis present with acute onset of fever, headache, confusion, and sometimes seizures. Younger children or infants may present with irritability, anorexia and fever.

Neurological examinations usually reveal a drowsy or confused patient. Stiff neck, due to the irritation of the meninges covering the brain, indicates that the patient has either meningitis or meningeoncephalitis.

Examination of the cerebrospinal fluid obtained by a lumbar puncture procedure (spinal tap) usually reveals increased amounts of protein and white blood cells with normal glucose, though in a significant percentage of patients, the cerebrospinal fluid may be normal. The cerebrospinal fluid can be analyzed for viral antigens and provide specimens for culture of the virus or bacteria that is present (Chamberlin and Narins 2005).

Blood tests can be used to detect antibodies to antigens of viruses.

CT scan often is not helpful, as cerebral abscess is uncommon. Cerebral abscess is more common in patients with meningitis than encephalitis. Bleeding is also uncommon except in patients with herpes simplex type one encephalitis.

Magnetic resonance imaging offers better resolution. In patients with herpes simplex encephalitis, electroencephalograph may show sharp waves in one or both of the temporal lobes. Lumbar puncture procedure is performed only after the possibility of prominent brain swelling is excluded by a CT scan examination. Diagnosis is often made with detection of antibodies against specific viral agent (such as herpes simplex virus) or by polymerase chain reaction that amplifies the RNA or DNA of the virus responsible.


Treatment is usually symptomatic (treating a patient's symptoms, rather than the disease or injury itself). Reduction of inflammation and brain swelling can be treated with corticosteroids and anti-convulsant drugs can be used to control seizures (Chamberlin and Narins 2005). Fever can be reduced with various anti-pyretic drugs, such as acetaminophen.

Reliably tested specific antiviral agents are available only for a few viral agents (e.g. acyclovir for herpes encephalitis) and are used with limited success for most infection except herpes simplex encephalitis. In patients who are very sick, supportive treatment, such as mechanical ventilation, is equally important.

Bacterial encephalitis can be treated with antibiotics.

Encephalitis lethargica

Encephalitis lethargica is an atypical form of encephalitis which caused an epidemic from 1917 to 1928. There have only been a small number of isolated cases since, though in recent years a few patients have shown very similar symptoms. The cause is now thought to be either a bacterial agent or an autoimmune response following infection.

Limbic system encephalitis

In a small number of cases, called limbic encephalitis, the pathogens responsible for encephalitis attack primarily the limbic system (a collection of structures at the base of the brain responsible for basic autonomic functions).

ISBN links support NWE through referral fees

  • Chamberlin, S. L., and B. Narins. 2005. The Gale Encyclopedia of Neurological Disorders. Detroit: Thomson Gale. ISBN 078769150X.
  • Krapp, Kristine M., and Jeffrey Wilson. 2005. The Gale encyclopedia of children's health: infancy through adolescence. Detroit: Thomson Gale. ISBN 0787692417.
  • Longe, J. L. 2005. The Gale Encyclopedia of Alternative Medicine. Farmington Hills, MI: Thomson/Gale.
  • Longe, J. L. 2006. The Gale Encyclopedia of Medicine. Detroit: Thomson Gale. ISBN 1414403682.

External links

All links retrieved February 13, 2024.


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