Encephalitis

Introduction

Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues

Primary Source Connection

BIBLIOGRAPHY

Introduction

Encephalitis is a type of acute brain inflammation, most often due to infection by a virus. When the inflammation occurs in the spinal cord, the condition is called myelitis, and when inflammation is in both the spinal cord and the brain, the condition is called encephalomyelitis. However, in reality, an infection in both areas is often referred to as encephalitis. The swelling in the brain that occurs in encephalitis can be serious and even life-threatening; brain damage, strokes, seizures, coma, and death can result. Encephalitis often accompanies bacterial meningitis (an infection of the lining of the brain known as the meninges).

Disease History, Characteristics, and Transmission

There are two types of encephalitis—a primary form and a secondary form. Primary encephalitis is directly due to a new viral infection. This form of encephalitis can be localized in just one region of the brain or spinal cord (focal infection) or can be more widely distributed (diffuse infection).

WORDS TO KNOW

ACUTE: An acute infection is one of rapid onset and of short duration, which either resolves or becomes chronic.

ARTHROPOD-BORNE DISEASE: A disease caused by one of a phylum of organisms characterized by exoskeletons and segmented bodies.

LATENT: A condition that is potential or dormant, not yet manifest or active, is latent.

MENINGITIS: Meningitis is an inflammation of the meninges—the three layers of protective membranes that line the spinal cord and the brain. Meningitis can occur when there is an infection near the brain or spinal cord, such as a respiratory infection in the sinuses, the mastoids, or the cavities around the ear. Disease organisms can also travel to the meninges through the bloodstream. The first signs may be a severe headache and neck stiffness followed by fever, vomiting, a rash, and, then, convulsions leading to loss of consciousness. Meningitis generally involves two types: non-bacterial meningitis, which is often called aseptic meningitis, and bacterial meningitis, which is referred to as purulent meningitis.

VECTOR: Any agent, living or otherwise, that carries and transmits parasites and diseases. Also, an organism or chemical used to transport a gene into a new host cell.

Secondary encephalitis, or post-infective encephalitis, arises as a consequence of an ongoing viral infection or from an immunization procedure that utilizes a virus. The latter uses a virus that has been altered to be incapable of causing harm. However, in rare cases, the vaccine itself becomes harmful. Secondary encephalitis, which is also termed acute disseminated encephalitis, typically appears 2–3 weeks following the first infection or the immunization injection.

Worldwide, encephalitis due to infections by the herpes simplex viruses causes only about 10% of all cases of the disease. However, over half of these cases result in death. Many of the cases involve the reactivation of an earlier infection by a herpes simplex virus that became latent (this occurs when the viral genetic material is incorporated into the host's genetic material). Upon reactivation, production of new copies of the virus resumes, and the symptoms associated with the infection appear.

The original infection may be with herpes simplex type 1, which commonly causes cold sores and facial blistering. Encephalitis related to this virus can occur in anyone, but is most prevalent in people under 20 years of age and older than 40. The disease is contagious, being spread most often by inhalation of water droplets expelled by a cough or sneeze. The person who contracts the infection develops a headache and fever that can last almost a week. Subsequently, changes in personality and behavior, seizures, and delusions may appear, and severe brain damage may result. Encephalitis due to herpes simplex type 2 is typically spread through sexual contact or, less commonly, a newborn can contract the virus from his or her infected mother during birth.

In the United States and Canada, Powassan encephalititis is transmitted to humans by ticks, which have previously acquired the virus from infected deer. The symptoms of Powassan encephalitis—headache, fever, nausea, and disorientation—begin within two weeks following the tick bite. Paralysis and coma also can occur. About 50% of those who contract Powassan encephalitis will have permanent brain damage and more than 15% of those who become infected die of the infection.

In the United States, there are four types of mosquito-borne viral encephalitis. These are: equine encephalitis, LaCrosse encephalitis, St. Louis encephalitis, and West Nile encephalitis.

Rarely, a form of the disease known as limbic system encephalitis occurs. The primary symptom of this form of encephalitis is memory impairment similar to what is seen in individuals suffering from Alzheimer's disease or Creutzfeldt-Jacob disease. A variation of limbic encephalitis called paraneoplastic limbic encephalitis is linked to the development of cancer.

Scope and Distribution

In the United States, there are several thousand reported cases of encephalitis every year. However, according to the U.S. National Institute of Neurological Disorders and Stroke, the actual tally is likely much higher, since many people do not seek medical help for cases that produce mild symptoms or no symptoms at all.

Encephalitis occurs in many regions of the world. For example, mosquito-borne forms of encephalitis are present in North and South America, Europe, Russia, Asia, India, northern Africa, and even Australia.

In the United States, encephalitis is usually caused by an enterovirus, by the herpes simplex virus types 1 and 2, by an arbovirus that is transmitted from an infected animal to humans via a vector such as a mosquito (an example is West Nile disease) or tick, or by the bite of a rabid animal such as a raccoon that is infected with the rabies virus. Lyme disease, which is caused by the bacterium Borrelia burdorferi, can also cause encephalitis.

One factor that has contributed to the global distribution of encephalitis is the fact that it is contagious, and can be passed from person to person by coughing or sneezing, releasing contaminated droplets into the air. In addition, the microorganisms that cause encephalitis can contaminate food and water.

Herpes simplex-mediated encephalitis is rare in the United States, occurring in about 1 person per 250,000– 500,000 population per year. The other forms of the disease can occur more frequently. For instance, children can develop encephalitis after a bout of measles, mumps, or rubella.

Viral encephalitis that is transmitted to humans via a vector, like a mosquito or tick, is more common in the United States. One such vector-borne encephalitis is equine encephalitis. As its name implies, equine encephalitis can affect horses. This disease in horses can be serious, and often comes before the detection of the disease in humans. A form called eastern equine encephalitis (EEE) is prevalent along the eastern coastal region of the United States and the coast of the Gulf of Mexico. Fever, muscles aches, and headache develop 3–10 days after being bitten by a virus-carrying mosquito. The headache becomes progressively worse, and, in severe cases, a person can lapse into a coma and die. The disease is still rare, despite having been known to occur in the United States since the 1930s. Only an average of five cases of eastern equine encephalitis appear in the United States each year; for these people, however, the consequences can be dire, as up to 50% may die.

The natural host of the EEE virus is still not precisely known, but the virus tends to infect birds that live near freshwater swamps. Whether the virus can survive the winter in northern climates is also unknown. Surveys of birds that live year-round in the northern climates have not detected the virus, and scientists suspect that returning migratory birds in the spring bring the virus back to these areas.

Western equine encephalitis is distributed in the western and central states of the United States. The virus was isolated in the United States in 1930 from an infected horse. Both horses and humans can be affected by this disease. The virus normally resides in a number of species of animals and birds, and is transmitted by mosquitoes. Symptoms begin about a week following infection. Children are particularly at risk of developing a severe form of the disease that can produce permanent brain damage.

The prevalence of western equine encephalitis has been influenced by agricultural practices. For example, the increasing irrigation of land has created more regions of stagnant water, which become breeding grounds for mosquitoes. In addition, the land becomes populated by bird species that naturally carry the virus.

Another form of equine encephalitis called Venezuelan equine encephalitis has killed thousands of people in epidemics in Central and South America. Survivors can have permanent brain damage.

LaCrosse encephalitis is another form of vector-borne encephalitis found in the United States. It is named for LaCrosse, Wisconsin, where the disease was first detected in 1963. It is typically distributed in midwestern states including Illinois, Indiana, Ohio, Iowa, and Wisconsin, but also has occurred in more eastern states. The virus is passed to mosquitoes from infected chipmunks and squirrels. Children and adolescents under 16 years of age are most at risk. Headache, fever, vomiting, and fatigue develop about one week following the mosquito bite. In more severe cases, a person can experience seizures. About 100 cases occur in the United States each year.

St. Louis encephalitis has been among the most common encephalitis diseases reported in the United States. Typically, there are around 130 cases reported to the U.S. Centers for Disease Control and Prevention (CDC) each year, although outbreaks can make thousands of people ill. There have been approximately 4,500 reported cases since 1964, with an average of almost 200 cases each year. The disease is transferred to mosquitoes from infected birds. In contrast to other forms of encephalitis, adults are affected more severely than are children. Symptoms include headache, fever, and, in more serious cases, mental disorientation, muscles tremors, convulsions, and unconsciousness.

IN CONTEXT: INFLAMMATION AS A NON-SPECIFIC DEFENSE

Inflammation is a localized, defensive response of the body to injury, usually characterized by pain, redness, heat, swelling, and, depending on the extent of trauma, loss of function. The process of inflammation, called the inflammatory response, is a series of events, or stages, that the body performs to attain homeostasis (the body's effort to maintain stability). The body's inflammatory response mechanism serves to confine, weaken, destroy, and remove bacteria, toxins, and foreign material at the site of trauma or injury. As a result, the spread of invading substances is halted, and the injured area is prepared for regeneration or repair. Inflammation is a nonspecific defense mechanism; the body's physiological response to a superficial cut is much the same as with a burn or a bacterial infection. The inflammatory response protects the body against a variety of invading pathogens and foreign matter, and should not be confused with an immune response.

SOURCE: Centers for Disease Control and Prevention

The final mosquito-borne encephalitis is West Nile disease. Its geographical distribution in the United States has expanded since it was first detected in 1999. The disease is also found in Canada, Africa, the Middle East, Russia, India, and Indonesia. People whose immune systems are impaired are most at risk. In addition to transmission by mosquito, the virus can be present in transplanted organs or transfused blood and blood products.

Japanese encephalitis is the most common cause of encephalitis worldwide. Approximately 50,000 cases and 15,000 deaths occur each year, according to the World Health Organization (WHO). This form of encephalitis is common in certain regions of Asia, including China, Korea, Japan, Taiwan, Sri Lanka, and the south of India. It also occurs on some Pacific islands. The disease is especially prevalent where rice production and pig rearing occur. This is because the mosquitoes that spread the disease can breed in the rice paddies and pigs are a host of the virus. The mosquitoes acquire the virus when taking a blood meal from a pig and then can spread the virus to humans.

Treatment and Prevention

The diagnosis of encephalitis involves an assessment of nerve function, hearing, speech, vision, balance and coordination, mental capability, and changes in behavior. The examination of body fluids, such as urine and blood, and a swab from the throat can be useful in revealing a bacterial or viral infection.

Tests that rely on growth of bacteria or the appearance of clear zones in a layer of bacterial growth (the clear zones are places where virus production has destroyed the bacteria) take 2–3 days. Antibody-based tests to detect protein components of the target bacteria or viruses and the use of polymerase chain reaction to amplify and detect specific regions of the viral or bacterial genetic material can produce results in as little as a day.

Other diagnostic procedures rely on imaging the brain or spinal cord. The two most widely used imaging procedures are computed tomography (CT) and magnetic resonance imaging (MRI). These techniques can be sensitive enough to detect inflammation of the meninges. These examinations need to be done promptly, since the inflammation associated with encephalitis can cause damage rapidly.

If viral encephalitis is suspected, treatment usually involves the antiviral drugs acyclovir and ganciclovir. Both drugs are similar in their three-dimensional structure to certain building blocks of the viral genetic material. Incorporation of the drug into the replicating genetic material instead of the normal building block inhibits the activity of an enzyme that is vital for the continued replication of the virus.

Milder cases of encephalitis are treated with bed rest and over-the-counter medications to relieve headache and make the person feel as comfortable as possible. In more severe cases, hospitalization may be necessary, and drugs may be given to control or prevent seizures. The swelling of the meninges can be reduced using corticosteroids, which are usually administered intravenously (into a vein) to get the drug to the site of swelling quickly and to maintain an effective concentration of the drug.

The best way to prevent encephalitis is to minimize contact with the vectors of the disease. Examples of preventative measures include the use of mosquito repellent, wearing protective clothing when outdoors, and eliminating sources of stagnant water (which can become breeding grounds for mosquitoes). In reality, however, these and other preventative measures are difficult to consistently maintain.

As of 2007, no vaccine for the forms of encephalitis that are prevalent in the United States is available, although development is underway. A vaccine for Japanese encephalitis is available in the United States. In Europe, a vaccine for tick-borne encephalitis is available. In addition, vaccines to protect horses from various forms of the disease are available.

Impacts and Issues

Encephalitis can be a devastating disease when it causes lasting effects, such as brain damage. A person can be incapable of resuming work or study, and can require assistance to perform routine daily tasks. This can place a burden on caregivers and can affect the person's capabilities as a family member and worker.

Sizeable outbreaks of encephalitis can occur. For example, a 1995 outbreak of Venezuelan equine encephalitis in Venezuela and Colombia sickened an estimated 90,000 people. The size of such an outbreak imposes yet another burden on developing countries, particularly those where acquired immunodeficiency syndrome (AIDS, also cited as acquired immune deficiency syndrome) is prevalent. The impaired immune system function that is a characteristic of AIDS makes individuals with the disease more susceptible to a wide range of other maladies, including encephalitis.

Even in a developed country with a relatively high access to medical care, like the United States, the costs of encephalitis are considerable. The CDC has estimated the cost of medical care, disease detection, and efforts such as spraying programs that are intended to control mosquito and other vector populations, at approximately $150 million a year. In addition, developed countries are becoming increasingly affected as the population ages and immune-compromising diseases, such as AIDS, become more prevalent. It is also likely that encephalitis will become more common in the more northerly regions of the United States, Canada, and Europe as global warming continues, since the warmer temperatures will be more favorable for the breeding of vectors, such as mosquitoes.

West Nile encephalitis is an emerging health hazard in the United States. The disease was first detected in the United States in 1999, and, in the following year, 284 Americans are known to have died of the disease. Since then, West Nile has increased in its geographical range and in the number of people affected. It has replaced St. Louis encephalitis as the most prevalent form of the disease in the United States. The CDC reported almost 1,300 cases of West Nile encephalitis in 2005, more than double the number of cases in 2002.

While vaccines are available for some forms of encephalitis, such as Japanese equine encephalitis, the high cost of the vaccines can make them prohibitively expensive for poorer nations. A number of agencies, such as WHO, are working to make encephalitis vaccines more widely available. For example, one of the priorities of the World Health Organization's Programme for Immunization Preventable Diseases in cooperation with the government of Nepal is the control of Japanese encephalitis. Over 8,000 cases, mainly involving children, have been reported in Nepal from 1998–2003.

Efforts also are underway to control encephalitis in North America. The CDC's Division of Vector-Borne Infectious Diseases conducts surveillance programs that monitor the occurrence of the disease and manages programs that try to control the disease in these hotspots.

IN CONTEXT: SCIENTIFIC, POLITICAL, AND ETHICAL ISSUES

Centers for Disease Control and Prevention, National Center for Infectious Diseases, Division of Vector-Borne Infectious Diseases argues that “Mosquito-borne encephalitis offers a rare opportunity in public health to detect the risk of a disease before it occurs and to intervene to reduce that risk substantially. The surveillance required to detect risk is being increasingly refined by the potential utilization of these new technologies which allows for rapid identification of dangerous viruses in mosquito populations. These rapid diagnostic techniques used in threat recognition can shorten public health response time and reduce the geographic spread of infected vectors and thereby the cost of containing them.”

SOURCE: Centers for Disease Control and Prevention, National Center for Infectious Diseases, Division of Vector-Borne Infectious Diseases

Primary Source Connection

This newspaper article details the temporary closing of an elementary school in Rhode Island in early 2007 after a student at the school died from an unusual form of encephalitis that was caused by a common bacteria. Two other students contracted the disease during the outbreak, prompting the school district to close all its schools for almost a week, and the CDC to investigate the outbreak.

See AlsoArthropod-borne Disease; Climate Change and Infectious Disease; Climate Change and Infectious Disease; Eastern Equine Encephalitis; Emerging Infectious Diseases; Encephalitis; Japanese Encephalitis; Meningitis, Viral; Mosquito-borne Diseases; Vector-borne Disease; West Nile.

BIBLIOGRAPHY

Books

Bloom, Ona, and Jennifer Morgan. Encephalitis. London: Chelsea House Publications, 2005.

Booss, John, and Margaret M. Esiri. Viral Encephalitis in Humans. Washington, DC: ASM Press, 2003.

Web Sites

Medline Plus. “Encephalitis.” <http://www.nlm.nih.gov/medlineplus/encephalitis.html> (accessed March 20, 2007).

Brian Hoyle

Encephalitis

Definition

Encephalitis is an inflammation of the brain, usually caused by a direct viral infection or a hypersensitivity reaction to a virus or foreign protein. Brain inflammation caused by a bacterial infection is sometimes called cerebritis. When both the brain and spinal cord are involved, the disorder is called encephalomyelitis. An inflammation of the brain's covering, or meninges, is called meningitis .

Description

Encephalitis is an inflammation of the brain. The inflammation is a reaction of the body's immune system to infection or invasion. During the inflammation, the brain's tissues become swollen. The combination of the infection and the immune reaction to it can cause headache and a fever , as well as more severe symptoms in some cases.

The viruses causing primary encephalitis can be epidemic or sporadic. The polio virus is an epidemic cause. Arthropod-borne viral encephalitis is responsible for most epidemic viral encephalitis. The viruses live in animal hosts and mosquitoes that transmit the disease. The most common form of non-epidemic or sporadic encephalitis is caused by the herpes simplex virus, type 1 (HSV-1) and has a high rate of death. Mumps is another example of a sporadic cause.

Demographics

Approximately 2,000 cases of encephalitis are reported to the Centers for Disease Control and Prevention in Atlanta, Georgia, each year. Encephalitis can strike anyone, at any age, although some kinds of encephalitis are more common in children. Other kinds of encephalitis can affect anyone, but may affect children more severely.

Causes and symptoms

There are more than a dozen viruses that can cause encephalitis, spread by either human-to human contact or by animal bites . Encephalitis may occur with several common viral infections of childhood. Viruses and viral diseases that may cause encephalitis include:

• chickenpox
• measles
• mumps
• Epstein-Barr virus (EBV)
• cytomegalovirus infection
• HIV
• herpes simplex
• herpes zoster (shingles)
• herpes B
• polio
• rabies
• mosquito-borne viruses (arboviruses)

Primary encephalitis is caused by direct infection by the virus, while secondary encephalitis is due to a post-infectious immune reaction to viral infection elsewhere in the body. Secondary encephalitis may occur with measles, chickenpox, mumps, rubella , and EBV. In secondary encephalitis, symptoms usually begin five to ten days after the onset of the disease itself and are related to the breakdown of the myelin sheath that covers nerve fibers.

In rare cases, encephalitis may follow vaccination against some of the viral diseases listed above. Creutzfeldt-Jakob disease, a very rare brain disorder caused by an infectious particle called a prion, may also cause encephalitis.

Mosquitoes spread viruses responsible for equine encephalitis (eastern and western types), St. Louis encephalitis, California encephalitis, and Japanese encephalitis. Lyme disease , spread by ticks, can cause encephalitis, as can Colorado tick fever. Rabies is most often spread by animal bites from dogs, cats, mice, raccoons, squirrels, and bats and may cause encephalitis.

Equine encephalitis is carried by mosquitoes that do not normally bite humans but do bite horses and birds. It is occasionally picked up from these animals by mosquitoes that do bite humans. Japanese encephalitis and St. Louis encephalitis are also carried by mosquitoes. The risk of contracting a mosquito-borne virus is greatest in mid- to late summer, when mosquitoes are most active, in those rural areas where these viruses are known to exist. Eastern equine encephalitis occurs in eastern and southeastern United States; western equine and California encephalitis occur throughout the West; and St. Louis encephalitis occurs throughout the country. Japanese encephalitis does not occur in the United States but is found throughout much of Asia. The viruses responsible for these diseases are classified as arbovirus, and these diseases are collectively called arbovirus encephalitis.

Herpes simplex encephalitis, the most common form of sporadic encephalitis in western countries, is a disease with significantly high mortality. It occurs in children and adults and both sides of the brain are affected. It is theorized that brain infection is caused by the virus moving from a peripheral location to the brain via two nerves, the olfactory and the trigeminal (largest nerves in the skull).

Herpes simplex encephalitis is responsible for 10 percent of all encephalitis cases and is the main cause of sporadic, fatal encephalitis. In untreated people, the rate of death is 70 percent while the mortality is 15 to 20 percent in persons who have been treated with acyclovir. The symptoms of herpes simplex encephalitis are fever, rapidly disintegrating mental state, headache, and behavioral changes.

The symptoms of encephalitis range from very mild to very severe and may include:

• headache
• fever
• lethargy (sleepiness, decreased alertness, and fatigue)
• malaise
• nausea and vomiting
• visual disturbances
• tremor
• decreased consciousness (drowsiness, confusion, delirium, and unconsciousness)
• stiff neck
• seizures

Symptoms may progress rapidly, changing from mild to severe within several days or even several hours.

When to call the doctor

A physician should be called whenever a headache does not respond to medication or when a person experiences a fever over 104°F (40.0°C), nausea and vomiting , visual disturbances, a stiff neck, or seizures.

A doctor should be called when an infant's temperature rises above 100°F (37.8°C) and cannot be brought down within a few minutes. Infants whose temperatures exceed 102°F (38.9°C) should be sponge-bathed in cool water while waiting for emergency help to arrive.

Diagnosis

Diagnosis of encephalitis includes careful questioning to determine possible exposure to viral sources. Tests that can help confirm the diagnosis and rule out other disorders include:

• blood tests (to detect antibodies to viral antigens and foreign proteins)
• cerebrospinal fluid analysis, or spinal tap (to detect viral antigens and provide culture specimens for the virus or bacteria that may be present in the cerebrospinal fluid)
• electroencephalogram (EEG)
• CT and MRI scans

A brain biopsy (surgical gathering of a small tissue sample) may be recommended in some cases in which treatment has thus far been ineffective and the cause of the encephalitis is unclear. Definite diagnosis by biopsy may allow specific treatment that would otherwise be too risky.

Treatment

Choice of treatment for encephalitis depends on the cause. Bacterial encephalitis is treated with antibiotics . Viral encephalitis is usually treated with antiviral drugs , including acyclovir, ganciclovir, foscarnet, ribavirin, and AZT. Viruses that respond to acyclovir include herpes simplex, the most common cause of sporadic (non-epidemic) encephalitis in the United States.

The symptoms of encephalitis may be treated with a number of different drugs. Corticosteroids, including prednisone and dexamethasone, are sometimes prescribed to reduce inflammation and brain swelling. Anticonvulsant drugs, including phenytoin, are used to control seizures. Fever may be reduced with acetaminophen or other fever-reducing drugs.

A person with encephalitis must be monitored carefully, since symptoms may change rapidly. Blood tests may be required regularly to track levels of fluids and salts in the blood.

Prognosis

Encephalitis symptoms may last several weeks. Most cases of encephalitis are mild, and recovery is usually quick. Mild encephalitis usually leaves no residual neurological problems. Overall, approximately 10 percent of those with encephalitis die from their infections or complications such as secondary infection. Some forms of encephalitis have more severe courses, including herpes encephalitis, in which mortality is 15 to 20 percent with treatment, and 70 to 80 percent without. Antiviral treatment is ineffective for eastern equine encephalitis, and mortality is approximately 30 percent.

Permanent neurological consequences may follow recovery in some cases. Consequences may include personality changes, memory loss, language difficulties, seizures, and partial paralysis.

Prevention

Because encephalitis is caused by infection, it may be prevented by avoiding the infection. Minimizing contact with others who have any of the viral illnesses listed above may reduce one's chances of becoming infected. Most infections are spread by hand-to-hand or hand-to-mouth contact; frequent hand washing may reduce the likelihood of infection if contact cannot be avoided.

Mosquito-borne viruses may be avoided by preventing mosquito bites. Mosquitoes are most active at dawn and dusk and are most common in moist areas with standing water. Covering skin and using mosquito repellents on exposed skin can reduce the chances of being bitten.

KEY TERMS

Cerebrospinal fluid analysis —A laboratory test, important in diagnosing diseases of the central nervous system, that examines a sample of the fluid surrounding the brain and spinal cord. The fluid is withdrawn through a needle in a procedure called a lumbar puncture.

Computed tomography (CT) —An imaging technique in which cross-sectional x rays of the body are compiled to create a three-dimensional image of the body's internal structures; also called computed axial tomography.

Electroencephalogram (EEG) —A record of the tiny electrical impulses produced by the brain's activity picked up by electrodes placed on the scalp. By measuring characteristic wave patterns, the EEG can help diagnose certain conditions of the brain.

Inflammation —Pain, redness, swelling, and heat that develop in response to tissue irritation or injury. It usually is caused by the immune system's response to the body's contact with a foreign substance, such as an allergen or pathogen.

Magnetic resonance imaging (MRI) —An imaging technique that uses a large circular magnet and radio waves to generate signals from atoms in the body. These signals are used to construct detailed images of internal body structures and organs, including the brain.

Vaccine —A substance prepared from a weakened or killed microorganism which, when injected, helps the body to form antibodies that will prevent infection by the natural microorganism.

Virus —A small infectious agent consisting of a core of genetic material (DNA or RNA) surrounded by a shell of protein. A virus needs a living cell to reproduce.

Vaccines are available against some viruses, including polio, herpes B, Japanese encephalitis, and equine encephalitis. Rabies vaccine is available for animals; it is also given to people after exposure. Japanese encephalitis vaccine is recommended for those traveling to Asia and staying in affected rural areas during transmission season.

Nutritional concerns

Adequate nutrition and fluids improve the chances for a full recovery from encephalitis.

Parental concerns

Parents should carefully monitor their infants and young children for symptoms of fever. Any fever that exceeds 103°F (39.4°C) for more than a few minutes should be promptly treated. Any complaints of a stiff neck, loss of consciousness, unexplained vomiting, or seizure activity should be promptly brought to competent medical attention.

Resources

BOOKS

Halstead, Scott A. "Arbovirus Encephalitis in North America." In Nelson Textbook of Pediatrics, 17th ed. Edited by Richard E. Behrman et al. Philadelphia: Saunders, 2003, pp. 1086–8.

Johnston, Michael V. "Encephalopathies." In Nelson Textbook of Pediatrics, 17th ed. Edited by Richard E. Behrman et al. Philadelphia: Saunders, 2003, pp. 2023–8.

Nath, Avindra, and Joseph R. Berger. "Acute Viral Meningitis and Encephalitis." In Cecil Textbook of Medicine, 22nd ed. Edited by Lee Goldman, et al. Philadelphia: Saunders, 2003, pp. 2232–5.

Tyler, Kenneth L. "Viral Meningitis and Encephalitis." In Harrison's Principles of Internal Medicine, 15th ed. Edited by Eugene Braunwald et al. New York: McGraw-Hill, 2001, pp. 2471–80.

PERIODICALS

Arciniegas, D. B., and C. A. Anderson. "Viral encephalitis: neuropsychiatric and neurobehavioral aspects." Current Psychiatry Reports 6, no. 5 (2004): 372–9.

Cunha, B. A. "Differential diagnosis of West Nile encephalitis." Current Opinions in Infectious Disease 17, no. 5 (2004): 413–20.

Lyle, P., et al. "Evaluation of encephalitis in the toddler: what part of negative don't you understand?" Current Opinions in Pediatrics 16, no. 5 (2004): 567–70.

Morgan, R. "West Nile viral encephalitis: a case study." Journal of Neuroscience of Nursing 36, no. 4 (2004): 185–8.

Savas, L., et al. "Full recovered meningoencephalomyelitis caused by mumps virus." European Journal of Neurology 11, no. 9 (2004): 639–40.

ORGANIZATIONS

American Academy of Neurology. 1080 Montreal Avenue, St. Paul, MN 55116. Web site: <www.aan.com>.

American Academy of Pediatrics. 141 Northwest Point Blvd., Elk Grove Village, IL 60007–1098. Web site: <www.aap.org/>.

American College of Emergency Physicians. PO Box 619911, Dallas, TX 75261-9911. Web site: <www.acep.org/>.

WEB SITES

"Arboviral Encephalitides." Centers for Disease Control and Prevention. Available online at <www.cdc.gov/ncidod/dvbid/arbor/> (accessed January 5, 2005).

"Encephalitis." Mayo Clinic. Available online at <www.mayoclinic.com/invoke.cfm?id=DS00226> (accessed January 5, 2005).

"Encephalitis." MedlinePlus. Available online at <www.nlm.nih.gov/medlineplus/encephalitis.html> (accessed January 5, 2005).

"Encephalitis." World Health Organization. Available online at <www.who.int/topics/encephalitis/en/> (accessed January 5, 2005).

Encephalitis Information Resource. Available online at <www.encephalitis.info/> (accessed January 5, 2005).

"NINDS Encephalitis and Meningitis Information Page." National Institute of Neurological Disorders and Stroke. Available online at <www.ninds.nih.gov/health_and_medical/disorders/encmenin_doc.htm> (accessed January 5, 2005).

L. Fleming Fallon, Jr., MD, DrPH

Encephalitis

What Is Encephalitis?

How Common Is Encephalitis?

Is the Disease Contagious?

How Is Encephalitis Spread?

What Are the Signs and Symptoms?

How Do Doctors Make the Diagnosis?

How Do Doctors Treat Encephalitis?

How Long Does the Illness Last?

What Are the Complications?

How Is Encephalitis Prevented?

Resources

Encephalitis (en-seh-fuh-LYE-tis) is inflammation of the brain.

KEYWORDS

for searching the Internet and other reference sources

Arboviruses

Herpes simplex virus

Immunizations

La Crosse encephalitis

Meningitis

Mosquito-borne illnesses

St. Louis encephalitis

Tick-borne illnesses

West Nile virus

What Is Encephalitis?

There are several different causes of encephalitis. The most common is infection, usually by a virus. The condition can range from mild to severe, depending on the type of germ producing the infection. Encephalitis can occur with certain childhood viral illnesses, such as mumps, measles, varicella (chicken pox), rubella (German measles), or mononucleosis.

A much more serious type of encephalitis is caused by the herpes simplex virus* (HSV). HSV rarely infects the brain, but when it does, it can be life threatening. West Nile virus, which first arrived in the United States from other parts of the world in the late 1990s, can cause encephalitis as well. Infected birds carry this virus, but mosquitoes can pick up the virus when they bite infected birds and then spread the virus to humans through a bite.

*herpes simplex

(HER-peez SIM-plex) virus is a virus that can cause infections of the skin, mouth, genitals, and other parts of the body.

Encephalitis may develop in a person who has meningitis (meh-nin-JY-tis), an inflammation of the membranes surrounding the brain and spinal cord, called the meninges (meh-NIN-jeez). It also can be a complication of other infectious diseases, such as rabies, cytomegalovirus* infection, listeriosis*, syphilis*, or Lyme disease. In people with a weakened immune system, for instance, those with HIV/AIDS, infection by parasites can lead to encephalitis, especially the parasite that causes toxoplasmosis*.

*cytomegalovirus

(sy-tuh-MEH-guh-lo-vy-rus), or CMV, infection is very common and usually causes no symptoms. It poses little risk for healthy people, but it can lead to serious illness in people with weak immune systems.

*listeriosis

(lis-teer-e-O-sis) is a bacterial infection that can cause a form of meningitis in infants and other symptoms in children and adults.

*syphilis

(SIH-fih-lis) is a sexually transmitted disease that, if untreated, can lead to serious lifelong problems throughout the body, including blindness and paralysis.

*toxoplasmosis

(tox-o-plaz-MO-sis) is a parasitic infection that usually causes no symptoms in healthy people, but it can cause serious problems in unborn babies and people with weak immune systems.

How Common Is Encephalitis?

Each year, several thousand cases of encephalitis are reported to the U.S. Centers for Disease Control and Prevention. Health officials believe that many more cases go unreported when the symptoms are mild.

Is the Disease Contagious?

Although the brain inflammation itself is not contagious, viruses that cause encephalitis may be contagious. When someone contracts the same virus that a person with encephalitis has, however, it does not mean that he or she also will develop encephalitis.

How Is Encephalitis Spread?

Because several different germs can cause encephalitis, spread of the infection may take place in different ways. For example, mosquitoes transmit viruses that cause West Nile encephalitis, St. Louis encephalitis, and western equine encephalitis. Other viruses (such as herpes simplex virus or varicella-zoster, var-uh-SEH-luh ZOS-ter, virus, which causes chicken pox) are spread in fluids from the mouth, throat, or nose; tiny drops of these fluids, such as saliva and nasal mucus*, may be sprayed into the air

*mucus

(MYOO-kus) is a thick, slippery substance that lines the insides of many body parts.

by a cough or sneeze from someone who is infected. Ticks spread Lyme disease, and humans can contract rabies from the bite of infected animals, such as raccoons and bats.

What Are the Signs and Symptoms?

Many people who are infected with a virus that can cause encephalitis have only mild symptoms. Symptoms in people who develop encephalitis may appear suddenly and include headache, fever, sensitivity to light, loss of appetite, and a stiff neck and back. In more serious cases there may be high fever, nausea (NAW-zee-uh), vomiting, confusion, double vision*, personality changes, problems with hearing and speech, hallucinations*, sleepiness, clumsiness, muscle weakness, loss of sensation, and irritability. In the most severe cases, seizures* and loss of consciousness may occur. A person who has any of these symptoms requires immediate medical attention.

*double vision

is a vision problem in which a person sees two images of a single object.

*hallucinations

(ha-loo-sin-AY-shuns) occur when a person sees or hears things that are not really there. Hallucinations can result from nervous system abnormalities, mental disorders, or the use of certain drugs.

*seizures

(SEE-zhurs) are sudden bursts of disorganized electrical activity that interrupt the normal functioning of the brain, often leading to uncontrolled movements in the body and sometimes a temporary change in consciousness.

How Do Doctors Make the Diagnosis?

Doctors use several tests to diagnose encephalitis. Imaging tests, such as computerized tomography* scans or magnetic resonance imaging*, provide special views of the brain that allow doctors to check for swelling, bleeding, or other abnormalities. An electroencephalogram (e-lek-tro-en-SEF-a-loh-gram), which measures brain electrical activity, will show abnormal patterns in a person with encephalitis. Doctors also might order blood tests to look for the microorganism in the blood and tests that can tell whether the person’s body is producing antibodies* in response to a specific virus or bacterium. A spinal tap, also called a lumbar puncture, often is performed. In this procedure a needle is inserted into the lower end of the spine and a small sample of cerebrospinal (seh-ree-bro-SPY-nuhl) fluid, which surrounds the brain and spinal cord, is removed. This fluid is examined under a microscope and tested for signs of infection. In addition, cotton swabs can be used to take fluid samples from the nose, throat, and rectum* to test for certain viruses that might be causing the infection.

*computerized tomography

(kom-PYOO-ter-ized toe-MgAH-gruh-fee), or CT, also called computerized axial tomography (CAT), is a technique in which a machine takes many X rays of the body to create a three-dimensional picture.

*magnetic resonance imaging

(MRI) uses magnetic waves, instead of X rays, to scan the body and produce detailed pictures of the body’s structures.

*antibodies

(AN-tih-bah-deez) are protein molecules produced by the body’s immune system to help fight specific infections caused by microorganisms, such as bacteria and viruses.

*rectum

is the final portion of the large intestine, connecting the colon to the anus.

How Do Doctors Treat Encephalitis?

Encephalitis is a very serious disease that can be life threatening. Although very mild cases can be treated at home, it is often necessary for patients to be hospitalized. Patients usually are admitted to an intensive care unit, where doctors carefully monitor blood pressure, heart rate, and ability to breathe. Such patients may require a ventilator (VEN-tuh-layter), a machine that does the breathing for very ill people until they can breathe on their own again. Doctors also monitor the body fluids to help prevent or control swelling of the brain, which can lead to a dangerous increase in the pressure within the skull.

Several medications may be given to people with encephalitis. Over-the-counter medicines such as acetaminophen (uh-SEE-teh-MIH-noh-fen) may be used to treat minor symptoms such as fever and headache. Antiviral medications sometimes can help prevent the spread of the virus and are very important in the treatment of encephalitis caused by herpes simplex virus. If a patient is having seizures, anticonvulsants* may help control them. Anti-inflammatory medications called corticosteroids (kor-tih-ko-STIR-oyds) may lessen swelling in the brain.

*anticonvulsants

(an-tie-kon-VULsents) are medications that affect the electrical activity in the brain and are given to prevent or stop seizures.

How Long Does the Illness Last?

The acute phase of the disease, the time when symptoms are most severe, usually lasts up to a week. Full recovery can take much longer, often several weeks or months.

What Are the Complications?

Most people recover from encephalitis completely. For some people, however, swelling of the brain may lead to permanent brain damage. These patients may face long-term complications, such as learning disabilities, seizures, speech problems, memory loss, lack of muscle control, paralysis*, or coma*. In the rare cases where brain damage is severe, death can result. Infants younger than 1 year and adults older than 55 have the greatest risk of permanent brain damage and death from encephalitis.

*paralysis

(pah-RAH-luh-sis) is the loss or impairment of the ability to move some part of the body.

*coma

(KO-ma) is an unconscious state in which a person cannot be awakened and cannot move, see, speak, or hear.

How Is Encephalitis Prevented?

Some of the viral infections that can cause encephalitis, including measles, mumps, and chicken pox, can be prevented with vaccines given in childhood. To prevent encephalitis caused by West Nile virus or other viruses transmitted by mosquitoes, people are encouraged to avoid being outside at dawn and dusk, when mosquitoes are most active. If they do go out, they are advised to wear light-colored, long-sleeved shirts and long pants and to use insect repellent. Mosquitoes breed in places where there is standing water, such as in buckets, birdbaths, and flower pots, so draining these containers frequently can help control the mosquito population and decrease the risk of infection.

Causes OF Encephalitis

Encephalitis may be caused by a variety of viruses, bacteria, and other organisms. These agents include:

• Measles, chicken pox, rubella (German measles), mumps, polio, and other viral illnesses, which generally lead to a mild form of encephalitis known as postinfectious or para-infectious encephalitis.
• Enteroviruses (en-tuh-ro-VY-ruh-sez), viruses that typically infect the intestines and then may spread to other parts of the body, including the brain.
• Herpes simplex virus, a virus that can infect the mouth, skin, and other parts of the body.
• HIV (human immunodeficiency virus), the virus that causes AIDS (acquired immunodeficiency syndrome) and is transmitted when an infected person’s blood or body fluids are introduced into the bloodstream of a healthy person.
• Arboviruses (ar-buh-VY-ruh-sez), which multiply in and are transmitted by blood-sucking insects (such as ticks and mosquitoes) when they bite infected birds, rodents (chipmunks and squirrels, for example), and other small animals and then bite humans.
• Animal-borne illnesses, such as rabies, toxoplasmosis, cat scratch disease, and Lyme disease, that are transmitted to humans by contact with an infected animal’s saliva (through a bite or lick, for example), contact with an infected animal’s feces (FEE-seez, or bowel movements), or an insect that bites an infected animal and then bites a person.

See also

AIDS and HIV Infection

Cytomegalovirus (CMV) Infection

Herpes Simplex Virus Infections

Immune Deficiencies

Lyme Disease

Measles (Rubeola)

Meningitis

Mononucleosis, Infectious

Mumps

Rabies

Rubella (German Measles)

Syphilis

Varicella (Chicken Pox) and Herpes Zoster (Shingles)

West Nile Fever

Resources

Organization

U.S. Centers for Disease Control and Prevention (CDC), 1600 Clifton Road, Atlanta, GA 30333. The CDC provides fact sheets and other information on encephalitis at its website.

Telephone 800-311-3435 http://www.cdc.gov

Website

KidsHealth.org. KidsHealth is a website created by the medical experts of the Nemours Foundation and is devoted to issues of children’s health. It contains articles on a variety of health topics, including encephalitis.

http://www.KidsHealth.org

Encephalitis

Definition

Encephalitis is an inflammation of the brain, usually caused by a direct viral infection or a hyper-sensitivity reaction to a virus or foreign protein. Brain inflammation caused by a bacterial infection is sometimes called cerebritis. When both the brain and spinal cord are involved, the disorder is called encephalomyelitis. An inflammation of the brain's covering, or meninges, is called meningitis.

Description

Encephalitis is an inflammation of the brain. The inflammation is a reaction of the body's immune system to infection or invasion. During the inflammation, the brain's tissues become swollen. The combination of the infection and the immune reaction to it can cause headache and a fever, as well as more severe symptoms in some cases.

Approximately 2,000 cases of encephalitis are reported to the Centers for Disease Control in Atlanta, GA each year. The viruses causing primary encephalitis can be epidemic or sporadic. The polio virus is an epidemic cause. Arthropod-borne viral encephalitis is responsible for most epidemic viral encephalitis. The viruses live in animal hosts and mosquitos that transmit the disease. The most common form of non-epidemic or sporadic encephalitis is caused by the herpes simplex virus, type 1 (HSV-1) and has a high rate of death. Mumps is another example of a sporadic cause.

Causes and symptoms

Causes

There are more than a dozen viruses that can cause encephalitis, spread by either human-to human contact or by animal bites. Encephalitis may occur with several common viral infections of childhood. Viruses and viral diseases that may cause encephalitis include:

• chickenpox
• measles
• mumps
• Epstein-Barr virus (EBV)
• cytomegalovirus infection
• HIV
• herpes simplex
• herpes zoster (shingles )
• herpes B
• polio
• rabies
• mosquito-borne viruses (arboviruses)

Primary encephalitis is caused by direct infection by the virus, while secondary encephalitis is due to a post-infectious immune reaction to viral infection elsewhere in the body. Secondary encephalitis may occur with measles, chickenpox, mumps, rubella, and EBV. In secondary encephalitis, symptoms usually begin five to 10 days after the onset of the disease itself and are related to the breakdown of the myelin sheath that covers nerve fibers.

In rare cases, encephalitis may follow vaccination against some of the viral diseases listed above. Creutzfeldt-Jakob disease, a very rare brain disorder caused by an infectious particle called a prion, may also cause encephalitis.

Mosquitoes spread viruses responsible for equine encephalitis (eastern and western types), St. Louis encephalitis, California encephalitis, and Japanese encephalitis. Lyme disease, spread by ticks, can cause encephalitis, as can Colorado tick fever. Rabies is most often spread by animal bites from dogs, cats, mice, raccoons, squirrels, and bats and may cause encephalitis.

Equine encephalitis is carried by mosquitoes that do not normally bite humans but do bite horses and birds. It is occasionally picked up from these animals by mosquitoes that do bite humans. Japanese encephalitis and St. Louis encephalitis are also carried by mosquitoes. The risk of contracting a mosquito-borne virus is greatest in mid- to late summer, when mosquitoes are most active, in those rural areas where these viruses are known to exist. Eastern equine encephalitis occurs in eastern and southeastern United States; western equine and California encephalitis occur throughout the West; and St. Louis encephalitis occurs throughout the country. Japanese encephalitis does not occur in the United States, but is found throughout much of Asia. The viruses responsible for these diseases are classified as arbovirus and these diseases are collectively called arbovirus encephalitis.

Herpes simplex encephalitis, the most common form of sporadic encephalitis in western countries, is a disease with significantly high mortality. It occurs in children and adults and both sides of the brain are affected. It is theorized that brain infection is caused by the virus moving from a peripheral location to the brain via two nerves, the olfactory and the trigeminal (largest nerves in the skull).

Herpes simplex encephalitis is responsible for 10% of all encephalitis cases and is the main cause of sporadic, fatal encephalitis. In untreated patients, the rate of death is 70% while the mortality is 15-20% in patients who have been treated with acyclovir. The symptoms of herpes simplex encephalitis are fever, rapidly disintegrating mental state, headache, and behavioral changes.

Symptoms

The symptoms of encephalitis range from very mild to very severe and may include:

• headache
• fever
• lethargy (sleepiness, decreased alertness, and fatigue)
• malaise
• nausea and vomiting
• visual disturbances
• tremor
• decreased consciousness (drowsiness, confusion, delirium, and unconsciousness)
• stiff neck
• seizures

Symptoms may progress rapidly, changing from mild to severe within several days or even several hours.

Diagnosis

Diagnosis of encephalitis includes careful questioning to determine possible exposure to viral sources. Tests that can help confirm the diagnosis and rule out other disorders include:

• Blood tests. These are to detect antibodies to viral antigens, and foreign proteins.
• Cerebrospinal fluid analysis (spinal tap). This detects viral antigens, and provides culture specimens for the virus or bacteria that may be present in the cerebrospinal fluid.
• Electroencephalogram (EEG).
• CT and MRI scans.

A brain biopsy (surgical gathering of a small tissue sample) may be recommended in some cases where treatment to date has been ineffective and the cause of the encephalitis is unclear. Definite diagnosis by biopsy may allow specific treatment that would otherwise be too risky.

Treatment

Choice of treatment for encephalitis will depend on the cause. Bacterial encephalitis is treated with antibiotics. Viral encephalitis is usually treated with antiviral drugs including acyclovir, ganciclovir, foscarnet, ribovarin, and AZT. Viruses that respond to acyclovir include herpes simplex, the most common cause of sporadic (non-epidemic) encephalitis in the United States.

The symptoms of encephalitis may be treated with a number of different drugs. Corticosteroids, including prednisone and dexamethasone, are sometimes prescribed to reduce inflammation and brain swelling. Anticonvulsant drugs, including dilantin and phenytoin, are used to control seizures. Fever may be reduced with acetaminophen or other fever-reducing drugs.

A person with encephalitis must be monitored carefully, since symptoms may change rapidly. Blood tests may be required regularly to track levels of fluids and salts in the blood.

Prognosis

Encephalitis symptoms may last several weeks. Most cases of encephalitis are mild, and recovery is usually quick. Mild encephalitis usually leaves no residual neurological problems. Overall, approximately 10% of those with encephalitis die from their infections or complications such as secondary infection. Some forms of encephalitis have more severe courses, including herpes encephalitis, in which mortality is 15-20% with treatment, and 70-80% without. Antiviral treatment is ineffective for eastern equine encephalitis, and mortality is approximately 30%.

Permanent neurological consequences may follow recovery in some cases. Consequences may include personality changes, memory loss, language difficulties, seizures, and partial paralysis.

Prevention

Because encephalitis is due to infection, it may be prevented by avoiding the infection. Minimizing contact with others who have any of the viral illness listed above may reduce the chances of becoming infected. Most infections are spread by hand-to-hand or hand-to-mouth contact; frequent hand washing may reduce the likelihood of infection if contact cannot be avoided.

Mosquito-borne viruses may be avoided by preventing mosquito bites. Mosquitoes are most active at dawn and dusk, and are most common in moist areas with standing water. Minimizing exposed skin and use of mosquito repellents on other areas can reduce the chances of being bitten.

Vaccines are available against some viruses, including polio, herpes B, Japanese encephalitis, and equine encephalitis. Rabies vaccine is available for animals; it is also given to people after exposure. Japanese encephalitis vaccine is recommended for those traveling to Asia and staying in affected rural areas during transmission season.

Resources

ORGANIZATIONS

Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435, (404) 639-3311. 〈http://www.cdc.gov〉.

KEY TERMS

Cerebrospinal fluid analysis— A analysis that is important in diagnosing diseases of the central nervous system. The fluid within the spine will indicate the presence of viruses, bacteria, and blood. Infections such as encephalitis will be indicated by an increase of cell count and total protein in the fluid.

Computerized tomography (CT) Scan— A test to examine organs within the body and detect evidence of tumors, blood clots, and accumulation of fluids.

Electroencephalagram (EEG)— A chart of the brain waves picked up by the electrodes placed on the scalp. Changes in brain wave activity can be an indication of nervous system disorders.

Inflammation— A response from the immune system to an injury. The signs are redness, heat, swelling, and pain.

Magnetic Resonance Imaging (MRI)— MRI is diagnostic radiography using electromagnetic energy to create an image of the central nervous system (CNS), blood system, and musculoskeletal system.

Vaccine— A prepartation containing killed or weakened microorganisms used to build immunity against infection from that microorganism.

Virus— A very small organism that can only live within a cell. They are unable to reproduce outside that cell.

Encephalitis

Types of the disease

Symptoms and treatment

Reye’s syndrome

Prevention

Resources

Encephalitis, or aseptic meningitis, is an inflammatory disease of the brain. It is caused by a virus that either has invaded the brain, or a virus encountered elsewhere in the body that has caused a sensitivity reaction in the brain. Most cases of encephalitis are considered secondary to a virus infection that stimulates an immune reaction. The inflammation is a reaction of the body’s immune system to infection or invasion. During the inflammation, the brain’s tissues become swollen. The combination of the infection and the immune reaction to it can cause headache and fever, as well as more severe symptoms in some cases. According to the U.S. Centers for Disease Control and Prevention (CDC), the occurrence and intensity of encephalitis in the United States in any one year varies from 150 to 3, 000 cases.

An infection that involves the membranes associated with the spinal cord is called meningitis. This is a less-serious condition than encephalitis in that it usually has few long-term effects. Encephalitis is an infection of the brain tissue itself, not the surrounding protective covering (meninges), so the consequences are more serious.

Types of the disease

Among the many forms of encephalitis are those that occur seasonally (Russian spring-summer encephalitis, for example), those that affect animals (bovine, fox, and equine), and a form that is carried by a mosquito. Viruses that have been directly implicated in causing encephalitis include the arbovirus, echovirus, poliovirus, and the herpes virus. Encephalitis occurs as a complication of, for example, chickenpox, polio, and vaccinia, which is a cowpox virus used in smallpox vaccinations, as well as the common flu virus. The herpes simplex virus, responsible for the common cold sore, eczema, and genital herpes; the measles (rubeola) virus; some of the 31 types of echoviruses that also cause a paralytic disease or an infection of the heart muscle; the coxsackievirus responsible for infections of the heart and its covering and paralysis; the mumps virus; the arboviruses that normally infect animals and can be spread by mosquito to humans—all have been implicated as causal agents in human encephalitis.

The virus responsible for the infection can invade the cranium and infect the brain via the circulatory system. The blood-brain barrier, a system that serves to protect the brain from certain drugs and other toxins, is ineffective against viruses. Once it has gained entrance into the brain the virus infects the brain tissue. The immediate reaction is an inflammation causing the brain to swell and activating the immune system. The tightly closed vault of the cranium leaves little room for the brain to enlarge, so when it does expand it is squeezed against the bony skull. This, with the active immune system can result in loss of brain cells (neurons), which can result in permanent post-infection damage, depending upon the location of the damage.

Symptoms and treatment

The individual who is developing encephalitis will have a fever, headache, and other symptoms that depend upon the affected area of the brain. He/she may fade in and out of consciousness and have seizures resembling epileptic seizures. The patient may also have rigidity in the back of the neck. Nausea, vomiting, weakness, and sore throat are common. Certain viruses may cause symptoms out of the nervous system as well. The mumps virus will cause inflammation of the parotid gland (parotitis), the spleen, and the pancreas as well as of the brain, for example. An infection by the herpes virus can cause hallucinations and bizarre behavior.

Treatment of encephalitis is difficult. It is important that the type of virus causing the infection be identified. Drugs are available to treat a herpes virus infection, but not others. Mortality (the death rate) can be as high as 50% among patients whose encephalitis is caused by the herpes virus. Infection by other viruses, such as the arbovirus, may have a mortality rate as low as 1%. Treatment is supportive of the patient. Reduction of fever, as well as treatment for nausea and headache are needed. Unfortunately, even those who survive viral encephalitis may have remaining neurologic defects and seizures.

Reye’s syndrome

Reye’s syndrome is a special form of encephalitis coupled with liver dysfunction seen in young children and those in their early teens.

Invariably, the individual who develops Reye’s syndrome has had an earlier viral infection from which they seemingly have recovered. Hours or days later they will begin to develop symptoms such as vomiting, convulsions, delirium, and coma. A virus such as the influenza virus, varicella (measles), and coxsackie virus are responsible. For reasons unknown, giving a child aspirin tablets to reduce fever accompanying a cold or flu can trigger Reye’s syndrome.

At the time the nervous system begins to show signs of infection, the liver is also being affected.

KEY TERMS

Blood-brain barrier— A blockade of cells separating the circulating blood from elements of the central nervous system (CNS); it acts as a filter, preventing many substances from entering the central nervous system.

Meninges— The tough, fibrous covering of the brain and spinal cord.

Spinal cord— The long cord of nervous tissue that leads from the back of the brain through the spinal column, from which nerves branch to various areas of the body. Severing the cord causes paralysis in areas of the body below the cut.

Fatty deposits begin to replace functional liver tissue. Similar fatty tissue can be found in the heart muscle and the kidneys. The relationship between the viral effects on the brain and the parallel liver damage is not known.

Treatment is not specific to the virus, but is directed at relieving pressure on the brain and reducing symptoms. The head of the bed can be elevated and the room left very cool. Care is taken to maintain blood sugar level at normal and not let it drop. Other blood factors such as sodium and potassium also fall quickly and must be corrected.

The mortality rate for Reye’s syndrome can be as high as 25 to 50%. Early diagnosis and initiation of treatment play an important part in keeping the mortality low. Other factors, including age and severity of symptoms, affect the outcome. Some children who survive Reye’s syndrome will show signs of brain damage such as impaired mental capacity or seizures.

Thus, it is important that children who contract one of the common childhood diseases of viral origin, such as mumps, measles, or chickenpox be watched closely to insure they do not develop symptoms of a brain infection from the same virus.

Prevention

Because encephalitis is due to infection, it may be prevented by avoiding the infection. Minimizing contact with others who have any of the viral illness listed above may reduce the chances of becoming infected. Most infections are spread by hand-to-hand or hand-to-mouth contact; frequent hand washing may reduce the likelihood of infection if contact cannot be avoided.

Mosquito-borne viruses may be avoided by preventing mosquito bites. Mosquitoes are most active at dawn and dusk, and are most common in moist areas with standing water. Minimizing exposed skin and use of mosquito repellents on other areas can reduce the chances of being bitten.

Vaccines are available against some viruses, including polio, herpes B, Japanese encephalitis, and equine encephalitis. Rabies vaccine is available for animals; it is also given to people after exposure. Japanese encephalitis vaccine is recommended for those traveling to Asia and staying in affected rural areas during transmission season.

Resources

BOOKS

Booss, John. Viral Encephalitis in Humans. Washington, DC: ASM Press, 2003.

Langford, Rae. Mosby’s Handbook of Diseases. St. Louis, MO: Elsevier Mosby, 2005.

Tamparo, Carol D. Diseases of the Human Body. Philadelphia, PA: F.A. Davis Company, 2005.

Larry Blaser