Encephalitis

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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.