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Viral encephalitis

Description

An in-depth report on the causes, diagnosis, treatment, and prevention of viral encephalitis.

Alternative Names

West Nile virus

Causes

Arboviruses

Arboviruses, including the West Nile virus, are transmitted by blood-sucking insects such as mosquitoes and ticks. Most of the time, the viral infections initially develop in birds. Insects that feed on the infected blood from a diseased bird (or reservoir ) carry the virus, and transmit it when they bite a susceptible host (such as an animal or a human). Because these insects play a role in the disease-transmission process, they are referred to as vectors .

Arboviruses multiply in blood-sucking vectors, nearly always mosquitoes. There is no evidence that these infections can be transmitted casually from one infected person or animal directly to another uninfected person without passing through a mosquito (or tick) first. (Although, a small number of West Nile virus cases have occurred through blood transfusions, organ transplantation, and possibly breast feeding.) It should be stressed that only about 10% of people who are infected by an arbovirus develop encephalitis and that symptoms arise in only about 1% of those infected.

Arboviruses that cause encephalitis are primarily found in three virus families: Togaviridae , Bunyaviridae , and Flaviviridae . In the United States, the main mosquito-borne encephalitis strains are: Eastern equine, Western equine, St. Louis, La Crosse, and West Nile. Equine encephalitis causes disease in both humans and, as its name implies, horses. Powassan encephalitis is a less common tick-borne flavivirus that occurs primarily in the northern United States. Japanese encephalitis is the most common form of viral encephalitis to occur outside of the United States. It is endemic in rural areas in east, south, and southwest Asia, especially China and Korea. Venezuelan equine encephalitis is found in South and Central America.

Different arboviruses cause different forms of encephalitis. Although the overall disease is the same, there are subtle differences in symptoms and the type of brain damage they produce.

Common Forms of Mosquito-Borne Arbovirus Encephalitis
Eastern Equine Encephalitis
Virus Family	Togaviridae (genus Alphavirus )
U.S. Geographic Areas	Atlantic and Gulf coasts, in New England, and around the Great Lakes. States most affected are Florida, Georgia, Massachusetts, and New Jersey.
Symptom Onset	Symptoms appear 4 - 10 days following infection and can range from mild flu-like symptoms to full-blown encephalitis.
Incidence and Mortality Rates	The most serious of the U.S. arboviruses are fortunately rare. About 220 cases have been confirmed since 1964 with an average rate of 5 cases per year. About a third of people who contract EEE die from it. Children are more likely to survive but also to suffer complications afterward.
Age Risk Groups	Adults over age 50 and children under age 15.
Western Equine Encephalitis
Virus Family	Togaviridae (genus Alphavirus )
U.S. Geographic Areas	Farming areas in western and central Plains and Rocky Mountain states west of the Mississippi.
Symptom Onset	5 - 10 days following infection.
Incidence and Mortality Rates	Very rare. There was only one case reported between 1995 and 2000. Mortality rate is 3 - 4%; 30% of survivors have complications afterward. Most severe in children, especially those younger than 1 year. Infants may suffer permanent neurological damage.
Age Risk Groups	Infants younger than 12 months.
St. Louis Encephalitis
Virus Family	Flaviviridae (genus Flavivirus )
U.S. Geographic Areas	Takes its name from an epidemic in St. Louis, but outbreaks have occurred in wider geographic areas, especially in midwestern and southeastern states, and can occur in rural or urban areas. As of 2000, the highest numbers of total cases have been reported in Texas (970), Illinois (695), Ohio (440), Indiana (368), and Florida (379).
Symptom Onset	7 - 10 days following infection.
Incidence and Mortality Rates	Although over 4,500 cases have been reported since 1964, the average number of cases has been declining with a yearly average of only 11 cases between 1995 and 2000. Mortality rate of between 3 - 30%, with about 5% of survivors suffering complications afterward.
Age Risk Groups	Elderly adults (over age 60) are at highest risk and the disease is most severe in this age group. Younger people usually experience mild, flu-like symptoms.
La Crosse Encephalitis
Virus Family	Bunyaviridae (genus Bunyavirus )
U.S. Geographic Areas	Occurs most frequently in upper Midwestern, southeastern (Appalachia), and mid-Atlantic states. Most cases have occurred in Ohio and Wisconsin. Unlike other encephalitis viruses which originate in birds, La Crosse encephalitis is transmitted to mosquitoes from infected chipmunks and squirrels.
Symptom Onset	5 - 10 days following infection.
Incidence and Mortality Rates	An average of 70 - 100 cases reported per year. Mortality rates are less than 1%. More common and severe in children under age 16.
Age Risk Groups	Children younger than 16 years.
West Nile Encephalitis
Virus Family	Flaviviridae (genus Flavivirus ).
U.S. Geographic Areas	Cases have been reported throughout the mainland United States. In 2006, majority of West Nile encephalitis cases occurred in Idaho, Colorado, Texas, California, and Nebraska.
Symptom Onset	3 - 14 days following infection.
Incidence and Mortality Rates	In 2006, 4,180 cases of WNV were reported to the CDC, with 149 deaths. Of all the reported cases, 61% were due to West Nile fever. Thirty-four percent of those who contracted WNV had more severe conditions, such as meningitis and encephalitis. However, most cases of West Nile virus do not produce symptoms, and are not reported, so these numbers imply a more worrisome picture than actually exists. In fact, less than 1% of people who infected with WNV go on to develop neurological disease.
Age Risk Groups	Adults over age 50.

West Nile Virus (WNV). Until 1999, the West Nile virus was generally restricted to Africa, the Middle East, southwestern Asia, eastern Europe, and Australia. It emerged in the United States with the first outbreak in New York City in 1999. WNV is now found in birds and mosquitoes in every state except Alaska and Hawaii. Human cases of West Nile encephalitis have been reported throughout the continental United States.

How WNV Is Transmitted. WNV, discovered in Uganda in 1937, circulates primarily between birds and mosquitoes and can be carried long distances by migrating birds. In a given geographic area, the appearance of the virus among birds and mosquitoes generally precedes infection in humans. WNV has infected over 110 species of birds. In addition to mosquito-to-human transmission, other causes of human infection include blood transfusions and organ transplantation. In 2005, the FDA announced the first West Nile virus blood test to screen donors of blood, organs, cells, and tissues. The test will be used by blood banks throughout America to help protect patients who receive transfusions. There have also been cases of mother-to-child transmission during pregnancy. However, a 2006 study reported that most pregnant women who are infected with WNV deliver healthy babies. It is still not clear if WNV can be transmitted through breast milk.

Severity of WNV . About 80% of people infected with WNV will not have any symptoms. Twenty percent will develop West Nile fever (which includes fever, headache, and occasional skin rash). Less than 1% of infected people will develop neuroinvasive disease, the most severe form of WNV.

Neuroinvasive disease affects the nervous system and includes encephalitis, meningitis, and poliomyelitis. People over age 50 and those with weakened immune systems are at the greatest risk for neuroinvasive disease. The fatality rate for those afflicted ranges from 3 - 15%. Neuroinvasive disease symptoms include high fever, headache, stiff neck, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. Preliminary research is currently being conducted on vaccines to prevent WNV and antiviral drugs to treat it.

Although West Nile fever is considered to be less serious than West Nile neuroinvasive disease, an important 2006 study found that both conditions can cause long-term health complications. Researchers found that more than a year after being diagnosed with WNV, half of patients complained of neurological and psychological symptoms, including fatigue, memory problems, headaches, depression, and tremors. Patients who had West Nile fever were as likely to experience these problems as those who had WNV-associated encephalitis or meningitis.

Tick-Borne Encephalitis Viruses

Tick-borne encephalitis (TBE) is commonly found in many countries throughout Europe, Asia, and the former Soviet Union, but it is reported only rarely in the U.S. Powassan encephalitis is the main tick-borne encephalitis found in the United States and Canada. Cases of tick-borne encephalitis have also been reported from Rocky Mountain spotted fever, but this is a bacterial (not viral) infection.

Enteroviruses

Enteroviruses account for between 10 - 20% of viral encephalitis cases. For example, the group A coxsackievirus has been detected in infants and children with encephalitis and is among the important viruses in the class. (It should be noted that the enteroviruses are nearly as common as cold viruses and are rarely serious.) Enteroviruses can be spread through food or water contaminated by trace amounts of fecal material and through sneezing and coughing.

Herpes Viruses

The herpesvirus group includes a number of common infections, including herpes simplex, varicella-zoster (the cause of chickenpox and shingles), cytomegalovirus, herpesvirus 6, and Epstein-Barr (EB) virus (the cause of mononucleosis). About 2,100 people are hospitalized each year from herpes-associated encephalitis. These viruses share certain features, including the capacity to cause an infection and then to go into hiding. They can lie dormant for periods of time as short as months or as long as a lifetime. In a few cases, when the viruses reactivate, they cause encephalitis. In fact, some evidence suggests that varicella-zoster, cytomegalovirus, and Epstein-Barr (EB) virus may be more common causes of encephalitis than previously thought. In most cases, however, encephalitis from these viruses occurs in people with impaired immune systems, such as people with HIV or organ transplant patients.

Herpes Simplex Virus. Herpes simplex virus (HSV) is the most common cause of encephalitis in developed countries and is responsible for about 10 - 20% of all adult cases of viral encephalitis. There are two distinct types of the herpes simplex virus: HSV-1 (which is commonly associated with oral herpes) and HSV-2 (which usually causes genital herpes, although HSV-1 can also cause this form). HSV-2 causes 70 - 90% of encephalitis cases in neonatal infants; the virus is transmitted through the mother's genital secretions. Although HSV-1 is the primary culprit in most adult cases of herpes encephalitis, HSV-2 may also cause a small number of these cases.

Herpes simplex encephalitis is the only effectively treatable form of encephalitis but treatment (typically intravenous acyclovir), must be administered within the first few days of symptom onset. If left untreated, the mortality rate for patients with HSV-1 is approximately 70%; if treated, the mortality rate declines to 30%. The mortality rate for neonatal HSV-2 encephalitis ranges from 15-57%. [For more information, see In-Depth Report #52: Herpes simplex . ]

Varicella-Zoster Virus. The varicella-zoster virus is responsible for both chickenpox (when the virus is called varicella ) and shingles (when it is referred to as herpes zoster ). chickenpox is the initial infection, after which the virus remains dormant, often for a lifetime. If it erupts, usually years later, is does so in the form of shingles. Encephalitis caused by varicella can occur in both children and adults and be very serious. If it occurs as a result of herpes zoster in adults, the brain inflammation tends to be mild except in immunocompromised patients. In such cases, symptoms can appear weeks to months after an attack of shingles and resemble those of a stroke. Fortunately, encephalitis is rare with both varicella and zoster. [For more information, see In-Depth Report #82: Shingles and chickenpox (varicella-zoster virus).]

Epstein-Barr Virus. Epstein-Barr virus is the cause of infectious mononucleosis, which is most common in children and young adults. Symptoms of the disease are severe fatigue, headache, sore throat, and fever. In 1% of cases, neurological complications occur about 1 - 3 weeks after the onset of the infection. If encephalitis develops, it is almost always mild with full recovery.

Cytomegalovirus Encephalitis. Cytomegalovirus is also very common and usually mild. In immunocompromised patients, such those with AIDS, it can be dangerous, with severe complications including encephalitis.

Other Viral Causes of Encephalitis

Rabies. The rabies virus is transmitted from the saliva of an infected animal. The encephalitis it causes is virtually always fatal but is very rare in the U.S. Only one or two cases are typically reported each year, often from contact with bats.

Encephalitis Associated with Childhood Diseases. Encephalitis occurs rarely after common childhood infections, such as rubella, measles, and mumps. Immunizations have almost completely eliminated these infections in developed countries. Measles encephalitis still sometimes occurs in immunocompromised children. Rarely, influenza has caused acute encephalitis, usually in children. (Flu vaccinations are important in preventing these events.) Although there used to be concern that diphtheria-pertussis-tetanus and measles-mumps-rubella vaccines could cause encephalitis, recent research indicates that these childhood vaccines are very safe and do not increase encephalitis risk.

Adenoviruses. Adenoviruses were first identified in 1953 from infected tonsils and adenoids. The viruses can cause respiratory or gastrointestinal infections that are usually mild. In rare cases, adenoviruses can cause encephalitis or meningoencephalitis, which can be fatal in 30% of cases. Symptoms include lethargy, confusion, coma, and symptoms of meningitis (stiff neck, headache, and vomiting).

Parasitic Causes of Encephalitis

Toxoplasmosis. Encephalitis from toxoplasmosis, which is transmitted in a cat's fecal matter, results in 2,100 hospitalizations a year, which rivals herpes as the most common infectious cause of encephalitis. However, this condition causes very mild symptoms in most people. People with HIV and impaired immune systems are at risk for more severe forms. In addition, the effects on the fetus in a pregnant women infected with toxoplasmosis can be devastating. It can be treated with antibiotics, particularly those that treat parasites.

Raccoon Roundworm. Raccoon roundworm ( Baylisascaris procyoni s) is a large parasitic worm that lives in the intestines of raccoons. In one Wisconsin study, half the raccoons tested were infected. Humans usually become infected by ingesting the worm's eggs through accidental contact with soil, wood chips, or tree bark contaminated with raccoon feces. The worm is harmless in raccoons but can produce severe central nervous system disease, including encephalitis, in people. At least 12 severe cases have been reported in the U.S. since 1981, most in children younger than 6 years of age (who are at higher risk because of their tendency to put their fingers or other objects into their mouths). Prompt treatment with larvae-killing drugs such as albendazole or anti-inflammatory drugs is not consistently effective, so it is extremely important to avoid infection. Raccoons should not be kept as pets. Eliminate access to food sources, like garbage cans and bird feeders, which will attract raccoons. Raccoon nests should be sealed off while raccoons are absent. Burning any contaminated materials is the most effective method of disposal. If burning is not feasible, contaminated substances should be buried deeply in a location remote from human activity. Wearing disposable gloves, boots, and a dust mask is important. Decks, woodpiles, and other surfaces can be decontaminated with boiling water.

Other Parasitic Infections. Encephalitis may be caused by other parasitic infections, such as toxocariasis (from roundworms found in dogs and cats); or cysticercosi (from food or water contaminated with pork tapeworm eggs). These infections usually cause only chills, fever, and swelling of lymph nodes, though seizures and headaches can occur.

Bacterial and Fungal Organisms

In very rare circumstances, encephalitis may be caused by bacterial or fungal organisms.

Acute Disseminated Encephalomyelitis (ADEM)

Acute disseminated encephalomyelitis (ADEM), also called noninfectious encephalitis, constitutes one-third of all known cases of encephalitis. It is not caused by a virus, although it most often develops in patients 2 - 3 weeks after recovery from a viral illness. (It does not affect children under 2 years old.) Damage to nerve cells in such cases is caused not by the viral infection, however, but most likely by an autoimmune reaction, in which the body's immune system attacks its own brain tissue.

Acute disseminated encephalomyelitis has been reported as a rare complication of childhood illness, including chickenpox, mumps, or measles. Vaccination reduces these risks to nearly insignificant levels. It is a complication of the rabies vaccine in one out of 30,000 cases. Nonspecific respiratory infections are now the most common causes of ADEM, but such cases are also extremely rare.

The inflammation occurs predominantly in the white matter of the brain rather than the gray matter (the usual target of infectious encephalitis). The nerve cells do not die as they do in a viral infection. Rather, the nerve cell coating (called a myelin sheath) is partially destroyed in much the same way as it is in multiple sclerosis. Indeed, the two conditions may at first be difficult to distinguish. Recurrences may occur several months to years after the initial episode.

• Review Date: 1/18/2007
• Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital

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