Encephalitis: Viral

Description

Causes

Viral infections that are caused by the arboviruses, which include West Nile Virus, are important causes of encephalitis. They are transmitted by mosquitoes or ticks. Insects that carry disease-causing microorganisms are known as vectors, and the diseases they cause are called vector-transmitted infections. A number of arboviruses, named for the area where they were first discovered, carry different forms of encephalitis. The encephalitis they cause is generally the same disease, although there may be subtle differences in symptoms and the type of brain damage they produce.

Arboviruses multiply in blood-sucking vectors, nearly always mosquitoes. There is no evidence that these infections can be transmitted from one infected person or animal directly to another uninfected person without passing through a mosquito (or tick) first. 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.

An important arbovirus subgroup consists of flaviviruses, which include West Nile, Japanese encephalitis (JE) virus, St. Louis encephalitis (SLE) virus, California serogroup, and Powassan. West Nile virus, at this time, is the most important cause of mosquito-born encephalitis. The incidences from other arboviruses are very low, although outbreaks with larger numbers can always occur.

Incidence of West Nile Virus (WNV). West Nile Virus (WNV) has now emerged as an important problem, causing over 4000 cases and 284 deaths in 2002. It is now is now second only to Lyme disease as the most commonly reported insect-born illness in America.

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 has now occurred in all states except Oregon, Nevada, Utah, and Arizona. The incidence in 2002 was highest in Illinois (884), Michigan (614), and Ohio (441). Other states reporting more than 100 cases included Indiana, Mississippi, Missouri, Nebraska, and Texas. (New York, where it was first reported in 1999, had 82 cases that year.)

How WNV Is Transmitted. WNV, discovered in Uganda in 1937, circulates primarily between birds and Culex 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. It has infected over 110 species of birds. Blue jays and crows can die from the infection but other birds generally survive. Reports in 2002 also found evidence of transmission from blood transfusions, organ transplantation, and breast milk. (The infant who contracted the virus from its infected mother is healthy at the time of this report.)

Severity of WNV. Most symptoms are mild; encephalitis or meningitis occurs in only 1 in 150 infected people. After effects include mild weakness and memory loss that usually clear up within a few weeks. Recovery is nearly always complete, but less rapid in adults than in children. There is a low incidence of seizures. Of concern, however, are reports in 2002 of polio-like symptoms in survivors. Symptoms include painless weakness in the legs and arms in seven patients and, in one patient, weakness in the neck and respiratory muscles.

Enteroviruses

Enteroviruses account for between 10% and 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 chicken pox 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 encephalitis in developed countries and is responsible for about 10% to 20% of all 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). Both HSV-1 and HSV-2 can cause encephalitis in infants. Although HSV-1 has been thought to be the primary culprit in most adult cases of herpes encephalitis, current evidence suggests that either form can cause this infection in adults.

Herpes simplex encephalitis is the only effectively treatable form of encephalitis. Treatment is typically intravenous acyclovir, which is very effective but must be administered within two days of symptoms for the best outcome. [For more information, see the Well-Connected Report Herpes Simplex.]

Varicella-Zoster Virus. The varicella-zoster virus is responsible for both chicken pox (when the virus is called varicella) and shingles (when it is referred to as herpes zoster). Chicken pox 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 the Well-Connected Report Varicella-Zoster Virus (Chicken Pox and Shingles).]

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 one to three weeks after the onset of the infection. If encephalitis develops, it is almost always mild with full recovery.

Tick-Borne Encephalitis Viruses

Some tick-borne encephalitis cases have been reported, but only rarely in the US. Most notable is Rocky Mountain spotted fever. Ticks can also carry related viruses that cause brain inflammation, particularly one known as Russian spring-summer encephalitis. This encephalitis is most common in China, Korea, and the eastern areas of Russia. Only a very few viral tick-borne cases of encephalitis have been reported in the US.

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 US. 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 wiped out these complications in developed countries. Measles encephalitis still sometimes occurs in immunocompromised children. Rarely, influenza has caused acute encephalitis, usually in children. (Flu vaccinations would be important in preventing these events.) Because of its rarity, encephalitis associated with childhood disease is not discussed in this report.

Epstein-Barr Encephalitis. Epstein-Barr, the cause of mononucleosis, is a very common virus. In rare cases, it has been associated with encephalitis, which is usually mild.

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.

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 cats fecal matter, results in 2,100 hospitalizations a year, which rivals herpes as the most common infectious cause of encephalitis. It should be noted, however, that this condition causes very mild symptoms in most people. People with HIV and impaired immune systems, however, are at risk for more severe forms. (Of note: 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 procyonis) 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 worms 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 US since 1981, most in children younger than six 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 agents 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 two to three weeks after recovery from a viral illness. (It does not affect children under two.) 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 chicken pox, 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.