Medical Health Encyclopedia

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.

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 US. Only one or two cases are typically reported each year, often from contact with bats.

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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.

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. 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. 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 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 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 US 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 two to three 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 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.