Encephalitis: Viral - Causes

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.

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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.