Encephalitis: Viral - Causes

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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. The first human encephalitis fatality caused by deer tick virus, which is closely related to Powasson virus, was reported in 2009. Cases of tick-borne encephalitis have also been reported from Rocky Mountain spotted fever, but this is a bacterial (not viral) infection.

Enteroviruses

Enteroviruses include various viruses that enter the body through the gastrointestinal tract. They account for between 10 - 20% of viral encephalitis cases. The group A coxsackievirus has been detected in infants and children with encephalitis and is among the important viruses in the class. (However, 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.

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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 raccoons, bats, or other wild animals.

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 patients. Symptoms include lethargy, confusion, coma, and symptoms of meningitis (stiff neck, headache, and vomiting).

Parasitic Causes of Encephalitis

Toxoplasmosis. Encephalitis from toxoplasmosis, which can be transmitted through a cat's feces or by eating contaminated food, 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 symptoms. In addition, in pregnant women toxoplasmosis can cause severe problems in the fetus’ central nervous system and eyes Toxoplasmosis 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. 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. 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.

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) is a condition that can develop in patients recovering from a viral infection. (Less commonly, it is associated with childhood vaccinations for measles, mumps, and rubella.) Although it is not caused directly by a viral infection, ADEM has symptoms similar to viral encephalitis, such as fever, fatigue, nausea and vomiting. In severe cases, ADEM can cause seizures and coma.

The inflammation associated with ADEM 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 and they can share common symptoms of myelin damage. These symptoms include vision problems and muscle paralysis.

Review Date: 01/26/2011
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org).