Encephalitis

What Is It?

Encephalitis means inflammation of the brain. This inflammation usually is triggered by a viral infection, although sometimes it can be caused by a bacterial infection of the brain, such as Lyme disease. In some cases, symptoms are caused by direct infection of the brain. In other cases, the brain inflammation is casued by the immune system's response to the brain infection. Even if the immune system attack succeeds in eliminating the infection, it may injure the brain in the process. This is called post-infectious encephalitis. Encephalitis can be fatal, but it also may produce only a short, mild illness.

Often, viruses that cause encephalitis also cause inflammation of the delicate tissues that cover the brain and spinal cord, which are called the meninges. This condition is meningitis. When encephalitis and meningitis occur together, which is relatively common, it is called meningoencephalitis.

Of the many different viruses that can cause meningoencephalitis, enteroviruses (particularly coxsackievirus and echovirus) are the most common cause in the United States, particularly if the illness occurs in the summer or fall. Encephalitis also can be caused by the herpes simplex virus, which also causes cold sores and genital herpes. This type of encephalitis is less common but tends to be more severe. The mumps and measles viruses also can cause encephalitis, with mumps occurring most often in the winter or spring. All of these viruses are transmitted directly from one person to another. Other viruses that can cause encephalitis include several related viruses: varicella-zoster virus (the cause of chickenpox and chingles), cytomegalovirus, Epstein-Barr virus (the most common cause of infectious mononucleosis) and human herpesvirus-6 (a cause of transient encephalitis in very young childn). HIV also can cause encephalitis, particularly in the early stages of infection.

Other viruses that cause encephalitis are spread by animals. Equine viruses (which also are found in horses) and arboviruses are transmitted from animals to humans by the bites of insects, typically mosquitoes. Lymphocytic choriomeningitis (LCM) virus often is spread by contact with small animals. The West Nile virus, one of the arboviruses, is widespread in Africa, Central Europe, the Middle East and Asia. Since 1999, it has become increasingly common in the United States. West Nile virus does not cause encephalitis in most humans who are infected, but when it does, the symptoms can be severe.

Symptoms

The symptoms of encephalitis range from mild to severe, and can be life threatening. Fortunately, most cases are not severe. Possible symptoms, beginning with the two most common, include the following:

• Sudden fever
• Headache
• Vomiting
• Stiff neck and back
• Drowsiness
• Extreme sensitivity to light
• Seizures
• Coma
• Paralysis
• Convulsions
• Abnormalities of the peripheral nerves

Many of these symptoms also occur in other conditions that affect the brain, including migraine headaches and life-threatening conditions such as bleeding in the brain. When a person has a fever along with the other symptoms, some kind of infection is most likely. Severe muscle weakness or paralysis and abnormalities of the peripheral nerves are more common with West Nile virus infection. If a person is behaving in a strange way, smelling things that are not there, or having difficulty speaking or understanding speech, infection with the herpes simplex virus is most likely.

Diagnosis

If your doctor suspects you have encephalitis, he or she will order tests such as a computed tomography (CT) scan or magnetic resonance image (MRI) of the brain. A procedure called a lumbar puncture or spinal tap may be used to draw fluid from the spine and test it to determine what virus is causing the encephalitis.

Expected Duration

Encephalitis can last from a few days to several months, depending on the virus involved and the severity of the case.

Prevention

When outbreaks of insect-borne encephalitis occur, people in the affected communities should remove pools of standing water, where mosquitoes can breed, and should use insect repellant. The most effective insect repellants contain a chemical called DEET. Travelers can be given vaccines against one cause, Japanese B encephalitis, which is common in Japan and other parts of Asia.

Treatment

Antiviral drugs are available for several causes of encephalitis, although those for the herpes simplex virus are the most effective. Treatment of symptoms seizures, pain and fever also may be helpful.

When To Call A Professional

Contact a doctor if someone in your family appears to be suddenly confused, is very hard to arouse, seems to have lost consciousness or has severe headaches. Whether these symptoms are caused by encephalitis or another condition, they require medical attention. In babies, a bulge in the fontanelle (the soft spot of the skull) is another important warning sign.

Prognosis

Encephalitis is most dangerous in babies and senior citizens, but it can be serious and even fatal in people of all ages. With severe cases, recovery is slow and may involve therapy to regain certain skills. The prospects for a complete recovery vary depending upon the type of virus involved. Encephalitis from the herpes virus can cause permanent injury. Eastern equine encephalitis is rare, with fewer than 10 cases per year in the United States. However, 50% to 60% of cases are fatal, and most survivors have permanent brain damage.

Additional Info

Centers for Disease Control and Prevention (CDC)

Division of Vector-Borne Infectious Diseases

P.O. Box 2087

Fort Collins, CO 80522

Toll-Free: (800) 311-3435

http://www.cdc.gov/ncidod/dvbid/index.htm