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 caused 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.

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, 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.

Other viruses that can cause encephalitis include several related viruses: varicella-zoster virus (the cause of chickenpox and shingles), cytomegalovirus, Epstein-Barr virus (the most common cause of infectious mononucleosis) and human herpesvirus-6 (a cause of transient encephalitis in very young children). HIV also can cause encephalitis, particularly in the early stages of infection.

Other viruses that cause encephalitis are transmitted directly or indirectly from animals to humans. Arboviruses are indirectly transmitted from animals and birds to humans by insects, especially mosquitoes and ticks.

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. The virus commonly infects birds. Mosquitoes that bite an infected bird and then bite a human can transmit the virus. West Nile virus does not cause encephalitis in most humans who are infected. This virus does not spread directly from human to human.

Arboviruses that can infect horses are called equine viruses, such as Eastern equine encephalitis (EEE or triple E). A mosquito that bites an infected horse can carry the virus to a human. Fortunately, human infection is rare because unlike West Nile virus infection in humans, triple E infection is often much more serious. Like West Nile, triple E does not spread through direct contact with an infected human.

Lymphocytic choriomeningitis (LCM) virus rarely infects humans. When it does, it can occur through contact with small animals.


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
  • Confusion, which can be mild at first
  • Vomiting
  • Stiff neck and back
  • Drowsiness
  • Extreme sensitivity to light
  • Seizures

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.


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.


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 repellents 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.


Antiviral drugs, such as intravenous acyclovir, are often given when encephalitis is initially diagnosed, even before the cause is known. Acyclovir is the best treatment for herpes simplex encephalitis. If the drug can be started soon after symptoms begin, the chance of full recovery is much better. Without acyclovir treatment, herpes encephalitis can cause severe permanent brain damage. Antiviral drugs don't have much effect on the other viral agents that cause encephalitis.

Encephalitis caused by Lyme disease is treated with intravenous antibiotics, usually ceftriaxone.

Other treatments are known as supportive therapies. These include drugs to reduce fever, relieve headache and treat seizures if they occur.

When To Call a Professional

Contact a doctor if someone in your family suddenly becomes 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 immediate medical attention. In babies, a bulge in the fontanelle (the soft spot of the skull) is another important warning sign.


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 on 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 percent to 60 percent of cases are fatal, and most survivors have permanent brain damage.