Viral encephalitis

Encephalitis is a severe inflammation of the brain. Encephalitis is often accompanied by meningitis (an inflammation of the membranes surrounding the brain and spinal cord). This condition is sometimes referred to as meningoencephalitis.


Causes/associated factors
Encephalitis is usually caused by a virus, as described below. The virus can enter your body through the bite of an arthropod (such as a mosquito or tick) or animal or through contact with an infected person. Less frequently, encephalitis is caused by fungi, parasites and bacteria. An infected mother can also pass the infection to her baby before or during birth.
When the virus enters the body, it often causes an infection somewhere other than the brain. When transported through the bloodstream, however, some viruses may travel to the brain. Some viruses can travel along the nerves to the brain.

You're at risk for encephalitis if you've been exposed to infected carriers (such as mosquitoes and ticks) or travel to areas where the disease is prevalent. Newborns, elderly adults and people infected with HIV (the virus that causes AIDS) are also at increased risk.


Types of encephalitis
Herpes simplex encephalitis (HSE)
The most common cause of viral encephalitis in the United States is the herpes simplex virus (HSV). Herpes simplex encephalitis (HSE) spreads through direct contact with drainage or discharge from the rash of someone who has herpes. HSE occurs year-round and affects people of all age groups. Left untreated (or ineffectively treated), it's often fatal.

Arboviral encephalitis
Arboviruses are viruses transmitted to humans and other animals through the bite of an infected arthropod (such as a mosquito or tick). The arthropod itself can become infected when it feeds on an infected bird or animal. In turn, female arthropods pass the virus in their eggs. These eggs develop into adult insects that are also capable of spreading the virus. Humans with arboviral encephalitis can't pass the infection to others through casual contact. Arboviral encephalitis is most common from June through September, the most active time of year for mosquitoes and ticks. In areas with mild weather, however, cases of encephalitis have been reported into the winter months. The main types of arboviral encephalitis in the United States include:

St. Louis encephalitis (SLE): The SLE virus is transmitted to humans through the bite of certain infected mosquitoes. The mosquitoes themselves become infected when they bite an infected bird. Outbreaks have occurred most recently in the Midwest and Southeast, but SLE can occur anywhere in the United States. Most people infected with the virus do not develop encephalitis. In fact, most develop no symptoms at all. An average of about 130 cases of SLE are reported in the United States each year. Most people survive the infection. Elderly adults are at the greatest risk of severe or fatal SLE. The infection is usually milder in children.

Lacrosse encephalitis (LAC): The LAC virus is transmitted to humans through the bite of certain daytime biting mosquitoes. The mosquitoes themselves become infected when they bite an infected animal, usually a chipmunk or tree squirrel. LAC occurs more often in the Midwest and mid-Atlantic states. There are about 75 cases reported each year in the United States. Most cases of LAC develop in children younger than age 16. Although the infection can be severe, it's not often fatal.

Western equine encephalitis (WEE): The WEE virus is transmitted to humans through the bite of certain infected mosquitoes. The mosquitoes themselves become infected when they bite an infected bird. Areas with irrigated agriculture provide a good habitat for this cycle. WEE occurs most often in the western United States. Children, especially those younger than age 1, are more often seriously affected by WEE. Most people survive the infection.

Eastern equine encephalitis (EEE): The EEE virus is transmitted to humans through the bite of certain infected mosquitoes. The mosquitoes themselves become infected when they bite an infected bird. Swampy areas provide a good habitat for this cycle. EEE occurs most often in the United States along the East Coast, the Gulf Coast and some inland Midwestern locations. Human outbreaks of EEE may be preceded by horse outbreaks. In fact, the infection is more common in horses than humans. EEE is a severe form of encephalitis for humans, however. About one-third of all people who are infected do not survive. Those who recover may experience permanent brain damage.

Powassan encephalitis (POW): POW virus is a rare cause of viral encephalitis. It's transmitted to humans through the bite of an infected deer tick. It is usually found in the upper United States and Canada. Only 20 cases have been reported in humans. It's considered to be a severe form of encephalitis and those who recover may have lasting neurological difficulties.

West Nile encephalitis (WNE): The WNE virus was previously thought to be present only in Africa, Asia and the Middle East. In 1999, however, it was identified in an outbreak of encephalitis in New York. Since then, virus activity has been identified in most of the continental United States. WNE spreads to humans through the bites of certain infected mosquitoes. The mosquitoes themselves become infected when they bite an infected bird. Most people survive the infection, but people older than age 50 are at the highest risk of developing a severe or even fatal infection.
Many other types of arborviral encephalitis occur throughout the world, including Venezuelan equine encephalitis, Japanese encephalitis and Murray Valley encephalitis. Most of these diseases are problematic only to the countries where the virus is found and for travelers to those countries.

Other types of encephalitis
Less common types of viral encephalitis are those caused by:

the varicella zoster virus, which spreads through contact with someone who has shingles or chickenpox
the Epstein-Barr virus, which spreads through contact with an infected person
the rabies virus, which most commonly spreads through the bite or saliva of an infected animal (often bats, wild animals or dogs)

Transmission
Again, viral encephalitis typically spreads through contact with an infected person or the bite of a mosquito, tick or animal. The method of transmission and incubation period (time from exposure until symptoms appear) depends on the type of virus. It's also important to remember that the arbovirus cannot be spread from human to human through casual contact, although suspected transmission to recent organ organ transplant and blood transfusion recipients has been reported. This is not true for other types of viral encephalitis, however.

Signs/symptoms
Although certain differences occur, most infections are characterized by nonspecific symptoms at first, which may progress slowly or rapidly. For all types of viral encephalitis, symptoms may include:
personality changes
impaired memory
confusion
hallucinations
trouble with speech
tremors
difficulty with movement
seizures
coma
If the infection involves the membranes surrounding the brain and spinal cord, symptoms may also include:

fever
headache
stiff neck and/or stiff back
muscle aches
aversion to bright light
nausea and vomiting

Diagnosis
Diagnosis is initially based on your medical history, current symptoms and a physical exam. Sporadic cases of encephalitis are often difficult to distinguish from other fever-producing illnesses. Your doctor will do a lumbar puncture or spinal tap (taking samples of cerebrospinal fluid, the fluid that surrounds the brain and spinal cord, through a needle placed in your lower back). In an attempt to isolate the disease-causing organism, the fluid will be studied for cell count and type, bacteria, glucose (sugar) level and protein count. Viral and fungal cultures and other studies may also be done. With all types of encephalitis, the pressure measured during the spinal tap will be elevated. White blood cells and protein levels may also be higher than normal. Another test called a polymerase chain reaction can be done to help diagnose herpes simplex virus, Epstein-Barr and enterovirus infections.
You'll also need blood tests, as well as possible studies of swabs or fluid samples from affected areas. Depending on the circumstances, you may also need one or more of the following tests:

electroencephalogram (a recording of the electrical activity of the brain) to identify abnormalities
computed tomography (CT scan, an imaging technique) to rule out the possibility of cerebral hematoma (a swelling or mass of blood in the brain) or sores in the temporal lobe (part of the brain)
magnetic resonance imaging (MRI, another imaging technique) to check for sores characteristic of herpes simplex encephalitis or signs that would indicate encephalitis caused by a parasite, such as toxoplasmosis
In severe cases, a brain biopsy (taking a tissue sample for further examination) may be recommended. Although an extremely invasive procedure, a brain biopsy is a highly sensitive and specific diagnostic tool for herpes simplex encephalitis.


Treatment
Early diagnosis and treatment are essential. Antiviral medications are effective for some infections. For example, herpes simplex and varicella zoster encephalitis are treated with the antiviral medication acyclovir for 14 to 21 days. Currently, although encephalitis treatment recommendations are constantly evolving, there are no effective antiviral medications for arboviral encephalitis. For any type of encephalitis, the doctor may recommend drinking fluids and replacing electrolytes (substances that transmit electrical impulses) to prevent dehydration. The doctor may prescribe medications to help prevent seizures, relieve pain, and reduce brain swelling and inflammation, as well as restlessness and fever. Antibiotics may also be prescribed for any associated infections.

Complications
Complications of encephalitis may include mental deterioration, seizures and coma. Long-term effects may include hearing loss, visual impairment, memory loss, seizures and behavioral changes. In some cases, encephalitis is fatal. Recently, there have been a few cases of polio-type paralysis associated with the West Nile virus.

Pregnancy-specific information
If you're infected with a virus that causes genital herpes, rubella, chickenpox or western equine encephalitis, the infection can spread to your baby before or during delivery and cause viral encephalitis.

Senior-specific information
Some types of viral encephalitis, such as St. Louis encephalitis and West Nile encephalitis, can be more serious for older adults.

Prevention
To prevent herpes simplex encephalitis, avoid contact with anyone who has active herpes sores, especially if there is any drainage or discharge. Seek prompt medical attention if you develop a lesion after contact with an infected person.
To prevent arboviral encephalitis, reduce the amount of time you spend outdoors, especially during the early evening hours. When you are outdoors, wear long pants and long-sleeved shirts, and apply mosquito repellent to exposed skin. Be sure that windows and doors are properly protected with intact screens.

Discourage the breeding of mosquitoes by emptying water out of any containers kept outside. Discarded tires that collect water are a good example. On the community level, chemicals can be sprayed to stop larvae from growing into infected adult mosquitoes, as well as kill adult mosquitoes. Any cases of encephalitis should be reported to the state or local health department.

If you're HIV-positive, discuss individual risk factors and additional precautions with your doctor.

In the United States, a human vaccine is available for Japanese encephalitis and a horse vaccine is available for eastern, western and Venezuelan equine encephalitis. Unfortunately, there are currently no approved vaccines for other types of encephalitis in the United States.