Signs and symptoms may include:
- Drowsiness
- Confusion and disorientation
- Seizures
- Sudden fever
- Severe headache
- Nausea and vomiting
- Tremor
- Bulging in the soft spots (fontanelles) of the skull — in infants
- Stiff neck — occasionally
Emergency signs and symptoms may include altered levels of consciousness. In infants, the key symptoms are a stiff neck and a bulging in the soft spots of the skull (fontanelles). In older children, the initial symptoms may be a severe headache and sensitivity to light. In adults, mental disturbances may be more prominent as an initial symptom.
Organisms that transmit disease from one animal host to another are called vectors. Mosquitoes are vectors for the transmission of encephalitis from small creatures, usually birds and rodents, to humans.
Birds who live near bodies of standing water, such as freshwater swamps, are susceptible to infection with an encephalitis virus. When a bird is infected with encephalitis, it carries high levels of the virus in its blood for a short time before recovering from the infection and developing immunity to the disease. If a mosquito feeds on an infected bird, the mosquito will become a lifelong carrier of the disease. The mosquito will transmit the infection to the next bird it feeds on, which will in turn give it to more mosquitoes.
Usually, this transmission pattern cycles through without serious impact on either creature and without affecting humans. This is partly because mosquitoes' primary hosts are birds and small mammals, and they bite humans only as a second choice. However, sometimes environmental disasters, unusual weather or other climate changes cause an increase in the number of infected birds, as well as mosquitoes that feed on both birds and humans. Under these conditions, humans may be affected. The risk of occurrence is highest during the warm months when birds and mosquitoes reproduce.
In the United States, encephalitis is caused by these types of mosquito-borne viruses:
- Eastern equine encephalitis. Eastern equine is the most serious encephalitis virus in North
America. As the name suggests, it afflicts horses. But it also can affect humans. Eastern equine
encephalitis outbreaks occur most commonly in the Eastern United States. This virus infects birds that
live near freshwater swamps. Although some people experience it only as a mild illness, eastern equine
encephalitis is fatal in 30 percent to 60 percent of infected people. However, less than five cases
are usually reported annually. Most cases occur in late summer, though they can happen year-round in
southern states. Symptoms of eastern equine encephalitis usually appear 4 to 10 days after a bite by
an infected mosquito.
- Western equine encephalitis. Like eastern equine encephalitis, this virus affects horses
and humans. Most cases of western equine encephalitis are reported in the central and western Plains
of the United States. This virus flourishes in birds that live near irrigated fields and farming
areas. Western equine encephalitis is less often fatal than its eastern cousin, but it's still
serious. Brain damage and other major complications occur in about 13 percent of people of all ages
infected with the disease, and in one-third of infants. About 3 percent of those who develop severe
symptoms will die of western equine encephalitis. This virus also is rare, with less than five cases
reported each year. Human infections are usually first detected in June or July. Symptoms appear
between 5 to 10 days after being bitten.
- St. Louis encephalitis. This virus causes the most common mosquito-borne disease in the
United States. Like most types of viral encephalitis, it's transmitted to mosquitoes by birds. The
mosquito vector of St. Louis encephalitis breeds in areas of standing water, including such places as
discarded tires, polluted pools, roadside ditches, and containers such as birdbaths and flowerpots.
Although many young people have mild or no symptoms when infected, up to 20 percent of older people
die when infected by St. Louis encephalitis. An average of 128 cases are reported each year in the
United States, although severe outbreaks have affected thousands of people in certain years. Symptoms
appear within 1 week to 10 days.
- La Crosse encephalitis. This virus was named for La Crosse, Wis., where the first outbreak
was recognized in 1963. It's most common in the hardwood forest areas of the upper Midwest and in
Appalachia. Unlike other forms of viral encephalitis, mosquitoes acquire it from chipmunks and
squirrels. La Crosse encephalitis usually affects children and has a mortality rate of about 1
percent. An average of 70 cases are reported annually. Symptoms appear 5 to 15 days after being bitten
by an infected mosquito.
- West Nile encephalitis. This virus first appeared in the United States in 1999. It's
normally found in Africa and the Middle East and in parts of Europe, Russia, India and Indonesia. The
virus is very similar to the St. Louis virus in that birds are its main animal hosts. Although
symptoms of West Nile encephalitis are generally mild, the disease can become severe, especially in
older people and those with weakened immune systems. Symptoms generally appear within 5 to 15 days of
being bitten by an infected mosquito. Mortality rates range from 3 percent to 15 percent. In 2000, the
virus infected 19 people in the United States, resulting in two deaths.
| When to seek medical advice
|
Because encephalitis is potentially serious and life-threatening, see your doctor if you or your child experiences the signs and symptoms that may occur with encephalitis. In infants, the key symptoms are a stiff neck and a bulging in the soft spots of the skull. In older children, the initial symptoms may be a severe headache and sensitivity to light. In adults, mental disturbances may be more prominent as an initial symptom.
Diagnosing encephalitis may involve:
- Spinal tap (lumbar puncture). The most common way to diagnose encephalitis is to analyze
the cerebrospinal fluid surrounding your brain and spinal cord. A needle inserted into your spine
extracts a sample of fluid for laboratory analysis. Analysis of the fluid may reveal presence of an
infection or an increased white blood count, a signal that your immune system is fighting an
infection. If hemorrhages have occurred, your cerebrospinal fluid may be slightly bloody. Diagnosis of
herpes simplex encephalitis is sometimes difficult. Advances using sensitive deoxyribonucleic acid
(DNA) methods have allowed detection of virus in the spinal fluid, confirming the diagnosis.
- Electroencephalography (EEG). This procedure, which takes about a half-hour, measures the
waves of electrical activity produced by your brain. It's often used to diagnose and manage seizure
disorders. Between 16 and 30 small electrodes may be attached to your scalp with paste or an elastic
cap as you recline. You remain still during the test, but at times you may be asked to breathe deeply
and steadily for several minutes or to stare at a patterned board. At times, a light may be flashed in
your eyes. These actions are meant to stimulate your brain. The electrodes pick up the electrical
impulses from your brain and send them to the EEG machine, which records your brain waves on a moving
sheet of paper.
- Brain imaging. A computerized tomography (CT) or magnetic resonance imaging (MRI) scan may
reveal swelling of your brain. This swelling may be localized in herpes simplex encephalitis. The scan
may reveal another condition with signs and symptoms similar to encephalitis, such as a concussion.
- Brain biopsy. Rarely, if diagnosis of herpes simplex encephalitis wasn't possible using DNA
methods or by CT or MRI scans, a small sample of brain tissue may be taken. This sample will be
analyzed in the laboratory to see if the virus is present. You may receive treatment first with an
antiviral agent. If you don't improve with antiviral treatment, then your doctor may suggest a brain
biopsy.
To treat herpes simplex encephalitis, an antiviral agent such as acyclovir may be prescribed in the early stages of the illness. In some cases an anticonvulsant medication is prescribed. Anti-inflammatory drugs or medications that reduce pressure within the skull also may be used.
Because viruses that cause encephalitis don't respond to antibiotics, treatment mainly consists of rest and a healthy diet including plenty of liquids to let your immune system fight the virus. In some cases you may need physical and speech therapy as part of your treatment.
Even though viral encephalitis is rare, a good way to prevent secondary encephalitis is to make sure you and your children are immunized against viral infections that may lead to encephalitis — chickenpox, measles (rubeola), mumps and rubella (German measles).
To protect yourself and your family against mosquito-borne encephalitis during an outbreak of the disease:
- Wear long sleeves and pants if you're outside between dusk and dawn.
- Apply mosquito repellent that contains DEET to your skin and clothing. Exercise caution when using
repellents on children. The American Academy of Pediatrics recommends choosing a repellent made for
children — containing no more than 10 percent DEET — and applying it to their clothing rather than
skin when possible.
- Repair all holes in screens on doors and windows.
- Eliminate standing water by emptying buckets, scrap tires, birdbaths, drains, wheelbarrows,
flowerpots and similar containers. Be sure to change your pet's water dish daily.
- Drain puddles when possible.
- Fill ornamental pools with mosquito-eating fish.
- Clean your gutters and drain flat roofs regularly.
- Fill holes in trees and stumps with mortar.
- Always cover the arms and legs of children playing near swampy areas. Cover your baby's carriage
or playpen with mosquito netting when outside. It's best to avoid applying DEET products to infants.
- Screen rain barrels and openings to water tanks.
—
September 24, 2001 —
