Encephalitis means "inflammation
(irritation and swelling)
brain," but it usually refers to brain inflammation
caused by a virus. This severe and potentially
life-threatening disease is rare.
Encephalitis takes two forms, categorized by the
two ways that viruses can infect your brain:
encephalitis. This occurs when a virus
directly invades your brain and spinal cord. It
can happen to people at any time of the year
(sporadic encephalitis), or it can be part of an
outbreak (epidemic encephalitis).
(post-infectious) encephalitis. This
form occurs when a virus first infects another
part of your body and secondarily enters your
The primary form of the disease is more serious,
while the secondary form is more common. But because
of the milder nature of secondary encephalitis,
doctors actually see more cases of primary
See your doctor if you experience signs and
symptoms of primary or secondary encephalitis
because the course of the disease is unpredictable.
Signs and symptoms
Most people infected with viral encephalitis have only mild or no
symptoms, and the illness doesn't last long. Serious cases can cause:
Stiff neck —
Bulging in the
soft spots (fontanels) of the skull in infants
Emergency signs and symptoms may include altered levels of
consciousness. In infants, the key signs are a stiff neck and a bulging
in the soft spots (fontanels) of the skull. In older children, watch for
severe headaches, lethargy, confusion and sensitivity to light. In
adults, mental disturbances may be more prominent.
Three broad categories of viruses — herpes viruses, childhood infections
and arboviruses — typically trigger primary and secondary encephalitis.
Some herpes viruses that cause common infections may also cause
encephalitis. These include:
There are two types of herpes simplex virus (HSV) infections. HSV
type 1 (HSV-1) causes cold sores or fever blisters around your
mouth. HSV type 2 (HSV-2) causes genital herpes. HSV is the most
common cause of sporadic encephalitis, with HSV-1 being the more
common culprit. However, you're not more likely to contract
encephalitis if you suffer from cold sores. When untreated, the
mortality rate from herpes simplex encephalitis is between 60
percent and 80 percent. That number drops to 15 percent to 20
percent with treatment.
This virus is responsible for chickenpox and shingles. It can cause
encephalitis in adults and children, but tends to be mild.
This herpes virus causes infectious mononucleosis (mono). If
encephalitis develops, it's usually mild but results in death in
about 8 percent of cases.
rare instances, secondary encephalitis occurs after common childhood
viral infections, including:
This type of occurrence of encephalitis may be due to a hypersensitivity
reaction — an overreaction of your immune system to a foreign substance.
Viruses that are transmitted by mosquitoes and ticks (arboviruses) are
the most common cause of epidemic encephalitis. In recent years, these
viruses have also produced the most publicized cases of encephalitis.
However, this kind of encephalitis is rare, even though encephalitis is
the most common mosquito-borne disease. Here's how
the transmission cycle works:
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
Birds that 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 becomes a lifelong carrier of the disease. The
mosquito transmits the infection to the next bird it feeds on, which in
turn passes 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 an increased number of
mosquitoes that feed on both birds and humans. Under these conditions,
humans may be affected.
You're at risk of these types of mosquito-borne
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 about
half of people who develop severe signs and symptoms. However, less
than five cases are reported in most years. Most cases occur in late
summer, though they can happen year-round in southern states.
Symptoms of eastern equine encephalitis usually appear four to 10
days after a bite by an infected mosquito.
Like eastern equine encephalitis, this virus affects horses and
humans. Most reports of western equine encephalitis come from 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 likely 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 people who develop severe signs and symptoms 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 five and 10 days
after a bite.
This virus is transmitted to mosquitoes by birds. The mosquito
vector of St. Louis encephalitis breeds in areas of standing water,
including polluted pools, roadside ditches and containers such as
birdbaths, flowerpots and discarded tires. Although many young
people have mild or no symptoms when infected, the disease can be
severe in seniors over age 60. The mortality rate is between 2
percent and 20 percent.
This virus was named for La Crosse, Wis., where the virus was first
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, this virus is passed to mosquitoes from chipmunks and
squirrels. La Crosse encephalitis usually affects children and has a
mortality rate of less than 1 percent. An average of 70 cases is
reported annually. Symptoms appear five to 15 days after a bite by
an infected mosquito.
This virus first appeared in the United States in 1999. It's also
found in Africa and the Middle East and in parts of Europe, Russia,
India and Indonesia. The virus is similar to the St. Louis virus in
that birds are its main animal hosts. However, in rare cases, it's
possible for the disease to spread from person to person through
organ transplant, blood transfusion or breast-feeding, or from
mother to unborn child. Symptoms of West Nile encephalitis are
generally mild, but the disease can be severe, especially in older
adults and those with weakened immune systems. Symptoms appear
within five to 15 days of being bitten by an infected mosquito.
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