West Nile Virus
West Nile virus is transmitted by mosquito's and causes an illness
that ranges from mild to severe. Mild, flu-like illness is often called West
Nile fever. More severe forms of disease, which can be life-threatening, may
be called West Nile encephalitis or West Nile meningitis, depending on where
Causes, incidence, and risk factors
West Nile virus is a type of organism called a flavivirus and is
similar to many other mosquito-borne viruses, including Japanese encephalitis
(which is found in Asia). Researchers believe the virus is spread when a mosquito
bites an infected bird and then bites a person.
West Nile virus was first identified in 1937 in the West Nile region
of Uganda, in eastern Africa. It was first identified in the US in the summer
of 1999 in the Queens borough of New York, NY. It caused 62 cases of encephalitis
and 7 deaths that summer. Since 1999 the virus has spread throughout the continental
US and as of July 2004 has been identified in 46 states.
Mosquito's carry the highest amounts of virus in the early fall,
thus there is a peak of disease in late August-early September. The risk of
disease then decreases as the weather becomes colder and mosquito's die off.
Although many people are bitten by mosquito's that carry West Nile
virus, most do not know they've been exposed. Few people develop severe disease
or even notice any symptoms at all.
Data from the outbreak in Queens suggests that although 2.6% of the
population was infected, only 1 in 5 infected people developed mild illness,
and only 1 in 150 infected people developed brain inflammation (meningitis
Risk factors for developing a worse form of the disease likely include
- Conditions that suppress the immune system
- recent chemotherapy
- recent organ transplantation
- Older age
West Nile virus may also be spread through blood transfusions and
organ transplantation. It is possible for an infected mother to transmit the
virus to her child via breast milk.
Mild disease, generally called West Nile,
has some or all of the following symptoms:
- Back pain
- Muscle aches
- Lack of appetite
- Sore throat
- Abdominal pain
These symptoms usually last for 3 to 6 days.
With more severe disease, the following symptoms can also be seen
and require prompt attention:
- Muscle weakness
- Stiff neck
- Confusion or change in clarity of thinking
- Loss of consciousness
Signs and tests
Signs of West Nile virus infection are
similar to those of other viral infections.
There is nothing that can be found on physical
examination to diagnose West Nile virus infection.
A rash is present in 20-50% of patients. True muscle weakness in
the presence of other related symptoms is suggestive of West Nile virus infection.
Diagnostic tests that may be used if West Nile virus is suspected
include the following:
- CBC -- may show a normal or elevated white blood cell (WBC) count
- Lumbar puncture and cerebrospinal
fluid (CSF) testing
- usually shows elevated WBC count (especially
- usually shows elevated protein level
- Head CT scan -- often normal
- Head MRI scan -- may show evidence
of inflammation (in about one-third of patients)
The most accurate way to diagnose this infection is serology, a test
to detect the presence of antibodies against West Nile virus in CSF or serum
(a blood component). This is considered the gold standard for diagnosis.
Rarely, a sample of blood or CSF may be sent to a lab to be cultured
to evaluate the presence of West Nile virus. The virus can also be identified
in body fluids using a technique called polymerase chain reaction (PCR). However,
these methods can provide false negative results.
Because this illness is not caused by bacteria, antibiotics do not
help treat West Nile virus infection. Standard hospital care may help decrease
the risk of complications in severe illness. There is no human vaccine available
at present, and it is likely there will not be one for several years.
Research trials are under way to determine whether ribavirin, an
antiviral drug used to treat hepatitis C, may be helpful.
In general, the likely outcome of a mild West Nile virus infection
For patients with severe cases of West Nile virus infection, the
outlook is more guarded. West Nile encephalitis or meningitis has the potential
to lead to brain damage and death. Approximately 10% of patients with brain
inflammation do not survive.
Complications from mild West Nile virus infection are extremely rare.
Complications from severe West Nile virus infection include permanent
brain damage or muscle weakness (sometimes similar to polio), and death.
Calling your health care provider
Call your health care provider if you have any of the symptoms of
West Nile virus infection, particularly if you may have had contact with mosquitoes.
If you are severely ill, seek attention in an emergency room.
If you have been bitten by an infected mosquito, there is no treatment
to avoid development of West Nile virus infection. People in good general health
generally do not develop a serious illness even if bitten by an infected mosquito.
The best way to prevent West Nile virus infection is to avoid mosquito
bites. Using mosquito-repellant products containing DEET and wearing long sleeves
and pants are the most effective
way to avoid bites. Community spraying for mosquitoes may also prevent mosquito
Mosquito's breed in stagnant water. If you notice pools of standing
water -- even in small areas such as trash bins and plant saucers -- drain
For More Information
West Nile Virus Tracking Maps are provide on our Epidemiology Page
State of California Site for West Nile
CDC West Nile Virus Page