West Nile Virus

West Nile Virus (WNV) is transmitted by mosquitos and causes an illness that ranges from mild to severe.  The mild, flu-like version is often called West Nile fever.  More severe forms of disease may be called West Nile encephalitis or West Nile meningitis, depending on where it spreads.

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 September 2002 had been identified in 42 states.

Incidence; Contributing Risk Factors

Data from the outbreak in Queens, New York suggested 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 or encephalitis).

Mosquitos carry the highest amounts of virus in late August to early September, causing a spike in the disease at this time.  The risk of disease decreases as the weather becomes colder and mosquitos die off.

Signs and Symptoms

Although many people are bitten by mosquitos 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.

With more severe disease, the following symptoms can also be seen and require prompt attention: Muscle weakness, stiff neck, confusion, loss of consciousness.

Diagnosis 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.

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 lymphocytes)
  • 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.

Treatment and Prevention

Because this illness is not caused by bacteria, antibiotics do not help.  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.

Prognosis; Complications

In general, the likely outcome of a mild West Nile virus infection is excellent.  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 but include permanent brain damage or muscle weakness (sometimes similar to polio), and death.