West Nile virus

Mild disease, generally called West Nile fever, has some or all of the following symptoms:

• Fever
• Headache
• Back pain
• Muscle aches
• Lack of appetite
• Sore throat
• Nausea
• Vomiting
• Abdominal pain
• Diarrhea

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 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 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.