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

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

These symptoms usually last for 3 - 6 days.

With more severe disease, the following symptoms can also occur, and need prompt attention:

• Confusion or change in ability to think clearly
• Loss of consciousness
• Muscle weakness
• Stiff neck

Signs of West Nile virus infection are similar to those of other viral infections. There may be no specific findings on a physical examination to diagnose West Nile virus infection.

About 20 - 50% of patients may have a rash. Muscle weakness with other related symptoms are signs of a West Nile virus infection.

Tests to diagnose West Nile virus include:

• Complete blood count (CBC)
• Head CT scan
• Head MRI scan
• Lumbar puncture and cerebrospinal fluid (CSF) testing

The most accurate way to diagnose this infection is with a serology test, which checks a blood or CSF sample for antibodies against the virus.

Rarely, a sample of blood or CSF may be sent to a lab to be cultured for 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 and are not commonly used.