West Nile virus is a disease spread by mosquitos. The condition ranges from mild to severe.

West Nile virus was first identified in 1937 in Uganda in eastern Africa. It was first identified in the United States in the summer of 1999 in New York. Since then, the virus has spread throughout the United States.

The West Nile virus is a type of organism called a flavivirus. Researchers believe West Nile virus is spread when a mosquito bites an infected bird and then bites a person.

Mosquitos carry the highest amounts of virus in the early fall, which is why the rate of the disease increases in late August to early September. The risk of disease decreases as the weather becomes colder and mosquitos die off.

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.

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 what part of the body is affected.

Risk factors for developing a more severe form of West Nile virus include:

• Conditions that weaken the immune system, such as HIV, organ transplants, and recent chemotherapy
• Pregnancy
• Older age

West Nile virus may also be spread through blood transfusions and organ transplantation. It is possible for an infected mother to spread the virus to her child through breast milk.

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.

Tests to diagnose West Nile virus include:

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

However, the most accurate way to diagnose this infection is 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 to look 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.