West Nile Virus Infection Symptoms and Risk Factors

West Nile virus was first observed in the U.S. during the summer of 1999 and is believed to be permanently established (endemic) in the U.S. at this time. A member of the flavivirus family, West Nile virus is related to the St. Louis encephalitis virus that is also found in the U.S. West Nile virus is commonly found in Africa, the Middle East, and in the western parts of Asia. It infects mosquitoes, birds, horses, humans, and some other mammals. In 2012, West Nile virus infections reached epidemic levels in Texas, and were reported in many other states.

Humans normally acquire the viral infection through a mosquito bite. The early fall, from late August to early September, is the most common time for infection to occur in the U.S. West Nile virus has the potential to cause a very serious illness, although 60%-80% of people infected will not develop any symptoms at all. The others most commonly develop a mild illness, sometimes termed West Nile fever, which is characterized by:

• fever,
• tiredness,
• headaches,
• body aches,
• swollen lymph nodes, and
• sometimes a rash.

West Nile fever develops two to 15 days following the bite of an infected mosquito and persists for a few days to a few weeks.

In less than 1% of cases, West Nile virus infection leads to severe illness that is referred to as "neuroinvasive" disease since it affects the nervous system. This severe form of West Nile virus infection results in an inflammation of the brain (encephalitis) or the meninges, tissues that cover the brain and central nervous system (meningitis). A combination of the two (meningoencephalitis) can also occur, and the disease can be fatal. People over 50 years of age, pregnant women, infants, and those with weakened immune systems due to medications, HIV, or cancer are at greatest risk for severe illness related to West Nile virus infection. Neuroinvasive West Nile virus infection is characterized by:

• high fever,
• neck stiffness,
• stupor,
• disorientation,
• coma,
• seizures,
• muscle weakness, and
• potentially permanent neurological disturbances.

West Nile virus infection cannot be spread by casual contact such as touching or kissing an infected person. In addition to transmission via mosquito bites, less common modes of transmission of the virus include organ transplantation, blood donation, and from mothers to their fetuses in the womb or to infants via breast milk.

The best way to avoid West Nile virus infection is to prevent mosquito bites. There is no human vaccine available, although a vaccine against West Nile virus has been licensed for use in horses. No specific treatment is available for the illness. West Nile fever generally resolves on its own, and those with severe infections must be hospitalized to receive supportive care.