West Nile Fever Symptoms


West Nile fever, also called West Nile virus, or WNV, is an emerging infectious disease which first appeared in the United States in 1999. Infected mosquitoes spread the virus, which belongs to a group of disease-causing viruses known as flaviviruses(1). The majority of people, about 4 out of 5, who contract WNV have no symptoms or mild symptoms.

About 20 percent of people infected develop a mild condition called West Nile fever.

Common symptoms of West Nile fever

  • Skin rash

  • Swollen lymph glands

  • Headache

  • Fever

  • Diarrhea

  • Nausea

  • Backache

  • Muscle ache

  • Lack of appetite

  • Vomiting

West Nile fever in humans usually is a fevered, influenzalike illness, typified by an abrupt onset (incubation period 3 to 6 days) of moderate to high fever (3 to 5 days, infrequently biphasic, sometimes with chills), headache (often frontal), sore throat, backache, myalgia, arthralgia, fatigue, conjunctivitis, retrobulbar pain, maculopapular or roseolar rash (in approximately half the cases, spreading from the trunk to the extremities and head), lymphadenopathy, anorexia, nausea, abdominal pain, diarrhea, and respiratory symptoms(2).

More Serious Symptoms

However, if West Nile virus enters the brain, it may be deadly. It can cause inflammation of the brain, called encephalitis, or inflammation of the tissue that surrounds the brain and spinal cord, called meningitis. About 1% of those infected with WNV will develop severe illness. The severe symptoms can includeâ€Â

  • acute aseptic meningitis or encephalitis (associated with neck stiffness, vomiting, confusion, disturbed consciousness, somnolence, tremor of extremities, abnormal reflexes, convulsions, pareses, and coma)

  • anterior myelitis

  • hepatosplenomegaly

  • hepatitis

  • pancreatitis

  • myocarditis

The virus can last in the blood for up to 10 days in healthy patients, as late as 22 to 28 days after infection in immune compromised patients. Recovery is usually complete, but less rapid in adults than in children, and can often be accompanied by long-term myalgias and weakness. Most fatal cases have been recorded in patients older than 50 years.


West Nile virus was first isolated from the blood of a woman with fever in the West Nile district of Uganda in 1937 (3).and was subsequently isolated from patients, birds, and mosquitoes in Egypt in the early 1950s (4-5).

The virus became recognized as a cause of severe human meningitis or encephalitis (inflammation of the spinal cord and brain) in elderly patients during an outbreak in Israel in 1957. Equine disease was first noted in Egypt and France in the early 1960s. WNV first appeared in North America in 1999, with encephalitis reported in humans and horses.

Outbreaks of WNV encephalitis in humans have occurred in Algeria in 1994, Romania in 1996-1997(7)., the Czech Republic in 1997(6)., the Democratic Republic of the Congo in 1998, Russia in 1999, the United States in 1999-2003, and Israel in 2000. Disease outbreaks in horses occurred in Morocco in 1996, Italy in 1998, the United States in 1999-2001, and France in 2000, and in birds in Israel in 1997-2001 and in the United States in 1999-2002.

West Nile virus was first discovered in the United States in 1999 during an outbreak in New York. During that outbreak, 62 people suffered serious illness and seven of those people died. By the end of 2002, the virus was confirmed in 44 states, including the District of Columbia. There were over 4100 confirmed human cases and 277 deaths attributed to the disease.

Risk Factors

Older people are most at risk. There are no specific vaccines or treatments for human WNV disease. The easiest and best way to avoid WNV is to prevent mosquito bites:

Use insect repellent

Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets or barrels

Stay indoors between dusk and dawn, when mosquitoes are most active

Use screens on windows to keep mosquitoes out


  1. Calisher CH, Karabatsos N, Dalrymple JM, Shope RE, Porterfield JS, Westaway EG, et al. Antigenic relationships between flaviviruses as determined by cross-neutralization tests with polyclonal antisera. J Gen Virol 1989;70:37-43.

  2. Peiris JSM, Amerasinghe FP. West Nile fever. In: Beran GW, Steele JH, editors. Handbook of zoonoses. Section B: Viral. 2nd ed. Boca Raton (FL): CRC Press; 1994. p. 139-48.

  3. Smithburn KC, Hughes TP, Burke AW, Paul JH. A neurotropic virus isolated from the blood of a native of Uganda. Am J Trop Med Hyg 1940;20:471-92.

  4. Melnick JL, Paul JR, Riordan JT, Barnett VHH, Goldblum N, Zabin E. Isolation from human sera in Egypt of a virus apparently identical to West Nile virus. Proc Soc Exp Biol Med 1951;77:661-5.

  5. Taylor RM, Work TH, Hurlbut HS, Rizk F. A study of the ecology of West Nile virus in Egypt. Am J Trop Med Hyg 1956;5:579-620.

  6. Hubálek Z, Halouzka J, Juricová Z. West Nile fever in Czechland. Emerg Infect Dis 1999;5:594-5.

  7. Antipa C, Girjabu E, Iftimovici R, Draganescu N. Serological investigations concerning the presence of antibodies to arboviruses in wild birds. Revue Roumaine de Medicine – Virologie 1984;35:5-9.

This article is for information only and is not meant to be used for self-diagnosis or as a substitute for consultation with a health care provider. If you have any questions about the disease described above or think that you may have this infection, consult a health care provider._

Last Updated on February 7, 2023