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West Nile Virus (WNV)

West Nile virus (WNV) is a potentially serious illness that seems to flare up in North America in the summer and fall. It is transmitted to humans mainly through mosquito bites.

West Nile virus has also been isolated from ticks, mites, and ked flies, but health researchers are not sure of the importance of these arthropods in passing WNV to humans. WNV also has been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby. These other methods of transmission represent a very small number of cases.


West Nile virus was first isolated in 1937 in a resident of what was then called West Nile province in the country of Uganda, Africa.

The virus has since been reported in people in other parts of Africa, and in Europe, the Middle East, West and Central Asia, and, more recently, the United States and Canada.

West Nile virus was first found in the United States in New York City in September 1999, and the virus was first detected in South Carolina in 2002.

The virus has been detected in humans, mosquitoes, birds, horses, and other domestic and wild animals.

Symptoms in Humans

Symptoms develop 3 to 14 days after the person is infected. Most people who are infected with WNV do not get sick, but a few experience severe symptoms. People over the age of 50 years appear to be at greater risk of becoming sick.

  • Serious Symptoms in a Few People (West Nile Virus Neuroinvasive Disease) – Approximately 1 in 150 people infected with the virus will get really sick. They may develop inflammation of the brain (West Nile encephalitis) or inflammation of the area surrounding the brain and spinal cord (meningitis). Symptoms could include headache, high fever, neck stiffness, stupor, confusion, coma, tremors, convulsions, muscle weakness, and paralysis. Symptoms may last several weeks, and effects on the nervous system may be permanent.
  • Milder Symptoms in Some People (West Nile Virus Fever) – About 1 in 5 people infected with WNV will have milder symptoms lasting several days. Symptoms can include fever, headache, and body aches, and occasionally, a skin rash on the trunk of the body and swollen lymph glands.
  • No Symptoms in Most People – About 4 in 5 people infected with WNV will not get sick or have any symptoms at all.

WNV in Horses

Symptoms of WNV in horses includes stumbling, incoordination, circling, head pressing, depression or apprehension, weakness of legs, partial paralysis, the inability to stand, muscle twitching or death. Horses may become infected without showing any symptoms. More than 1 in 3 horses infected with the virus die from it (35 percent death rate).

Virus Is Passed From Birds to Mosquitoes to Humans

Virus levels in horses and humans are generally too low to infect mosquitoes, so horses and humans are considered dead-end hosts incapable of continuing the virus transmission cycle.

Birds, however, can infect mosquitoes. West Nile virus exists primarily as an infection of birds, and it is transmitted from bird to bird by several species of mosquitoes.

Mosquitoes become infected when they feed on infected birds that may have had the virus in their blood for only a few days.

The virus eventually finds its way into the mosquito's salivary glands.

Ten to 14 days after feeding on an infected bird, a mosquito becomes capable of transmitting the virus to another bird or animal or to a human through its bite.

Many types of mosquitoes may carry the virus, including the Southern House Mosquito (Culex quinquefasciatus). This type of mosquito breeds anywhere foul water stands for more than a week, such as in rain barrels, tubs, catch basins, cesspools, ditches, ground pools, dairy drains, sewage lagoons, and other similar habitats. It bites at night and may enter homes.

Some Types of Birds Die Quickly from WNV

Certain birds, including American crows, blue jays, magpies, ravens, house finches, and house sparrows, can more easily infect mosquitoes with West Nile Virus than other birds.

Crows and blue jays usually die within about one week of being infected, so they are useful as early indicators for WNV (Emerging Infectious Diseases 9(3): 311-322).

Songbirds, shorebirds, owls, and hawks can also develop high enough levels of virus to infect most feeding mosquitoes.

West Nile virus rarely causes death in house sparrows, cardinals, catbirds, mourning doves, and rock doves. Pigeons, woodpeckers, and ducks do not develop sufficient levels of virus to infect most feeding mosquitoes.

See South Carolina's data on West Nile virus.

For more information please contact your local Mosquito Control Program.