West Nile Virus

  • Basics

    The West Nile virus (WNV) is a mosquito-borne virus that was initially discovered in the district of Uganda in 1937. The first detected case of West Nile virus in the western hemisphere occurred in New York, 1999.

    With the first U.S. outbreak of WNV in 1999, there were 62 cases of serious illness. Of these, seven people died from West Nile encephalitis. The number of states affected by WNV had been increasing for several years, but now seems to have levelled off. By the end of 2004, human illness from WNV was documented in every state. Human illness cases in the U.S. exceeded 2740 in 2005, and resulted in at least 88 deaths.

    Most humans infected with WNV have no symptoms of illness. Some develop mild, flu-like symptoms that are referred to as West Nile fever.

    With West Nile fever, the symptoms are often so mild that those affected don't feel it is necessary to seek medical attention. The symptoms can easily be mistaken by individuals as having the flu.

    There is a small risk of developing West Nile encephalitis when infected with WNV. This is a serious illness that requires hospitalization.

    Encephalitis occurs when an infection causes an inflammation of the brain. Cases of West Nile encephalitis in the U.S. are rare. However, hospitalizations and deaths due to West Nile encephalitis have been increasing. Statistically, there is only one case of West Nile encephalitis for every 150 cases of West Nile fever.

  • Causes

    The West Nile virus (WNV) is transmitted through mosquito bites.

    There are over 30 types of mosquitoes that carry WNV. The virus is most often transmitted through the bites of northern and southern House Mosquitoes. These mosquitoes are most active at dawn and dusk.

    Only a small percentage of the mosquito population carries WNV during an outbreak. This is why the size of the mosquito population has such a direct influence on the severity of a WNV outbreak in a particular area. If temperature and rainfall are favorable for mosquito breeding, there will be a large mosquito population. This results in more human and equine (horse) cases of West Nile illness.

    Mosquitoes pick up the virus from infected birds.

    Birds act as hosts to WNV. Infected birds carry a large amount of the virus in their bloodstream. This allows the virus to be passed on to mosquitoes that feed from infected birds. The mosquitoes then transmit the virus to other birds, animals, or humans. Although most WNV infected birds survive, an increase in dead bird sightings can indicate to health officials that WNV is in their state. Ravens, crows, and blue jays are the most likely birds to become ill or die when infected with WNV.

    Testing live chickens, or samples of dead birds (such as crows and blue jays) can provide scientists with early warning of WNV being in a particular region, before humans are infected.

    Humans cannot acquire the virus from insects other than mosquitoes, or from other infected animals.

    There is no evidence to suggest that West Nile virus can be transmitted to humans by ticks, or insects other than mosquitoes. The virus also cannot be transmitted to humans directly from infected birds, horses, or contact with other humans. Researchers have determined that pregnant women who contract West Nile virus can transmit the virus to their fetuses. Researchers are still trying to determine if infected mothers can transmit the virus to their babies via breast milk.

    Humans and domestic animals are referred to as “dead-end” hosts. They may be infected with WNV, but do not transmit the virus back to mosquitoes. There is also no evidence that WNV can be transmitted to people or horses from contact with infected people or horses. This is because the virus in the blood of humans and horses does not reach a high enough level to be infective to other mammals, or to mosquitoes.

    A special circumstance where humans may be vulnerable to WNV from other humans is when they receive a blood transfusion or an organ transplant. This risk, although small, peaks during times of WNV outbreaks in the affected regions of the country. If you need a blood transfusion or organ transplant, your physician will discuss with you the risks and the benefits of the treatment. Transmission in dialysis centers has also been reported.

    You cannot contract WNV or any other infectious disease through donating blood.

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