West Nile Virus

West Nile Virus, a virus passed from mosquitoes to humans

Bites from infected mosquitoes cause most West Nile virus infections in people. In rare cases, the virus also has been spread to people through blood transfusions, organ transplants and from mother to baby during pregnancy or breastfeeding. Pregnant or nursing women should see a doctor if they develop symptoms indicative of West Nile virus.

The first recorded outbreak of West Nile virus in North America occurred in New York City in 1999. By the summer of 2004, the virus had spread to California.

Mosquitoes become carriers of West Nile virus by feeding on the blood of infected birds.

Approximately 80% of people infected with West Nile virus have no symptoms, so most never realize they have the disease. The unfortunate minority who develop symptoms typically become ill 3 to 14 days after being bitten by an infected mosquito, but they also are unlikely to know they have the virus.

West Nile virus affects the central nervous system. Mild infections cause flu-like symptoms, including fever, headache, body aches, nausea, vomiting and sometimes swollen lymph glands or a skin rash. Symptoms can last from a few days to several weeks. People over the age of 50 are more likely to develop symptoms.

Acute symptoms—including severe headaches or confusion, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis— requiring hospitalization occur in less than 1% of those infected. These symptoms may last several weeks, and neurological effects may be permanent. Intravenous fluids and respiratory support are common hospital treatments, but no cure for West Nile virus has been discovered. In 2003, 262 deaths were attributed to West Nile virus in the United States.

Centers for Disease Control and Prevention (CDC) website at http://www.cdc.gov/ncidod/dvbid/westnile features up-to-date maps of West Nile virus activity for the United States, including state maps with county-level data.

Another online source of in-depth information on West Nile virus is the Cornell University Environmental Risk Analysis Program at http://environmentalrisk.cornell.edu/WNV/.

Up-to-date news articles about West Nile virus can be viewed at http://news.yahoo.com/fc?tmpl=fc&cid=34&in=health&cat=west_nile_virus.

Detailed Information:
What is West Nile virus and how is it transmitted?
The West Nile virus can infect mosquitoes, birds, people, horses and some other mammals. It is most commonly found in Africa, West Asia and the Middle East, but has emerged in the United States and Europe in recent years. It causes a spectrum of clinical diseases ranging from flu-like West Nile fever to potentially fatal West Nile meningitis (inflammation of the covering of the brain and spinal cord) and West Nile encephalitis (inflammation of the brain). Of the 4,269 cases reported in 2006, 1459 (34%) were reported as West Nile meningitis or encephalitis, while 2,616 (61%) were reported as West Nile fever and 194 (5%) were unspecified. These figures most likely underestimate the prevalence of West Nile infection as the less serious fever is probably not apt to be reported to health officials as frequently as cases of encephalitis and meningitis.

In the United States, the West Nile virus is transmitted by mosquitoes—primarily members of the Culex species, although other species have been implicated as well. The virus is maintained in a complex life cycle that involves birds as the reservoir of the virus. People are not the usual host and when infected, become “dead-end hosts” as the virus does not circulate in human blood. While 317 species of birds have been found to carry the West Nile virus, the majority do not usually show signs of illness and few die. However, West Nile virus infection can be fatal in crows and jays. It is also fatal for horses in approximately 40% of cases. Dogs and cats can also become infected but there is no evidence of transmission from these animals to people.

Culex tarsalis mosquito is the primary vector of West Nile virus.

The virus was first isolated from a woman in the West Nile region of Uganda in 1937. It was found to be the cause of equine encephalitis (swelling of the brain and spinal cord in horses) in France and Egypt in the early 1960s. West Nile virus first appeared in the United States in 1999 with reports of encephalitis in horses and people. Since then, it has been reported in all states except Alaska and Hawaii.

How many cases of West Nile virus infection occur in the United States annually?
Since first appearing on the East Coast in 1999, West Nile virus has spread westwards and now is endemic in all states except Alaska and Hawaii.

West Nile Virus Distribution Map, Year 2000


West Nile Virus Distribution Map, Year 2001


West Nile Virus Distribution Map, United States


West Nile Virus Distribution Map, Year 2002


West Nile Virus Distribution Map, Year 2003


West Nile Virus Distribution Map, Year 2004


West Nile Virus Distribution Map, Year 2005
West Nile Virus Distribution Map, Year 2006

How is West Nile virus infection treated?
As with other viral diseases for which there is no vaccine, West Nile virus infection has no specific treatment. Milder cases are usually self-limiting. Patients with encephalitis or meningitis are hospitalized, given intravenous fluids and respiratory support if necessary. These severe forms of West Nile virus infection are often life-threatening and require intensive medical treatment.

Is there a vaccine for West Nile virus?
There is a vaccine available to prevent West Nile virus infection in horses but not people. There are several initiatives in progress at present to develop a vaccine.

How can I prevent infection with West Nile virus?
Personal protection measures that prevent mosquito bites are key to reducing your risk. Eliminate potential mosquito breeding sites near your home by emptying containers that hold standing water. Your local government may have a mosquito control plan in place to eliminate mosquito breeding sites in public facilities such as stormwater drains and reservoirs.
Additionally, you should take measures to prevent mosquito bites. Insect Shield® Repellent Apparel is proven and registered to repel mosquitoes—including those that can carry West Nile virus.