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Deadly Diseases

West Nile Virus

In 1937, a woman in Uganda running a high fever was the first person diagnosed with West Nile virus. Since it was first identified in the United States in 1999, it has killed more than 650 people, and caused illness in more than 16,000.

West Nile, one of the flavivirus group, is carried in the blood of infected birds, notably crows, ravens, magpies, and jays. Mosquitoes that bite infected birds transmit the virus to humans through subsequent skin bites.

The disease is common in African, Middle Eastern, and Eurasian populations, but most who have the virus in their bloodstream experience only mild, flu-like symptoms such as skin rash, headache, and tiredness. Symptoms appear two to 14 days after the bite from an infected mosquito; the effects may last from a few days to several months. An estimated 80 percent of people infected, however, experience few or no ill effects at all.

In its severest form, though, the virus passes the blood-brain barrier and causes high fever, stiff neck, muscle tremors, paralysis, and brain swelling. This version, which mimics encephalitis (or "sleeping sickness"), arises in only 1 percent of those infected and can cause death within days. In recent years, there have been several outbreaks of West Nile virus in its lethal encephalitic form: in Algeria in 1994; in Romania in 1996; in the Czech Republic in 1997; in the Democratic Republic of the Congo in 1998; in Russia in 1999; and in Israel in 2000. There is no cure.

The disease was unknown in North America until 1999, when the bodies of dead crows dropped from the New York City sky. While this avian mystery was being investigated, human patients began to arrive in local hospitals complaining of fever, nausea, vomiting, and difficulty breathing. At first doctors suspected the mosquito-borne St. Louis encephalitis, but that disease doesn't kill birds so it wouldn't explain the crows' deaths. Blood tests and autopsies discovered that one pathogen had infected both birds and humans: West Nile virus.

Since its arrival, the virus has established a permanent foothold in the United States. In 2004, 2,535 cases were reported, covering all but nine states. In North America, the disease is most prevalent in late summer and early fall. In milder southern countries, West Nile virus occurs year round.

Household pets contract the disease in the same way as humans. There is no evidence that cats or dogs can transmit the virus to humans, nor is it passed through typical person-to-person contact, such as kissing or touching.

A promising new vaccine has been developed by the Cambridge, Massachusetts-based biotechnology company Acambis, which says it produced enough antibodies to fight off West Nile disease in all but one of the 60 people vaccinated in preliminary trials. A vaccine will not be ready for market for at least three years.

Because there is currently neither an approved vaccine available to prevent the disease nor treatment for it, public health officials focus on controlling the vector, or carrier. Local health departments may choose to spray insecticides in mosquito-breeding areas. Homeowners can eliminate standing water in basins, flowerpots, and other receptacles — breeding grounds for mosquitoes — as well as repair screens. Outside activity should be minimized in early mornings and evenings, when mosquitoes are most active. Repellents containing DEET deter mosquitoes when sprayed on clothes or skin. The recommended concentration of DEET is 10 percent for children and 30 percent for adults. Protective clothing covering the arms and legs is also recommended for those spending time in areas where mosquitoes are prevalent.

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From the Series

West Nile in the USA

Mosquitoes and birds rapidly spread the virus across the country.

Part I: West Nile Arrives (8:41)

Part II: West Nile in the Rockies (3:04)

Part III: West Nile in the Rockies, Continued (5:14)

Part IV: West Nile in Southern California (3:26)

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