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[November 11, 2002]

West Nile Virus

The West Nile Virus (WNV), was first identified in 1937. The disease is widespread in Africa, the Middle East, Europe, the former USSR, Asia, and Australia. Its first appearance in North America was in New York in 1999, and 62 people were infected in that initial outbreak, seven of whom died. The level of disease in the U.S. remained fairly steady in 2000 and 2001, but in 2002 many more cases have been reported � 3174 cases with 173 fatalities as of October, spread across 32 states and Washington, D.C.

The disease is spread by mosquitoes, which become infected when they ingest blood from birds (primarily crows and blue jays) that carry the virus. Humans and other animals, particularly horses, become infected when bitten by infected mosquitoes. WNV has now been found in more than 100 species of birds, and it is believed that migratory birds and mosquitoes will continue to spread the disease, possibly throughout the entire Western Hemisphere. The virus has been found in birds as far west as Montana and New Mexico.

WNV is not transmitted from person to person or from animals to humans. However, four U.S. cases became infected through organ transplants from a single infected donor, and WNV can also be transmitted by blood transfusion. Although the risk of such transmission is low, the Food and Drug Administration (FDA) has issued new guidelines for screening blood donors to reduce the risk that infected blood enters the blood supply. The FDA guidelines are available at

Although it appears that WNV can be transmitted in breast milk, it is not known what the risk of such transmission is, or how likely the infants of nursing mothers infected with WNV are to become infected themselves. Because the health benefits of breast-feeding are well established, breast-feeding recommendations are currently unchanged. Women who are breast-feeding who develop any illness that includes a fever should consult their physician.

The vast majority of people bitten by a WNV-infected mosquito do not become ill. Of the two-tenths of one percent (0.2%) of people bitten who do get sick, most have only mild symptoms that may include fever, headache, and body aches, and also may include a skin rash on the trunk and swollen lymph glands. A very small subset of those who become infected (about 1 in 150 persons infected) will develop a more severe form of disease. The symptoms of severe infection (West Nile encephalitis or meningitis) include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. The risk of serious disease is higher for people 50 years of age and older.

Unfortunately, there currently is no test that can rapidly identify the presence of WNV. Patients are diagnosed on the basis of their immune (antibody) response to the virus, which takes about 14 days from the time the infection occurs.

Mosquito control and protection against mosquito bites are currently the most effective means of preventing human WNV disease. There is no known effective antiviral therapy, and no licensed WNV vaccine for use in humans, though several experimental vaccines are under development. Research is also ongoing on anti-viral drugs against WNV disease, including alpha-interferon, a drug used in the treatment of hepatitis C.

Vaccines Against WNV
Because the Japanese Encephalitis virus (JEV) is similar to WNV, the inactivated JEV vaccine is being evaluated as one possible way to protect humans against WNV disease. Another approach being examined is using the current yellow fever vaccine as the template for a WNV vaccine. To make this vaccine, yellow fever vaccine genes are replaced with corresponding WNV genes. A weakened strain of the yellow fever virus is used as the backbone of the vaccine, and the gene that encodes the yellow fever virus's protein coat is replaced with the corresponding gene from the WNV. The yellow fever vaccine used as the basis for this process has been safely and effectively given to millions of people, and initial WNV trials in mice have shown promising results.

Somewhat similar efforts to produce a WNV vaccine based on the dengue type 4 vaccine are in process at The National Institutes of Health (NIH), and this vaccine appears to have some success in decreasing the severity of WNV disease in laboratory mice. The National Institute of Allergy and Infectious Diseases at the NIH is funding research on West Nile Virus vaccines and believes that one may be ready for human trials sometime in 2003.

1. Centers for Disease Control and Prevention (CDC), Division of Vector-Borne Infectious Diseases. (2002). Questions and answers: Blood transfusions and organ donations [webpage]. Available online:

2. CDC, Division of Vector-Borne Infectious Diseases. (2002). Questions and answers: Symptoms of West Nile Virus [webpage]. Available online:

3. CDC, Division of Vector-Borne Infectious Diseases. (2002). Questions and answers: West Nile Virus and breast-feeding [webpage]. Available online:

4. CDC, Division of Vector-Borne Infectious Diseases. (2002). Questions and answers: West Nile Virus and horses [webpage]. Available online:

5. CDC, Division of Vector-Borne Infectious Diseases. (2002). West Nile Virus�basics [webpage]. Available online:

6. CDC, Office of Communication. (2002). Fact sheet: West Nile Virus infection in organ transplantation and blood transfusion recipients [webpage]. Available online:

7. CDC, Public Health Practice Program Office. (2002). CDC responds: Update on West Nile Virus [webcast transcript]. Available online:

8. Cornell University, Center for the Environment, Environmental Risk Analysis Program. (2002). What�s going on with the West Nile Virus [webpage]. Available online:

9. Kramer LD and Bernard KA. (2001). West Nile Virus in the western hemisphere. Current Opinion in Infectious Diseases, 14(5), 519-525.

10. Monath TP, Arroyo J, Miller C, and Guirakhoo F. (2001). West Nile Virus vaccine. Current Drug Targets - Infectious Disorders, 1(1). Available online:

11. Pletnev AG, Putnak R, Speicher J, Wagar EJ, and Vaughn DW. (2002). West Nile virus/dengue type 4 virus chimeras that are reduced in neurovirulence and peripheral virulence without loss of immunogenicity or protective efficacy. Proceedings of the National Academy of Sciences of the United States of America, 99(5), 3036-3041. Available online:

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