West Nile Virus Prevention

by Food and Drug Administration (FDA)

Her entire lifetime may span only a few weeks, but the female Culex pipiens mosquito makes the most of it. Feeding on the blood of humans and animals gives this common carrier of West Nile virus the protein she needs to produce several hundred eggs every few days — eggs that evolve into biting adult mosquitoes seeking blood meals of their own. And with each bite, an infected mosquito may transmit West Nile virus (WNV).

Most people who become infected with WNV have no symptoms, according to the Centers for Disease Control and Prevention (CDC). About 20 percent develop West Nile fever with its mild, flu-like symptoms: fever, headache, body aches, and sometimes a rash and swollen lymph glands. In a small number of people with symptoms — about 1 in 150 — the virus causes life-threatening inflammation of the brain (encephalitis) or inflammation of the membrane surrounding the brain and spinal cord (meningitis).

There is no evidence to suggest that WNV can be spread through casual contact such as touching or kissing a person with the virus or by handling an animal with the virus, says the CDC. But public health officials are concerned that WNV may spread from person to person by other means.

"Based on investigations of cases previously reported, we believe that West Nile virus can be transmitted by both organ transplantation and blood transfusion," says Jesse Goodman, M.D., M.P.H., director of the Food and Drug Administration's Center for Biologics Evaluation and Research. It is not yet clear how commonly this occurs. In addition, there is limited evidence suggesting the virus may also be transmitted from mother to infant through breast milk.

No licensed tests exist yet for the large-scale screening of donor blood for WNV, nor are there any FDA-approved drugs to treat the virus or vaccines to prevent it. But the health and scientific communities, including federal agencies, public health departments, research facilities, and blood banks, are actively pursuing these medical advances.

An Insect-Spread Virus

WNV is one of a group of disease-causing viruses called flaviviruses, which are spread by insects, usually mosquitoes. Other flaviviruses include yellow fever, dengue, and St. Louis encephalitis viruses.

WNV primarily circulates between infected birds and mosquitoes that bite them. Only female mosquitoes bite and feed on blood; males feed on nectar. The infected mosquitoes can transmit the virus when they bite other animals or people. (See graphic at right.)

More than 130 species of birds have been reported to be infected with WNV, according to the CDC. The virus also can infect horses and some other animals.

Even in areas where WNV is circulating, not all mosquitoes become infected with it, and human infection does not occur in all individuals exposed to mosquitoes. A study done in 1999 among residents at the height of WNV activity in New York City showed that only 2.6 percent had been infected, says the CDC. People who do get West Nile fever typically develop symptoms within three to 14 days after being bitten by an infected mosquito. Symptoms in people whose disease is limited to West Nile fever often go away without treatment in three to six days and do not seem to cause any long-term health effects.

The risk of getting the more severe West Nile encephalitis from an infected mosquito is higher for people 50 years of age and older. Many recent cases in transplant and transfusion recipients have been severe and, although unproven, it is likely that people with compromised immune systems are at increased risk for the more severe symptoms of WNV infection.

Blood Safety

In 2002, the CDC, in collaboration with the FDA, state health departments, and blood organizations, concluded that 23 cases of WNV infection occurred from the transfusion of infected blood. In addition, WNV was identified in transplanted organs in at least four people who received organs from a single donor. That donor's blood was later confirmed to contain the virus at the time of organ transplantation.

The FDA and other organizations have taken precautions to reduce the risk of WNV transmission through blood transfusion and organ transplantation. One of these precautions has been to allow donor blood to be screened using rapidly developed experimental tests.

"This is a new challenge to blood safety," says Goodman. "There is a risk, and we have been taking it very seriously." The FDA has been working closely with diagnostics manufacturers, the blood industry and CDC to address the risk, says Goodman. "What is remarkable is that in only eight months since we recognized the new risk, tremendous progress has been made in protecting the blood supply from the threat of WNV, including the development and implementation of new screening tests to help keep blood safe."

In August 2002, the FDA alerted blood banks to be vigilant in excluding individuals from donating blood who may have early symptoms of WNV infection.

To further reduce the risk of transmitting WNV infection through transfusions, the FDA issued guidance to the blood industry in October 2002. The agency updated the guidance in May 2003 based on its experience with the 2002 outbreak. The guidance provides detailed information to help blood centers determine which potential donors should be excluded from giving blood, when to report donor illnesses that suggest WNV infection, and when to withdraw or quarantine blood products from these donors. Blood banks are not to take donations from people who have had a fever and headache in the week before they donate blood. In addition, donors are encouraged to report any illness suggestive of WNV infection that they contract within two weeks after donating blood.

The CDC has asked physicians to notify public health authorities of any patients who develop WNV infection within four weeks of receiving a blood transfusion or organ transplantation.

There is no risk that donors will acquire WNV by giving blood, and the FDA continues to encourage blood donations, as well as blood transfusions and organ transplants for those who need them.

Unlike other viruses, such as HIV and hepatitis B and C (which can be lifelong infections), WNV does not stay in the blood for a long period of time. "The available evidence supports the idea that most infected persons develop antibodies against the virus within one to two weeks," says Goodman. "These antibodies help clear the virus from the blood and likely protect the person against re-infection, but it's possible that immunity decreases in later years."

Although the virus disappears, the antibodies may remain in a person's blood for years, resulting in a positive blood antibody test. But there is no indication that such a person is chronically infected, says Goodman. "Current evidence supports the concept that such individuals who have recovered from West Nile virus can still be considered safe blood donors."

About the Author

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