West Nile Virus : Diagnostic and Blood-Screening Tests, Transmission

by Food and Drug Administration (FDA)

(Page 2 of 3)

Diagnostic and Blood-Screening Tests

In July 2003, the FDA cleared the first commercially available test that will help physicians diagnose cases of potentially deadly WNV earlier than current methods allow. The test, called the IgM Capture ELISA, detects the levels in the blood of a particular type of antibody (IgM) to WNV. The test is for use in people who have symptoms of viral encephalitis or meningitis (headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis). Results from the IgM Capture ELISA must be confirmed with other lab tests as part of a complete evaluation. Diagnostic testing, based on CDC methods, is also available through state health departments.

Although such tests can detect antibodies to WNV in the blood, the IgM Capture ELISA is not adequate for screening donated blood to prevent the spread of WNV through transfusion. WNV is usually only present in the blood before symptoms and antibodies develop and can be detected, according to Goodman. Blood-screening tests are required to identify the virus itself, and the most sensitive methods work by detecting its genetic material.

The FDA has been working with the blood and medical diagnostics industry to speed development of sensitive blood-screening tests for WNV. Several screening tests have been developed that can detect very small amounts of genetic material. The FDA is evaluating the sensitivity and clinical use of these nucleic acid tests (NAT) and has encouraged blood banks to use them to help protect the safety of the blood supply. As a result, while the tests are being studied, the nation's blood supply is also currently being screened for WNV. Over 800 potentially infective WNV-positive units of blood have already been removed before they could be transfused, undoubtedly greatly reducing the risk of transmission of WNV by transfusion and protecting the safety of the blood supply.

Pathogen inactivation is another promising area of technology under development that is being explored to help make donated blood safe from WNV and other infectious agents. This type of technology uses chemical treatments, often combined with other methods such as ultraviolet energy, to kill the virus in the blood. The FDA is evaluating several inactivation methods for their effectiveness and safety, says Goodman, who stresses the importance of studying the effects of these treatments, both on possible infectious agents and on the quality and safety of the blood products themselves. "If one treats large numbers of units of blood to eliminate an infection from a rare unit, one has to be very sure that the effects of the treatment on those other units are not potentially harmful or don't potentially affect the quality of blood for the majority of patients who are receiving it."

Transmission Through Breast Milk

Based on a case in Michigan in September 2002, it appears that WNV may be transmitted through breast milk. A new mother, unaware that she had been infected with the virus, possibly from a blood transfusion received shortly after giving birth, breast-fed her baby for 17 days. After the mother reported symptoms of nausea, malaise, and fever, she and her baby were tested for WNV. Laboratory tests confirmed that they both had WNV antibodies, indicating they both had been infected. The virus also appeared in the mother's breast milk.

Although the infant's exposure to the virus from a mosquito bite cannot be ruled out, it is unlikely, since the baby was outside only when transported from the hospital to the home, says Matthew Boulton, M.D., M.P.H., chief epidemiologist at Michigan's Department of Community Health. "We think the baby was exposed through the oral route from the breast milk," says Boulton, who adds that the infant showed no symptoms of illness from the virus and remains healthy.

These findings do not suggest a change in breast-feeding recommendations, according to the CDC. "The benefits of breast-feeding by far outweigh any potential risks of breast-feeding, including the risk of WNV transmission," says Lyle Petersen, M.D., deputy director of the CDC's Division of Vector-Borne Infectious Diseases. There is no evidence that WNV can be transmitted from mother to child during pregnancy or birth, he adds.

Prevention and Treatment

The only way to control the spread of WNV in people today is by controlling the mosquitoes that carry the virus and taking precautions to avoid getting bitten. But medical researchers are also working on drugs to treat WNV and a vaccine to prevent it.

More than 550 drugs have been screened to treat the disease, and about 3 percent have shown potential for additional testing in animals, according to the National Institute of Allergy and Infectious Diseases (NIAID).

NIAID is supporting the development of several vaccine approaches, including a live vaccine made by mixing WNV with the already existing yellow fever vaccine. Scientists are exploring additional approaches to developing effective WNV vaccines and the FDA is committed to helping to get these products developed and evaluated as quickly as possible.

About the Author

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FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.