Why Test for West Nile Virus?
In 1999, Americans found that they had a new reason to dislike mosquitoes. In that year, West Nile virus (WNV), which had previously only been found in the eastern hemisphere, made an appearance in Queens, New York. Once it established a foothold, WNV spread rapidly across North America via birds and mammals, including humans. In the United States, the number of human cases rocketed from 66 in 2001 to 4156 in 2002, then more than doubled again the following year1. Canada had a high of nearly 1500 cases in 2003, including 947 cases in a province that had no cases the year before2. Because of its rapid spread, some countries, such as France and Japan, are carefully monitoring WNV activity so that preparedness plans can be implemented quickly if necessary.
WNV is a mosquito-borne virus that is part of the same family as yellow fever, dengue fever, and Japanese encephalitis viruses. Its impact on a human host can range from no symptoms at all to severe neurological symptoms that can lead to coma or death. Currently, there is neither a cure nor a vaccine, so the only means of fighting the spread of the virus has been through mosquito control.
Although the number of cases of WNV reported by the Centers for Disease Control (CDC) dropped after a high point in 2003, WNV continues to be a concern in the United States. In 2006, 149 deaths were attributed to WNV, which was an increase over the previous two years.
After a number of documented transfusion transmissions in 20021, WNV proved itself to be a big concern for blood banks. Since it is mosquito-borne, the risk of transfusion transmission cannot be minimized using donor questionnaires related to high-risk behaviors. In addition, an infected donor may show no symptoms. Consequently, all residents of or travelers to endemic regions - which includes most of the United States - are a risk as donors.
In 2003, blood banks in the United States began screening for WNV using investigational assays, including Chiron's PROCLEIX® WNV Assay. These assays were developed at the urging of the FDA, in response to growing concern over potential transfusion transmissions. Different strategies have been employed within endemic areas to minimize the impact of an increased testing load. This ranges from yearly screening with guidelines for intensification testing in the United States to seasonal screening in Canada. As a result, testing centers have been able to significantly improve blood safety without exceeding testing capacities.
In 2005, the FDA approved the PROCLEIX WNV Assay, and it has continued to be an important means of keeping WNV out of the blood supply. Between 2003 and 2005, about 1,500 infected donations have been intercepted in the United States by nucleic acid testing. As a result, about 2200 blood recipients did not receive potentially infected blood3. Although WNV is still a threat to public health, the swift response of the government, the blood centers, and industry has ensured that blood safety has been preserved.
References
- Centers for Disease Control and Prevention Website
- Canadian Broadcasting Company
- Stramer, S. L. 2006. Impact of NAT on serological screening of transfusion-transmitted agents. ISBT Science Series, Vol. 1, No. 1: 194-202.