WEDNESDAY, Sept. 9, 2009 (HealthDay News) -- One in every 100 Americans has been infected with the West Nile virus, whether they know it or not.
That number has grown steadily since the virus first appeared in North America in 1999, according to a study in the October issue of the journal Emerging Infectious Diseases.
Several West Nile vaccine trials are under way or are being planned, and these findings could not only help yield needed antibodies to the virus, they may help determine future vaccines' effectiveness, said the study authors, from the U.S. Food and Drug Administration and Baxter Bioscience.
"It's important to know this because we don't test people routinely for West Nile virus," said Dr. Maria Alcaide, an assistant professor of infectious diseases at the University of Miami Miller School of Medicine.
The majority of people -- about 80 percent -- who become infected with West Nile virus have no symptoms at all. Up to 20 percent may experience flu-like symptoms such as fever, headache and body aches or even nausea and vomiting. Only about one in 150 people will suffer severe illness, resulting in meningitis or encephalitis. People aged 50 and older are at higher risk for developing severe complications.
The authors of the study tested lots of blood donations for the presence of antibodies to the West Nile virus, then extrapolated this number to the general population. Each lot contained blood donations from thousands of individuals, making the testing process much more efficient.
"If you looked at each individual, you would have to do thousands of tests to find one infection," Alcaide explained.
In 2003, about one half of 1 percent of the U.S. population had been infected with the virus. Today the number is approximately 1 percent, which corresponds well with other estimates, the study authors stated.
"As WNV [West Nile virus] infections often remain asymptomatic, the antibody levels in individual lots of IGIV [immune globulinintravenous], which are each derived from the plasma donations of thousands of healthy individuals, can also be an important public health tool to determine how many people have experienced past WNV infections from a very large sample size," said study senior author Thomas Kreil, senior director of viral vaccines and global pathogen safety at Baxter Bioscience in Vienna, Austria.
"Also, the findings are important because they show that intravenous immunoglobulin contains antibodies to West Nile virus, possibly providing protection against WNV infection for patients who take IGIV, but this has not yet been shown in clinical studies," he added.
The U.S. Centers for Disease Control and Prevention estimates that 3 million West Nile virus infections occurred between 1999 and 2008.
Despite its lowered visibility due to the new H1N1 swine flu, West Nile remains a public health threat. "It's true that we haven't talked about it as much, but it doesn't mean it's not going on," Alcaide said.
West Nile virus is normally passed from an infected mosquito to a bird, and then from the bird to other mosquitoes. The mosquitoes then pass the virus on to humans. Infections tend to peak in the summer.
Health officials recommend removing any standing water where mosquitoes are likely to breed. People should also wear insect repellant and long-sleeved clothing whenever outside.
In addition to the standard DEET-containing products, the CDC also endorses mosquito-repellent products containing oil of lemon eucalyptus and Picaridin.
The U.S. Centers for Disease Control and Prevention has more on West Nile virus.