West Nile Virus Becoming Less Virulent

By Pippa Wysong, Access Excellence

Houston, Texas (02/15/05)- There is evidence that the West Nile Virus (WNV) of today is less virulent than the version of the virus that hit North America in 1999. Research tracking the virus over the past few years has found not only changes in its genome, but tests on samples of the virus suggest it is now less deadly than it once was.

Mutations occurring in the virus mean that it is adapting to its environment in North America, said Alan Barrett (PhD), professor of pathology at the University of Texas. But more than that, the virus may be causing less neurological disease (such as encephalitis or swelling of the brain, seizures or other nerve-related conditions) than it once did. Dr. Barrett was co-author of two recently published studies on the virus in the journals Virology and Emerging Infectious Diseases.

"We've been studying what's called the molecular epidemiology of WNV in the US. We study the genes of the virus and changes in the virus as it moves across the country," he told Access Excellence in an interview. Newer samples of the virus are being compared to older samples. As part of those studies, researchers found that some of the samples of WNV obtained in 2003 were biologically different from earlier samples. Two main questions arose from this finding, one being that if there were changes in the phenotype, would there be changes in the virus genes as well; and the second being would the virus behave differently when it infected animals in tests?

Twenty-five different samples of WNV were taken from mosquitoes and birds in the Houston, Texas area. The samples were used in various studies including ones to see how they affected lab mice. Numerous species, including mice, are susceptible to infection from the virus, so the rodent provided a good model. In recent years, outbreaks in humans, horses and birds all have been associated with neurological disease, such as seizures, as the virus attacked the brain.

"We found that six of the 25 isolates were actually no longer able to invade the brains of mice. That's why we reported it, it's the first observation in the US that the virus had changed biologically," Dr. Barrett said. It had become attenuated, meaning weaker or less virulent. Plus, changes were evident at the genetic level. At this point the researchers are still trying to determine which part of the gene is associated with the decreased ability of the virus to infect the brain, something that could prove useful for developing treatments and vaccines. After the virus arrived in North America, cases and deaths among humans and animals rose each year. The year 2002 saw the largest recorded epidemic of encephalitis due to mosquito-borne viruses in the western hemisphere.

In North America, WNV is in a different environment than in other parts of the world and infects different animals. "It starts to evolve and change with the animal species found in this part of the world. It starts to change and evolve to suit the population," he said. However, the virus killed off many members of one of its main hosts, the American Crow. From a biological point of view, there isn't any benefit to WNV if it kills its hosts --it can't survive. Eventually the viruses that don't kill their hosts become more common, and the virus population overall begins to change.

Researchers tested the isolates in mice and found that, unlike with earlier versions of the virus, it did not cause neurological disease. While this is good news for mice, it doesn't mean the virus is any less of a problem in people. More studies are needed to see whether the action of the virus in infected people has changed or not.

The newer version of the virus is not yet found everywhere in the US, there are still other, older versions of it around, but this is a typical pattern. "You see a few viruses change, then you start to see that virus will be amplified in the country, then it will become a majority virus," he said. Prior to 1999, WNV was limited to Africa, the Middle East, India and western and central Asia. Since its arrival in the US, WNV is now present in 46 states, seven provinces in Canada, throughout parts of Mexico and even in several Caribbean islands.


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