Looking For West Nile

October 5, 2001

Heather Heilman
Phone: 865-5714

You can't get through summer in New Orleans without collecting a few mosquito bites. In fact, you probably can't get through spring, fall or winter in New Orleans without some itchy red bumps. Usually, mosquitoes are nothing more than an annoyance, but this summer when West Nile virus appeared in Louisiana, the insects seemed a little more menacing.

"People are afraid of West Nile. They don't know much about it but they think it's going to kill them," said Dawn Wesson, associate professor of tropical medicine and Tulane's resident mosquito expert. "But it's probably not. The chances are better that they'll win the lottery. But that doesn't mean we shouldn't be concerned about West Nile."

"West Nile, like most of the emerging disease problems, is of our own making," she said. "They're not occurring as part of the natural process." In the summer of 1999 Wesson was preoccupied with an outbreak of Eastern equine encephalitis in rural Louisiana. That mosquito-borne virus killed more than a hundred horses, and mosquito control experts were worried that it would spread to humans.

"In the meantime there were reports of an outbreak of St. Louis encephalitis in New York City. That was interesting because St. Louis encephalitis had never occurred up there. New York didn't even have a mosquito control program, because they'd never had problems with mosquito-borne diseases," Wesson said. It was a month before researchers at the Centers for Disease Control realized that they were not dealing with St. Louis encephalitis at all, but a similar virus called West Nile.

Even though the viruses were similar, St. Louis encephalitis has long been established and widespread in North America, while West Nile had been confined primarily to the Middle East and southern Europe. That may be why West Niles spread across America has been marked by large numbers of bird deaths. Birds in North America have no resistance to the disease.

Like St. Louis encephalitis and Eastern equine encephalitis, both of which occur in Louisiana, West Nile is usually spread from mosquitoes to birds and back again. These diseases can spread to humans who are bitten by infected mosquitoes. All of these viruses can cause encephalitis, or a swelling of the brain, but most people who become infected have no symptoms at all. "It's sort of a misnomer to call these viruses encephalitis. That's only one possible outcome of infection," Wesson said.

All of the strains of West Nile virus that were isolated in New York that first year were identical, which suggest that there was only one point of introduction. The strain was also identical to that identified in a dead goose in Israel the year before.

"There are different possibilities as far as how it got to New York," Wesson said. "It could have come in an infected vertebrate host that wasn't showing symptoms. A lot of frogs and other animals are imported for use in medical schools or laboratory research that don't go through the same quarantine procedures a dog or a horse might go through."

It's possible that an illegally imported animal, a parrot for example, could have been infected. An infected mosquito might somehow have survived a plane ride. Some people have mentioned bioterrorism, but it's not a very good weapon because it moves pretty slowly and depends on local conditions. The virus is spread by bird migration and has made it as far west as Iowa.

In August, a dead blue jay in Kenner was found to be infected with West Nile, and a horse in Vermilion Parish died of the disease. In September, a dead crow found in the same area of Kenner was also found to be infected. So far, there have been no known human cases in Louisiana. In order to control the virus, researchers need to figure out which species of birds and mosquitoes play the biggest roles in spreading it.

"We've found a lot of birds that are infected with West Nile, but we don't know how many of them will be a good host," Wesson said. "To be a good host they have to get infected easily and develop a high viremia without getting sick and dying from it."

And while a mosquito is a mosquito is a mosquito as far as most people are concerned, in fact there are many different species of mosquitoes with different potentials for transmitting West Nile. Researchers are investigating which species can be most easily infected in the lab, although susceptibility to the virus is not the only thing that makes a mosquito a good vector.

"The mosquito that's most commonly found to be infected in the Northeast is only an average host in the laboratory, but because of its feeding behavior it picks up a lot of viruses," Wesson said. "It likes to feed from birds and it's a nighttime feeder, when birds don't have anti-mosquito behavior."

The worrisome part is that the strain of that particular species that we have in the South also feeds on humans. Another disquieting thing is that at the same time West Nile showed up in the Northeast, so did a new species of mosquito that seems to have been carried in on ships from Southeast Asia, probably in the form of eggs laid in used tires. This new species is a very good vector of West Nile.

"This is about our modern way of life," Wesson said. "We do have quarantine laws in place, we check for sick individuals and sick animals. But we don't always know how to look for an unknown. Some things are going to slip through. We need to be prepared for them when they do."

Tulane University, New Orleans, LA 70118 504-865-5000