Knight Ridder/tribune Originally published November 17, 2002

DENVER - West Nile virus, relatively weak in its native Africa, evolved into an efficient killer just before coming to America, scientists have discovered. And it might be evolving again.

As colder weather brings the world's worst West Nile epidemic to an end for this year, scientists at the American Society for Tropical Medicine and Hygiene conference in Denver last week were trying to figure out how it got so bad so quickly and whether that bodes ill for next year.

The mosquito-borne disease killed 212 Americans and sickened 3,605 more this year, said geneticist Aaron Brault at the Centers for Disease Control and Prevention in Fort Collins, Colo.

"Nothing like this has been seen before," said Andrew Spielman, a Harvard University tropical health professor. "Charles Darwin would have loved this."

A picture is emerging of a virus that hit the Darwinian trifecta: Just the right evolutionary change in its genes, hosts (birds) that had no immunity to it, and favorable environmental conditions.

"You'll never see a better example of an emerging infectious disease than the experience of the Western Hemisphere with West Nile," said James Hughes, director of the CDC's National Center for Infectious Diseases.

What follows summarizes what scientists guess is the best biography of this virus, which has adapted and spread like no other.

The disease, first spotted in Uganda in 1937, didn't kill birds in Africa. They just passed the disease along, often not very efficiently.

"Something happened to that virus in the early 1990s that made it more easily epidemic and made it spread," said Dr. Duane Gubler, the director of the CDC's division of vector-borne infectious diseases, those spread by insects or animals.

The original version of West Nile, now dubbed Old World West Nile, spread across Europe and Africa, but killed no birds.

In 1998 a new strain, now dubbed New World West Nile, was spotted in Israel. Birds started dying.

In 1999, New World West Nile found its way to a Queens neighborhood in New York City. From there it spread, infecting people in five Northeastern states in 1999 and 43 states in 2002.

How the new strain sickens birds is key. When mosquitoes bite crows - the virus' primary U.S. hosts, along with house sparrows and blue jays - the birds get loaded with the new strain's virus. When infected with the old strain, crows don't acquire nearly so much of the virus. The more of the virus that's in the birds in the days before they die, the more likely they are to spread it.

While some laymen have pointed to West Nile's Middle Eastern origin and wondered whether it was engineered to be a biological weapon, CDC's Brault dismissed the idea. He said the genetic mutations he has found had all the hallmarks of the natural hit-or-miss changes that happen routinely in the fast-evolving world of viruses.

Those changes have wreaked havoc in America's bird and horse populations. This year, more than 91,000 dead birds have been reported. By the time more than 7,000 dead crows had tested positive for West Nile, most states were so overwhelmed that they stopped testing dead birds for the virus.

Nearly 13,000 horses have come down with West Nile this year, and about one-third of them died. Earlier strains of the virus had never sickened horses.

The genetic changes, which probably happened to the virus in the Middle East, most likely emerged via a mosquito biting a wayward bird, perhaps one blown off course by unusual westerly winds, Brault said. Other mosquitoes then spread the disease further.

An epidemic erupted in Israel. Scientists don't know for sure how that epidemic came to the United States, but it probably migrated in the blood of a traveler who settled in Queens and then was bitten by mosquitoes, Spielman said.

Genetic tests hint that the 2002 virus found in New York has genetically altered from the 1999 virus found there, said Laura Kramer, director of New York state's public health lab.

But the CDC's Gubler and Brault said it was too early to tell whether the virus was changing. If a genetic change is occurring, they said, it's probably not a significant one. Even so, given West Nile's recent history, Spielman said, "I'd accept that the virus is evolving rapidly."

If so, will that make it nastier? Maybe not. Gubler said viruses that made a second dramatic genetic change often were moderating themselves to avoid killing off all their hosts - in this case, birds.

But Brault isn't ready to assume that the threat of West Nile is ebbing. He's waiting to see what happens next summer, when mosquitoes and humans meet again.

"We have to be prepared for the possibility that next year it will spread to the West Coast, and there could be as many cases as this year," said infectious disease chief Hughes. "I hope not."

-- Anonymous, November 17, 2002