WASHINGTON - The strain of West Nile virus spreading rapidly across the country is a genetic match to one found in Israel, indicating the U.S. bug came from the Middle East, says a Centers for Disease Control and Prevention scientist.

"The virus that was introduced into New York in 1999 is closely related to a virus in Israel," which has infected hundreds of Israelis, said Harry Savage, a CDC researcher. "So it at least indicates that it came from that part of the world."

Federal scientists, however, are still trying to figure out how the mosquito-borne virus jumped the Atlantic, an unusual feat.

The mode of entry into the U.S. "is still unknown," Savage told WorldNetDaily.

Initially the CIA suspected bioterrorism, he says, but fears diminished after researchers found no traces of manufacturing in the U.S. strain.

"Bioterrorism is very remote because it's a wild virus," he said. "There's no engineering to it."

Also, West Nile wouldn't make an effective weapon since it kills relatively few and does not spread through human-to-human contact, Savage says. A blood-borne virus, its vector is mosquitoes, which can be controlled through water abatement and malathion spraying.

"There are lots of better weapons, if you want a weapon," he said.

Still, the death toll from West Nile has climbed to five in Louisiana alone, pushing total deaths nationwide to more than 20. And the CDC has rushed teams of doctors and scientists to the swampy state.

The brain-swelling disease has swept across more than half the states now and will hit all 50 within two years, epidemiologists predict. Relief from this year's outbreak isn't likely to come until October, when temperatures cool and viral growth slows.

So if not by bioterrorism, how was the virus introduced to America?

Epidemiologists and virologists offer three possibilities, all of which they admit are not highly feasible.

One is through an infected traveler. But that's not likely because humans usually don't have enough of the virus in their blood cells to act as a host for other mosquitoes, who in turn would spread it.

The second scenario is through an infected bird, which would provide a rich vector for the virus. But there are few birds imported from the Middle East.

The third way West Nile may have entered the U.S. is through a virus-carrying mosquito itself. Perhaps it came through a New York harbor. But that's not likely, either, since mosquitoes have a short life span.

Some point to government or university labs as the source.

Researchers were working on a West Nile virus vaccine in New York, Maryland and Washington, D.C., around the same time the virus first showed up in dead crows. Hundreds of infected crows have been found in the Washington area, including at the White House, which is near an Army medical facility that experimented with West Nile virus vaccines.

As part of experiments, researchers in New York and elsewhere injected crows with West Nile.

Some most affected by the virus (which is also quite lethal to horses), such as ranchers and horse trainers and breeders, can't understand why the government hasn't been able to solve the mystery.

"We have some of the best epidemiologists in the world. They have been assisting Europe with the Mad Cow (disease) problem," said Jim Garfinkel, a California equine veterinarian. "Why is it that we can't seem to get a handle on this disease?"

He says he was briefed about West Nile at a U.C.-Davis Veterinary Teaching Hospital meeting shortly after the virus first hit the U.S. and was told that it had been isolated to an Israeli strain of the virus.

Paul Sperry is Washington bureau chief for WorldNetDaily.