NEW ORLEANS (AP) -- The United States suffered the biggest reported outbreak of West Nile encephalitis in the world this year, a federal disease control expert said Wednesday.

Now officials are wondering if the situation will be as bad or worse next year.

Caused by a virus carried by mosquitoes, the 2,500 cases of West Nile encephalitis and meningitis surpassed by far earlier outbreaks in Israel, Russia and Romania, said Dr. Roy Campbell, an epidemiologist with the U.S. Centers for Disease Control and Prevention in Atlanta.

The virus killed 232 people across the country in 2002, including five deaths which were reported to Louisiana health officials only this week. State and local health departments have reported more than 3,800 West Nile virus cases to the CDC. But about a third of those are West Nile fever, a much less serious disease.

Nobody's sure why the West Nile virus hit the United States so hard. However, the fact that it is a new virus in this part of the world was a major factor, experts say. Most animals in this country had never been exposed to it -- nor had people.

In addition, the United States has a large elderly population, one of the most susceptible groups.

A big question is whether states that had small numbers of cases this year can expect a big jump next year, said Dr. Bob Shope, a professor at the University of Texas Medical Branch in Galveston, Texas.

That's what happened in Louisiana, which had one West Nile case in 2001 and 329 this year.

Shope and other experts will only speculate.

Many states have ramped up mosquito-control programs and made other efforts to stop the disease's spread.

In a telephone interview, Campbell also noted that 2002 also will go down as the largest outbreak of bug-borne viral encephalitis in U.S. history. The next-largest was in 1975, when about 2,000 people came down with St. Louis encephalitis, a mosquito-borne virus in the same family as West Nile. About 170 died then, Campbell said.

West Nile virus showed up first in New York in 1999, first striking crows, then people. By late November of this year, only four Western states in the Lower 48-- Oregon, Nevada, Utah and Arizona -- appeared free of the virus.

"We can say it has completed its transcontinental migration in a period of three years, which is quite remarkable," Campbell said.

Most people bitten by a West Nile-infected mosquito never notice more than the usual itch. Only one in 150 to 200 infected people get seriously ill; another 20 or so get the flu-like West Nile fever.

Encephalitis and meningitis, infections of the brain, spinal cord or membranes, are the most serious diseases caused by the virus. They produce fever, headaches, stiff neck, disorientation and tremors. About 10 percent of encephalitis patients die; others are left with brain damage.

Most who become seriously ill are elderly or have suppressed immune systems.

There is no treatment for the virus, although the Food and Drug Administration this year approved tests of a drug called alpha-interferon, which is used to treat hepatitis C.

In Africa, where the virus was first identified in 1937, it is so widespread that most people probably get mild cases as children and are immune for life.

"So you don't see it like you do in developed cities in the Northern hemisphere," Campbell said.

West Nile fever made tens of thousands of South Africans ill in 1974, but only one case of encephalitis was reported, he said.

Copyright 2002 The Associated Press. All rights reserved.