CHICAGO (Reuters Health) - This summer's West Nile virus outbreak in the US and Canada is the largest epidemic of mosquito-borne viral meningoencephalitis ever documented in the Western Hemisphere, according to the US Centers for Disease Control and Prevention (CDC).

Meningoencephalitis combines two conditions: meningitis, or inflammation of membranes that encase and protect the brain and spinal cord; and encephalitis, which involves inflammation of brain tissue.

In the US, the 2002 epidemic surpasses the 1975 epidemic of St. Louis encephalitis (SLE), in which 1,967 cases were reported to the CDC. About 170 people died in the outbreak, according to the CDC's Dr. Roy Campbell. But this summer brought even higher numbers.

"As of last Friday, the states had reported about 3,400 West Nile-related cases to us," Campbell told the annual meeting of the Infectious Diseases Society of America. "Of those, about 2,000 were the more severe meningoencephalitis forms, and another 1,400 were a combination of milder West Nile fever or a form not yet clinically categorized."

Campbell said West Nile has killed about 190 Americans so far this year. "That number will go up because of a lag in reporting, and some patients will die later of complications from the disease."

Both West Nile and SLE belong to the arbovirus family, which includes more than 250 different tick- and mosquito-borne viruses.

As in 1975, the nation's heartland bore the brunt of the infections. The hardest-hit states this time were Mississippi, Ohio, Indiana, Illinois, North Dakota, Kentucky and Louisiana. But in 1975, Louisiana didn't report a single case of SLE.

Campbell pointed out that the CDC has incomplete data on why these genetically similar viruses strike certain geographic areas and leave others alone.

"The appearance of epidemics of West Nile or SLE depends on an extremely complex and poorly understood constellation of natural factors," the medical epidemiologist said, explaining that pieces of the puzzle clearly include such factors as bird and mosquito abundance and species composition, as well as climate and weather conditions.

It's known that the adult northern house mosquito, which is most responsible for transmitting these viruses, can survive winters by going into warm, humid, protected areas. The creature can also survive in the egg stage, ready to hatch in the spring, Campbell noted.

After the 1999 New York City outbreak, Campbell and other CDC investigators found live West Nile virus in bird tissue, demonstrating that the bug can go "underground" as a non-lethal host in birds, and perhaps ride along during migration.

Campbell cannot predict what will happen next year. "But typically after you have a major epidemic you get a few less severe years and the thing goes away for a while and may pop up a few years later. So we may be on the down slope for now."

There is no specific treatment for human meningoencephalitis, but Campbell says advances in medical care have helped cut the death rate from the 20% recorded in the 1933 SLE epidemic to about 10% today.

He says the best defense against these outbreaks is good surveillance, including surveillance of bird deaths and mosquito testing to detect viral activity early in the spring. Early warning gives communities more time to put control measures in place and educate the public about reducing risk, he explained.