West Nile virus is here to stay, the CDC says.

There were more serious cases of West Nile virus this year than last. That, the CDC says, means we aren't going to see the mosquito-borne disease go away. It's now firmly planted in the U.S., becoming what the CDC calls an endemic disease.

"The increase in reported cases of West Nile virus disease … suggests that endemic transmission of West Nile virus in the United States will continue for the foreseeable future," states an editorial note in the Dec. 16 issue of the CDC's MMWR: Morbidity and Mortality Weekly Report.

Over the first 11 months of 2005, there were 385 more reported West Nile cases -- 2,744 -- than during the same period in 2004. In 2004 there were 98 West Nile deaths. As of Dec. 6, there were 98 West Nile deaths in 2005.

The 2005 West Nile virus season started in late May, peaked in the third week of August, and kept going into November.

West NileHot Spots

Generally about four out of five people who get West Nile virus infection don’t have symptoms. About one in five will come down with West Nile fever. Usually this is a very mild illness -- but it can be fatal, especially in the elderly.

Less than 1 percent of West Nile infections, however, are far more severe. It's called West Nile neuroinvasive disease (WNND) -- infection of the brain and central nervous system. And sometime patients may get a polio-like paralysis.

Because most infections will not produce a lot of symptoms and West Nile fever usually is so mild, most people don't see a doctor. This means the case doesn't get reported. That's why the CDC warns that the annual case count represents only a fraction of actual cases.

Even so, serious cases do get reported. And they show where the virus is most intense.

Nearly a third of U.S. cases were in California. Other focal points of serious disease were Illinois, Texas, and Louisiana.

But the likelihood of getting West Nile virus was highest in South Dakota, where there were 4.8 cases of WNND per 100,000 residents. Other hot spots were Nebraska and North Dakota (2.1 and 1.9 WNND cases per 100,000 residents, respectively).

Preventing West Nile Virus

Why won't West Nile virus go away? The problem is that it circulates in wild birds. Corvids -- crows, ravens, jays, and magpies -- carry a lot of virus in their blood. So do house finches and house sparrows. Mosquitoes that bite the birds carry the virus to humans, horses, and other animals.

The key to avoiding West Nile virus is to avoid mosquito bites. That means keeping your yard and house free of tires, flowerpots, clogged gutters, and other places where water stagnates. It also means wearing mosquito repellent and protective clothing -- and simply avoiding being outdoors at dawn and dusk when mosquitoes are most active.

By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: CDC, MMWR: Morbidity and Mortality Weekly Report, Dec. 16, 2005; vol 54: pp 1253-1256. CDC web site.