Through the end of July, 241 human cases have been reported in 22 states, including four deaths. Texas, especially around the Dallas area, has seen the bulk of them.
Health officials believe the mild winter, early spring and very hot summer have fostered breeding of the mosquitoes that spread the virus to people.
Most West Nile infections are reported in August and September, so it's not clear how bad this year will be. But it doesn't look good.
"Unless the weather changes dramatically, we'll see more cases (in 2012) than we have in the last couple of years," said Roger Nasci of the Centers for Disease Control and Prevention. He is chief of the CDC branch that tracks insect-borne diseases.
Mosquitoes pick up the virus from birds they bite and then spread it to people.
Only about one in five infected people get sick. One in 150 infected people will develop severe symptoms including neck stiffness, disorientation, coma and paralysis.
Of the 241 cases reported so far this year, 144 were severe cases in which the virus spread to the brain and nervous system and caused encephalitis or other problems. The last time so many serious cases were reported this early was 2004, when the number was 154.
West Nile virus was first reported in the United States in 1999 in New York, and then gradually spread across the country. Its peak occurred in 2002 and 2003, when severe illnesses numbered nearly 3,000 and deaths surpassed 260.
Last year was a mild one, with fewer than 700 human cases reported.
In recent years, the general pattern has been cases scattered across the country along with hot spots with more illnesses. The recurring hot spots include southeast Louisiana, central and southern California, and areas around Dallas, Houston, Chicago and Phoenix.
Those areas seem to have a combination of factors that include the right kinds of virus-carrying mosquitoes and birds, along with large numbers of people who can be infected, Nasci said.
The best way to prevent West Nile disease is to avoid mosquito bites. Insect repellants, screens on doors and windows and wearing long sleeves and pants are some of the recommended strategies. Also, empty standing water from buckets, kiddie pools and other places to discourage mosquito breeding.
CDC West Nile page: www.cdc.gov/ncidod/dvbid/westnile/index.htm
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