ATLANTA (AP) _ The nation's summer heat wave has spurred a jump in West Nile virus cases, and this will probably be the worst season for the mosquito-borne disease in three years, health experts said this week.
``That's the general trend,'' said Dr. Lyle Petersen of the U.S. Centers for Disease Control and Prevention.
Cases of the virus are down in many states, where West Nile has become customary and some birds have developed immunity. But activity has been heavier in some parts of the western United States with limited past exposure to the virus.
Idaho is hardest hit, with 440 human illnesses reported as of Friday, including five deaths. The state never before had more than 13 cases in a year. Idaho Gov. James Risch this month signed disaster proclamations for two counties in metropolitan Boise, in the southern part of the state, to allow state money to be spent on mosquito-killing measures such as nighttime spraying by airplanes.
``We're one of the last states to see West Nile enter our borders,'' said Tom Shanahan, a spokesman for the Idaho Department of Health & Welfare.
Nationally, 581 cases of human West Nile virus were reported to the CDC from 29 states as of Aug. 22. Officials consider that an undercount, because there is often a delay in diagnosing cases and in transferring reports from states to the CDC.
At the same time last year, the CDC received 501 reports from 25 states. That means cases are up 16 percent from last year.
West Nile season usually peaks in late August and doesn't end until November, so it's hard to say exactly how bad the season will be, said Petersen, the Colorado-based director of the CDC's Division of Vector-borne Infectious Diseases.
But it probably will be worse than 2005 and 2004, when the summers were relatively cool, Petersen said.
This year should not be as bad as 2003, when nearly 10,000 cases of human illness were reported nationwide, including about 2,000 in Nebraska and 3,000 in Colorado.
West Nile virus was first reported in the United States in 1999 in New York, then spread across the country.
Mosquitoes transmit the virus, often picking it up from birds they bite and then spreading 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.
West Nile has caused at least 19 deaths so far this year, and it caused 119 last year, according to CDC statistics.
The nation suffered through a scorching heat wave for more than two weeks in late July and early August that killed more than 200 people. The heat wave appears to have helped spread the virus in some parts of the country, said Laura Kramer, a research scientist with the New York State Health Department.
The virus replicates more quickly at higher temperatures, and mosquitoes breed more quickly, too, she said.
West Nile usually appears in a state or area in modest numbers one year and then can return at epidemic levels the following spring, said William Reisen, a research entomologist at the University of California, Davis.
A few ocean-bordering U.S. states with cooler climates, including Maine and Washington, have yet to report a human case, Reisen said.