DALLAS -- Federal health officials say the current West Nile outbreak is one of the largest in the U.S., with four times the usual number of cases for this time of year.

So far, 1,118 illnesses have been reported nationwide, about half of them in Texas. That's the most ever by this time of year according to health officials. Normally, fewer than 300 cases are reported by mid-August. There have also been 41 deaths this year, the Centers for Disease Control and Prevention says.

Cases seem to be accelerating, with about 400 of the cases reported in the last week.

Experts think the mild winter, early spring and very hot summer helped stimulate mosquito breeding and the spread of the virus.

No human cases have been detected so far in Washington state this year. But the state Department of Health reported in July that West Nile was found in Mosquitoes collected near Mesa in Franklin County. The virus was found in mosquitos in three Washington counties in 2011, but no human infections were reported.

In Oregon, the Klamath County Health Department reports that the West Nile virus was detected in a horse that died in the town of Keno.

Washington Dept. of Health information on West Nile Virus

WFAA's coverage of the West Nile public health emergency in Texas

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