SACRAMENTO " California health officials are concerned about the discovery of 133 birds in 18 counties this year that have died of West Nile virus.
At this time last year, they had found only 33 West Nile-infected dead birds in 12 counties.
While there have been no human cases reported so far, Department of Health spokeswoman Suanne Buggy says officials are closely monitoring the virus' spread.
Humans contract the virus through mosquito bites.
A spokeswoman for the Sacramento-Yolo monitoring region, Luz Maria Rodriguez, says she was surprised when a dead bird with the virus was found so early in the season " in March.
The mosquito season is temperature-dependent, but usually lasts from about April to September.