When West Nile virus (WNV) made its march from East Coast to West Coast starting in late 1999, California was one of the last states to be struck by the disease. In the past couple of years, California has one of the the highest rates of West Nile virus problems in the country, and it now has recorded its first horse death for 2009 due to the disease, which is spread by mosquitoes.

Nearly every county in California has had birds or mosquitoes discoverd this year that test positive for West Nile virus (WNV), according to the Centers for Disease Control. The first horse death due to West Nile virus was reported near Tracy, Calif., on Aug. 6. There has also been one human case reported in San Joaquin County, where the horse resided, according to an article on recordnet