"This year (2012) is now officially our worst year ever for West Nile Virus disease," said David Lakey, commissioner of the Texas Department of Health. Previously, the worst year was 2003 when Texas had 439 cases and 40 deaths, he said.

Officials say the virus lives in birds. Mosquitoes bite infected birds, become infected, and then pass the virus along to humans. Since 1999, more than 30,000 people in the United States have reported they became sick with West Nile virus. Of all the cases of WNV this year, 54 percent were classified as neuroinvasive disease (such as meningitis or encephalitis) and 46 percent were classified as non-neuroinvasive disease.

CDC officials say heavy rains from Hurricane Isaac have added to the breeding potential of infected mosquitoes across the Delta, and a warmer-than-usual winter may have added to the current outbreak.

In addition to human victims, 21 Texas counties are reporting WNV cases in animals. The specific number of each animal group was not available from the Texas Animal Health Commission (TAHC), but generally infected animals are limited to horses, cats, dogs, sheep, goats, bats, llamas, wolves, and rodents. Livestock, as a rule of thumb, are generally not affected by WMV.