West Nile Virus has arrived in Southern California, Dr. Ron Jensen warned the California Horse Racing Board July 22, and could strike at one of the region's racetracks this summer as it speeds its way through the state."Personally, I wouldn't be surprised to see a case at one of our racetracks," said Jensen, the CHRB's equine medical director. "We've done very well at vaccinating our horses, but it's not 100%. The disease won't stop at the (tracks') gates. It's important not to panic."Infected mosquitoes have killed hundreds of birds with their bites in several Southern California counties. The first case in the northern part of the state was confirmed in a dead crow found in Santa Clara County July 22, Jensen said. There have been 26 infected horses so far reported this year, all in Southern California, with 11 fatalies. Twenty-eight human cases have been uncovered in the state in 2004, none resulting in death.Mosquitoes become infected when they feed on infected birds and they pass on the disease by biting humans and other animals. Humans and horses are most susceptible, Jensen noted.Nationwide, West Nile virus has been detected in 34 states, he reported.Jensen said the CHRB is working closely with Del Mar to minimize the conditions that could lead to an outbreak. He said that vaccinations are highly effective in protecting horses. But he predicted there will be more horse cases throughout the state this summer.Among the measures that can be taken are to avoid spending time outside when mosquitos are most active, especially at dawn and after sunset, wearing long pants and long-sleeved shirts, using insect repellant, eliminating sources of standing water that support mosquito breeding, and contacting the vector control agency if there is a significant mosquito problem.Del Mar vice president Craig Fravel said the track is working with San Diego County vector control."I hope that we realize that horses are just victims," Fravel said. "There is no connection between horses and humans" transmitting the virus to each other.