Louisville, Ky. ? Churchill Downs’ primary weapon against the West Nile virus sits in a clear, plastic vial on the desk of track superintendent Butch Lehr.

The vial holds birdseed-like pellets that release a chemical that kills mosquitoes in their larval stage. The chemical is otherwise safe and is harmless to horses, making it an ideal mosquito repellent for the home of horse racing’s premier event, the Kentucky Derby.

“It does the trick,” said Lehr, superintendent since 1981.

Eight horses in Kentucky have been diagnosed with West Nile this summer and five have died. The mosquito-borne disease has also killed seven people this year, all in Louisiana.

The horse racing industry is trying to protect the animals from West Nile, and there is a vaccine available for horses. The industry is still recovering from an illness that killed hundreds of foals and caused pregnant mares to abort last year.

The cause of Mare Reproductive Loss Syndrome is still unknown, though scientists believe it’s connected to a breed of caterpillars that feed on cyanide-laced cherry tree leaves. About 15 percent of the foals that would have been born in spring 2001 were lost to the illness, and many live foals were sickly and needed intensive medical care to survive.

While West Nile doesn’t appear to pose the same sort of threat, Lehr and others aren’t taking any chances. A 2-year-old colt died last fall at Churchill Downs from the virus.

“That was a learning experience for all of us,” Lehr said.

The track already had a mosquito deterrent system angled, concrete ditches that drain water from horse barns. The ditches reduce standing water, where mosquitos breed.

Lehr wanted to do more. This spring, the track started using the vials of larvicide called Vectobac-G. The pellets were spread along the grassy perimeter of the track’s infield and put in water-collecting basins. The track also set up sticky, yellow rolls of plastic sheets that trap mosquitos like flypaper.

The track hasn’t had another case of West Nile.

“It’s never been a frantic situation, but we came to grips with this as a very real threat last fall,” said John Asher, the track’s vice president of communications. “There’s nothing to prevent a single infected mosquito from flying across the fence. But we think we’ve taken every precaution we can.”