Churchill Downs’ primary weapon against the West Nile virus (WNV) sits in a clear, plastic vial on the desk of track superintendent Butch Lehr, according to AP writer Chris Duncan.

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 WNV this summer and five have died. The mosquito-borne disease has also killed seven people this year, all in Louisiana