Chicago's Tree House Humane Society has deployed 1,000 feral cats onto the city streets to fight the city's rat infestation. Tree House began the annual "Cats at Work" program a decade ago as an alternative to the use of rat poison that can harm other wildlife. From WGN9:
[Tree House spokesperson Sarah] Liss said the cats generally do not eat a lot of rats, although the cats will kill some rats in the beginning when they arrive in a new location.
After they get acquainted to the space though, much less effort is required on the cat's behalf.
"They are actually deterring them with their pheromones. That's enough to keep the rats away," Liss said.