Castro is a 15-year-old Sumatran tiger who has lymphoma, a form of cancer. Castro also has kidney stones.
Today, surgeons inserted flexible tubes called "stents" to help drain urine from the tiger's kidney to his bladder.
Ray Wack is one of those surgeons. He's a senior veterinarian at the Sacramento Zoo and UC Davis Wildlife Health Center.
"He came through the operation very well. It took us about four hours to do a complete physical and his cancer evaluation and the surgery to place the stents in and he's now back up in his night house recovering from the anesthesia."
Bill Culp is one of the other surgeons. He's with the UC Davis School of Veterinary Medicine. Culp says the procedure on a large animal is pretty unusual.
"This, to my knowledge, has not been done in a large cat before. We do them very commonly in domestic cats at UC Davis. But in big cats, this is not something that's been done."
Culp and Wack are among a team of veterinary surgeons from both UC Davis and the Sacramento Zoo who performed the minimally-invasive surgery.
Sumatran tigers are endangered, fewer than 500 animals are believed to exist in the wild and about 200 live in zoos around the world.