In a joint effort, the SPCA and Seaside police served search warrants at the properties on Tuesday.
The company that manages the properties contacted the SPCA after seeing dead kittens on the property several days ago, SPCA humane investigation supervisor Capt. Judi Adams said.
The warrants were executed simultaneously, SPCA spokeswoman Beth Brookhouser said. The SPCA has not disclosed the locations of the properties but said they are near each other.
The remains of the kittens are still being examined to determine the causes of death and the exact number of deceased felines recovered, Brookhouser said. There were no kittens among the live cats seized.
All of the living cats were brought to the SPCA shelter, where they were given thorough medical exams and flea and worm treatments, Adams said. They were also vaccinated.
Five of the cats are pregnant, and two others needed emergency surgery for pyometra, a severe uterine infection that is fatal if untreated, Brookhouser said.
The 51 cats' coats were in poor condition, and all were suffering from upper respiratory infections, parasites and dental disease, Adams said.
They are currently in comfortable living confines while being treated, and are being given fresh food and clean water, Adams said.
The SPCA has received reports that the cats were taken from nearby streets. Any owners who are looking for missing cats are urged to contact the SPCA.
Upon completion of the investigation, the case will be sent to the Monterey County District Attorney's Office for a charging decision, Adams said.
The investigation is ongoing and no arrests have been made, Adams said.
Anyone interested in adopting a cat, or who wants to report animal cruelty, can call the SPCA at (831) 373-2631. All calls are confidential.
Donations for the seized cats' care can be made online at www.SPCAmc.org.
The Associated Press contributed to this report