On Saturday evening staff arrived at the shelter to discover eight kittens dumped in a sack adjacent to one of the outdoor dog pens. The kittens were distressed and were covered in condensation as the sack, which was tied by rope, was airtight.
In the nearby dog pen they found the bloody body of a dead kitten and another sack, ripped to shreds and blood-smeared.
Animal volunteer Helen Davies said: "The sacks had been left right outside a dog pen which was housing a Japanese Akita that hates cats. The sacks were left close enough to the wire so that the dog was able to pull one of the sacks through and kill one of the kittens.
A search of the premises led to the discovery of another kitten cowering in a corner with a puncture wound to its paw that needed surgery.
In all staff found 10 kittens between four and six weeks old while others are between 6 and 8 weeks old, all needing treatment for cat flu, ear mites and worms. Two also need badly ulcerated eyes removed.
A traumatised adult she cat has been spotted near the centre and when staff tried to catch her they discovered three more kittens.
"We do not know if there are any more kittens loose or if there was a second mother cat in the sack, but we do know that two sacks containing 13 kittens and a mother cat were dumped outside the shelter. All 12 kittens that survived are now being cared for, but the terrified and emaciated mother cat is still on the loose," said Helena.
Free neutering vouchers from the Cats Protection League are available at the Rainbow Rehoming Centre.