Weight loss and diarrhea in 9 year old male cat

[woofendog]

Sylvester has gone from 19 lbs in February 2013 to 12 lbs in October 2013 with vet bills approaching one thousand dollars. Blood analysis, urinalysis, radiology, and surgery revealed nothing. In the last month there has been constant and severe diarrhea. A stool sample revealed the presence of parasites. He was given an antibiotic injection and five antibiotic pills. He was also prescribed Hills Prescription ID diet food which has not solved the diarrhea problem. Apart from the weight loss and diarrhea, he continues to be as frisky and playful as ever. What could be causing this problem? My four other cats have also eaten the Costo Kirland brand food and are perfectly healthy. Thanks.