My cat would have been two next month if we had not had to put her down. We did use Hartz on her. Last year she developed severe diarrhea with blood and mucus. Then she lost a lot of weight. We took her to the vet and he gave her steroids. She gained the weight back, but she did not stop the bloody diarrhea. She got to her all time low weight of 4 pounds, so we took her to the vet this past Monday. All blood tests came back negative. She was anemic and her white blood cell count was low. She began to twitch and her pupils were twitching too. We decided to put her down on Wednesday. Do you think the Hartz could have permanently damaged my Kitty?