hi read about your cats diarohea and it sounds exactly what my cat has just been treated for with ronidazole.she had terrible diarohea for many months and was not diagnosed until she was referred to the animal university for many tests and was confirmed with Tritrichomonas. she was put on science plan veterinary food and given the ronidazole tablets for 2 weeks . After a further 2 weeks she had gone back to normal poos and back on her usual food Royal canin. She has had an all clear test and is going to be retested in 3 months time.
not many vets no about this problem as it is a new disease in multi cat homes or breeding catteries. the tests are done in scotland so this will be a bit nearer to home for you. It is a horrible problem but hopefully quite easy to sort out so long as you can get the tablets down your cats throat!!
hope this helps you and your cat