Hi there - new to the forum - quick question???
Is the parasite you mention also called giardia? I've been breeding for 40 years (not a profession and not constantly - as my life permits) and I've have only once gone through a bad bout of persistent diarrhea. Of course I am always warning about diet changes and certain types of food - food quality - but I'm referring to a health issue - not a fishy food.
The bad bout occurred the 1st time we had more than one litter at a time - 13 kittens. We went through the gamut - every test and many treatments - it was definitely contagious. Our vet in the US worked at a shelter and always looked at health through the eyes of what shelter or feral kittens get - treated for street infections and parasites. Other consults with country vets centered around country problems - so if you have well water they think giardia - it's in the water.
Also the tests are very inaccurate. Depends of freshness of samples tested, labs used. Somethings can't been seen by a vet with a scope. Get a lot of false negatives and false positives. (I also have a lot of experience with super-bugs caused by antibiotics, like MRSA and C-DIF - drug resistant and reoccurring). Must get the right tests.
It seems everything depends on where and how you live and diet to a certain extent. But also being really careful - because every time we did something that worked - kittens stool got solid - all it took was forgetting one thing. For example we took to boiling water - but if kittens had access to a sink or tub and could get a drink...
I also found letting visitors handle the kittens before they were completely well was a mistake - their immune systems are weak and they'll catch a new infection. And respiratory infections will also cause bowl problems. We had to ask everyone to remove shoes and wash hands.
In the end flagil was the antibiotic which saved the day for us - but it's a really harsh drug that is known not to be used with any kind of liver disease - and never use for a long period. Flagil, like vancomycin bleaches the gut - kills everything (all infections and giardia) and then you have to build up the good flora again. But for an illness that kept going on and on - recontamination - we had to resort to it.
Eventually - get the cat on a really good diet - I only use foods that have the best (organic if possible) ingredients and ethical practices. If they won't eat what you cook try Royal Canin, Burns, Applaws, etc. - keep litters pristine and avoid all chemical additives.
The second thing I would like to mention is that Flagyl(Metronidazole) does not "bleach" the gut and kill the good bacteria, this is a misconception I had also until my Vet had me research it. If you research Metronidazole, on thier website, it should tell you this. It can cause side effects especially over time, Vancomycin is seldom used here for most of the GI issues as there has been too much Vancomycin resistant bacteria issues. Metro is very effective for Clostridium diarrhea as well as other bacteria overgrowth which can be contagious and may be what you experienced. Hope this helped.