diarrhea problems need advice please

I have 2 cats with diarrhea that goes away on meds and comes back when meds are stopped. A little history: Lucy 10 months old. Had multiple problems when we got her at 12 weeks. She was infested with ring worm when we brought her home. She also had smelly diarrhea. Which we later found out was giardia. She did 2 rounds of meds for giardia and the smelly diarrhea went away. She had been having normal poop for at least 4 months. Jinx 7 months old. We found him outside at about 3 weeks old. No problems at all. Never had diarrhea. Lucy and Jinx had been separated from day 1 since Lucy had problems that were contagious.  In November, when Lucy was finally clear from any problems, I introduced them to each other and they have been friends ever since. They share food bowls, water, and litter box. In January I noticed my house would start to smell when the cats would use the litter box. Smelly like when Lucy had giardia. Way more smelly than normal. I saw Lucy had pure liquid stools and took her into the vet. They did not find anything in her poop under the microscope. Since she had giardia in the past, they gave me meds for it. When I got home from the the vet I noticed Jinx also had liquid stool and took his in to be tested since nothing was found in Lucy's. They found nothing in his stool either. Just said that they noticed yeast which meant that things were unbalanced. They gave me meds for giardia for him too since they both had diarrhea and Lucy had a history of giardia. They were given 5 days of fenbendazole and 10 days of metronidazole. A few days on meds and stools were formed. A week off of meds and diarrhea started to come back. Dr gave them another round of meds. They were off meds for about a week again and diarrhea started to come back. I sent in stool for a fecal float and fecal pathogens test for both cats stool. All came back negative. Ova and parasite, clostridium, giardia, and cryptococcus. I will also add that when bringing in a stool sample for Jinx that a ova and parasite was done then too and came back negative.  Now they want me to try tylosin.  This is an antibiotic for cats with IBD. I will also add that both cats are acting normal, eating and drinking normal, and fur is shiny. Nothing seems wrong besides the stool.

My questions are, wouldn't them both getting diarrhea at the same time indicate a parasite? If it was a bacteria overgrowth then wouldn't that show up on the microscope? Why are all the tests coming back negative? I had suggested that giardia is not always shedding but she assured me that it would show up on their tests. I'm not so sure. Any meds that have worked for other cats with this? Any one had this happen to them? I don't know what to do next. It worried me because Jinx has never had diarrhea and does not have a sensitive stomach so this makes me feel that something is really wrong with him. There stool is not full blown diarrhea yet but seems to slowly be getting there. Like, every other poop is super runny but not every single one yet. Any advice? I've heard that stools get better with metronidazole and sometimes go back to being runny after the med, but I don't remember why or what ppl did in that case. Help!

I would ask the vet to do a more in-depth fecal test that tests for tritrichomonas foetus to rule out any other parasites.  http://www.highgait.com/page/page/3485008.htm   I'm assuming that you've had them dewormed and all that??  Then, look at their food.  What are you feeding them?  I ask this because I prefer to feed my cats a grain-free diet, but my male cat cannot tolerate a totally grain-free diet.  After several rounds of metro, my vet suggested we add Royal Canin Gastrointestinal HE to his diet.  That cleared up his problem in a week or so.  That's been 3 years ago now and he's still on it and doing well.  We are able to mix his dry grain-free food with the RC food so that he has nice poopies.

Thank you for your response. The fecal they did was the most indepth they had for parasites but it did not include the tritichomonas. She had said something along the lines of needing to check for that separately. I will have to look into that. My cats are on natures variety chicken. They have been on this food since I have had them with the exception of Jinx needing formula bc he was only 3 weeks old. They also get a can of fancy feast kitten ocean whitefish or turkey. They have been fine on the food without diarrhea. I don't think the food would have all the sudden not agreed with them. Only thing I can think of if it was the food is that maybe they got a bad batch of it? But their bacteria is not unbalanced so I'm not sure what to think about all this and they have eaten out of more than just one bag since all this started. They have been dewormed multiple times. Jinx has all his shots and Lucy just needs one more set. She was not able to get all of hers due to the ringworm being so bad. Did not want to compromise her immune system. Both were also checked for FIV and leukemia. Both were negative. The most puzzling thing to me is that they both developed the diarrhea at the same exact time. 

Ohhhhh...it could be the whitefish!  Many kitties are allergic to and/or sensitive to canned fish food.  You could try a bland diet with boiled chicken (no salt, no seasonings).  Just boil and shred, using some of the broth to make it like oatmeal consistency.  Also, I would change their diet to mostly canned and give them mainly chicken and turkey canned food.  Wellness makes a good canned food for kitties the age of your guys.  Also, adding probiotics, such as acidophilus will help.  I get it from the health food store and ground up into wet food.  Add a bit of water to the wet food and the kitties don't even know it's in there.

Thank you for your replies. I don't think it is their food. They have been having this food since I have had them (about 7 months). I can't afford to give them mostly canned food but I do make sure they get one can a day in the morning. It's one of the small cans, I think 3 oz. They have not had a problem with their food. If it did happen to be food it could possibly have been a bad batch of food but I don't  think that is very likely because they have had different bags since then, and it should have been cleared up by now. I have not seen any recalls of natures variety now or in the past. If they were having a food intolerance I find it strange to have happened to both cats at exactly the same time, as with a bacterial imbalance. I can see one cat or the other having that, or maybe even a rare chance that they both would have it, but not at exactly the same time. They are not from the same litter either, so no genetic thing that would be in their genes. I was thinking of a probiotic. The vet wants me to put them on the antibiotic called tylosan, but I'm not sure that I agree.

I agree with you - them both having diarrhoea at the same time makes me think parasites.
Giardia testing is difficult and a negative result does not mean they don't have giardia. You are completely right that cats don't shed it all the time in their poop.

Here's a link that explains it

http://www.petmd.com/blogs/fullyvet...osing-giardia-in-cats-dogs-is-difficult-30990