Help! Cat gastroenteritis or possible parasites

Hi , me and my girlfriend have a 5 years old bengal male wich never goes outside got all his basic shots but nothing more . Recently we recently baught a second bengal cat a female kitten from a different breeder , we introduced both cats as introductions were going well about 1 week after we started introducing both cats our 5 years old stopped eating started to have bloody dihearas with vomitting became lethargic and vomited bile with pink bubbles as if it was blood or tissues in the vomit. So we took a stool sample and braught him to the nearby vet . The vet looked at him quickly saying he probbly had a gastroenteritis due to stress from new kitten . I Asked for a blood test bit the vet said it was too drastic so they sent us back home with metronidazole and anti acid pills back home that didnt did it the cat vomitted the pills so we took it back to the vet did the blood test ended up being a major inflammation/infection of the digestive system and on top of that the cat was deshydrated so they kept the cat for 48h under solute plus antibiotics 3 differents vets tossed to each others the case of our cat with different pronostics they tested feline lucemia and aids came back negative the stool sample came back negative as well they came with the pronostic of the cat maybe having krohm diease ? Or it was a gastro enteritis . The cat was sent home 48h after with metronidazole and antiacid for 5 days some cerenia and appetite stimulant pills if needed also with 5 days of panacure but we were told to wait after the 5 days of medications before using the panacure. We also changed his whole food with royal canin gastro canned soft food and a hard food bag the first days our cat eated his food and has semi hard stools but after few days he started to have diherea again no blood this time and started to eat less and drink less . We braught our girl for a checkup at a different vet this time and we started talking about what was happening with our cat and she came up saying that maybe the cat had tf parasite or giardia had it dormant or controlled and the stress from the new kitten might have started it . She asked me to take stools samples of eatch cats for 3 days and refrigirate it so we can test both for these parasite as one stool sample might not show .as the first vet never mentionnet anything about more than 1 stool or refrigerating it ... tomorrow i will be bringing the samples to the vet by the mean time our cat at home medication is finished and started the panacur and he had a normal stool the next morning wich was weird he came back home from the vet at 11.5 pounds and after 1 week is at 10.5 lbs he was at 12.5 lbs before this whole story . Any advices would be appreciated thanks!

If your resident cat isn't eating, that leads me to believe that it's gastroenteritis.  One of my cats went through that a few months ago. It is not fun!  It took about a week to get him back to eating normally and longer to get his stools back to normal consistency.  The Cerenia should help calm the tummy so they feel like eating.  It took us a couple of shots to get our boy back to eating.  The sudden change of dry food may be causing the continued diarrhea.  Dry food changes should be made gradually over a period of time. I always mix it with their regular food to aid in transition.  Have you tried the old food to see if that entices your kitty to eat?

The cat eats now mostly like a bird few bites then comes back later for few more , i was told by the vet to remove all his previous food and replace it by the gastro royal canin vet dry and soft food and not changing it until everything was back to normal. My cat was on orijen dry food before with almo nature either chiken or salmon in water in small cans as a side from his dry food wich were two good brands ... I think? We are pretty concerned as this cat never had any health issues in 5 years and dropped this quick.

some parasites are extremely hard to detect, even with multiple stool samples. I think since this correlates pretty clearly with getting a new kitten its likely something the kitten was carrying, dormant or otherwise. I would look into giving your cat some s boulardii, which is a probiotic that is known to help treat parasite infections. You can buy it at most health food stores or order it, there are lots of brands, I think even walmart carries one. But it must be this specific probiotic. You can then mix into wet food, baby food, or in a little water and syringe twice a day. It will help with diarrhea, too, and it doesn't interact with any medications. It's very safe and good for cats.

This post has instructions for how to use it. Dosage for an adult cat is 1/2 a cap AM and PM.

http://www.foodfurlife.com/my-cat-has-diarrhea---what-do-i-do.html

Oops. I meant to say that "bacterial infections" rather than parasites. 

I braught the samples to the vet today and both cats tested negative i also asked them to test for TF parasite wich i will have results in a few days it looks toward gastroenteritis our poor cat dont seem to like the vet food this much as he basicly begs for food when we eat something. His last two stools were solid his tummy looks still upset though i layed my ear on his tummy and its making many noises i might give a shot with probiotics . Still watching his weight carefully if he drops lower than 10.5 pounds its back to the vet for sure

The introduction of the new cat could also have your resident cat worked up and stressed out and that might be causing issues as well.  Do you run Feliway diffusers or use any other calming agents? That might help.  It's so important for your guy to eat, something!  Offer his old food and see if he'll eat that, especially since he's had some solid poops.

I gave the cat some of his old solid food this morning wich he ate with appetite around 1/3 cup to test it out he was fine for 2-3 hours he played fetch for a good hour...after he layed under bed curtains not sleeping just laying in that certain position with certain eyes looking like he has stomach or belly pain or discomfort ( I might be overreacting though ) I removed the old food and left him the gastro vet food.. now its the evening hes sleeping next to my girlfriend on the couch with us and looks well ( sleeping in his regular position) he took few bites of his vet food wet and dry so ill check up his stool tomorrow morning he usually goes around 10 am

Also to mention yesterday our cat was pretty insistant about going to see the new kitty in his room so we open the door and gave it a try both cats sniffed eatchothers for a good 30 second no hissing or fighting at all our older cat seemed calm they took turn chasing eatchothers still whithout hissing or meowing finally we separated them after 10 min as our cat didnt want to leave . And then this morning our old cat got dihera after 2 days of normal hard stools coincidence?