Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 16 years of experience. I look forward to working with you. I apologize for the delay in an expert responding but this is a very tough question.
I'm sorry your cat is experiencing this and that it has been going on for several months. This becomes complex and is certainly something probably more than just an intestinal parasite (assuming she has had a comprehensive stool screening for some of our atypical parasites like Tritrichomonas and giardia). At this point our top concerns become that she may have inflammatory bowel disease, chronic pancreatitis, or food intolerance (unlikely but on the list). There are other potential problems but these are our top concerns. As well, at this point there is a high chance she has Vitamin B12 deficiency which complicates the weight loss and chronic diarrhea (it happens due to poor absorption of the b-vitamins; and it's not something you can supplement via mouth but must be done through your vet via injection).
At this point the next steps in her diagnosis would require: 1)Screening for pancreatitis (PLI), 2)Screen vitamin B12 and folate levels (folate tells us about bacterial overgrowth in the gut), 3)Radiographs (X-rays)or abdominal ultrasound to rule out foreign object in the system and cancer (unlikely at her age but good starting point), 4)If not already done screening lab work that includes a complete blood count, chemistry profile with electrolytes and urinalysis (we need to make sure there is not an atypical infection or organ problem leading to diarrhea). And, again, if not already done she needs to be properly screened for Tritichomonas and giardia parasitism. 5) If already done all of that then you are to the point of biopsies of the intestinal tract (done surgically or via endoscopy).
It's a lot of testing and if your vet does not want to talk about these and work through it, then please ask for a referral to see an Internal Medicine Specialist which is really the point where you are.
I wish I could say there is an easy medication that will clear this but I suspect you have tried the easier medications such as metronidazole (or Tylosin) and probiotics which have not already resolved the problem. Diet changes might or might not help; but you should ask your vet what (s)he recommends - bland versus hypoallergenic. I am not very suspicious of food allergy or food intolerance at her age but it is certainly a possibility. The frustrating issue is that food trial takes 2-3 months to know if it's going to work, during which time we are not resolving the diarrhea (unless she is very responsive to metronidazole and probiotics; in that case then you could elect to continue those drugs longer term and do a food trial).
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Excellent that she is keeping up with her weight. The next step? I would advocate some testing. You do not need to do all the testing I mentioned but probably to start working through this. If she was my patient I would prioritize screening her Vitamin B12/folate and PLI to look for vitamin deficiency and pancreatitis, which are blood tests. As well, be sure, given she is a purebred, that she was screened for Tritrichomonas as this is a more common chronic parasite than we think and has a defined cure. (I am assume she already had normal screening lab work as mentioned above with a CBC, chemistry profile and urine check).
If not already using, she should be on a daily probiotic supplement (ideal to use one as recommended by your veterinarian that is efficacious and meant for cats).
You are most welcome. Please keep me posted.