I have been reading over on VIN and what everyone keeps saying over and over again is that there needs to be a firm diagnosis!
FIP is rare... and having multiple cats affected makes me wonder if this might be a parasite problem.
Giardia and Tritrichomonas are 2 parasites that could cause chronic diarrhea like this.
These are treatable, though treatment can be challenging!
It is very hard to diagnose Feline infectious peritonitis (FIP) without doing biopsies of the organs affected.
Quite often, when multiple animals are affected, it is suggested that if one of them unfortunately passes away, that we send their remains in to be tested so that they may yet save the others.
So, without examining your cats and without biopsies, the best we can do is make a tentative diagnosis of FIP. This could still be IBD (inflammatory bowel disease) which is more common in Siamese cats than in other breeds.
FIP is serious and destructive virus. I realize you probably know as much as I do about this virus by now, as you have surely had lots of time to read up on it with these years of sick cats.
The disease is most commonly seen in facilities housing large numbers cats, such as catteries and animal shelters.
Transmission occurs when a cat comes into contact with an infected cat's bodily secretions, primarily saliva and feces.
Unfortunately, the virus can survive a long time outside of the body and can remain a source of infection on a dirty food bowl or litter pan.
Initial symptoms include upper respiratory problems, depression, and weight loss.
Two types of the disease are recognized.
"Wet" type FIP-infected cats appear with large "pot-bellied" abdomens that are actually filled with fluid, eventually leaving the cat struggling to breathe.
"Dry" type FIP-infected cats have minimal fluid accumulation and exhibit weight loss, depression, anemia, and fever.
Unfortunately, FIP is hard to diagnose as test results are unreliable; by the time symptoms are identified as likely resulting from FIP, the disease has already significantly progressed.
The only way to care for an FIP-positive cat is to provide supportive care based upon the symptoms. A vaccine does exist for this virus but is quite controversial and is not frequently used. The best prevention is to minimize a cat's possibilities of exposure.
So, then we come to treating diarrhea. The things that I would try are:
1. Metronidazole -
this antibiotic kills the "bad" bacteria in the intestines and spares the good.
Here is more about metronidazole: http://www.petplace.com/drug-library/metronidazole-flagyl/page1.aspx
This would be what was in the yogurt you tried. Often, diarrhea in cats is due to an overgrowth of "bad" (anaerobic) bacteria in the intestines. If you give your cat a probiotic, it will help to re-establish "good" bacteria in her intestines. You can either get FortiFlora from your veterinarian, or you could use Culturelle (1/2 - 1 capsule per day for a cat, sprinkled on her food). You need to continue treatment for 1 month in order to have this be effective.
Here is more about them:
3. Diet changes
Diarrhea can be due to fibre in the food, so in some cats the problem is resolved by trying a low-residue food (low fibre). Purina makes a low-residue cat food you can get at your vet clinic.
Here is a link to it: http://shopping.msn.com/specs/cat-food-purina-veterinary-diets-en-gastroenteric-feline-formula-dry-cat-food/itemid933787345/?itemtext=itemname:cat-food-purina-veterinary-diets-en-gastroenteric-feline-formula-dry-cat-food
Other cats seem to have diarrhea due to food allergies. These cats' feces are normal if the cat eat a hypoallergenic diet. Again, these would be available from your vet.
You would need to commit to a 90 day trial where this was the ONLY thing your cat was allowed to eat, in order to test to see if this was going to help. These foods either have a unique protein (that your cat has NEVER eaten before – things like venison, duck, rabbit, salmon) or a hydrolyzed protein (that is partly broken down so the body does not recognize it and react to it).
Here is more about this issue, including some examples of foods:
4. Reduce carbohydrates (CHO's)
For many cats, the CHOs can make diarrhea worse.
So, it is a good idea to offer a high quality canned, low-carb diet, such as Evo 95.
Another option would be a lightly cooked raw diet (Nature's Variety or Companion or Primal) WITH the probiotic. This combination is often effective!
I wish there were more I could do to help you with this very challenging problem. I'm sorry I don't have a magic solution for you. It sounds as though your vet has been very thorough!
If this has been helpful, please "Accept" my answer and leave feedback. I will still be here tomorrow to provide more information if you need it!
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.