My name is***** have been a veterinarian for 11 years. I am very sorry to hear that Bella has diarrhea. I would like to help you with your concerns today. If you could let me know what your exact question is, I would be more than happy to answer specifically.
As to your question about new litter boxes, yes that may help as cats do not like old litter boxes. I recommend new litter boxes every year. With 3 cats, you should also have at least 4 litter boxes.
Metronidazole and FortiFlora both work very well, but you do have to make sure that they are not given within 1 hour of each other. The metronidazole will inactivate the FortiFlora if they are given too close together. It can take 3-4 days for the metronidazole to start working to decrease the level of bad bacteria that is often a part of the diarrhea.
If she is not improving within 4-5 days, then she may need some additional blood work as a 13 year old cat can have many problems, including kidney or thyroid disease, among other things. If you don't know what the underlying problem is, then you may end up spending even more money with treatments that don't work, because it is a guessing game. It is certainly reasonable to do a course of the metronidazole and FortiFlora prior to additional diagnostics though (which is what you are doing).
You may also want to speak to your veterinarian about a prescription food for a short period of time. My favorite for cats with diarrhea is Royal Canin Gastrintestinal Fiber Response. It helps with both diarrhea and constipation. In fact, this is what my cat is on due to chronic diarrhea. There are other options as well, for easily digestible foods, such as Hill's i/d and Iams Low Residue.
I do recommend confining her to a smaller room, such as a bathroom or laundry room that can be cleaned easily until the medication starts to work. I know that this can be upsetting as she may cry to be let out, but to keep peace within the home and to prevent potentially spreading whatever is going on to the other cats, confinement is recommended. The small room should have at least 1 litter box, water, food, and a soft, clean bedding area.
If you have additional questions, please let me know.
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I hope Bella feels better soon and that you have a safe 4th of July!