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It sounds as if you are doing all of the proper food supplementations that you can at this time. SLO will take at least 3 to 4 months to show response to OFA therapy.
Corticosteroids and antibiotics are also indicated in treating this condition if the OFA therapy does not work.
What you could try at home is to soak the nails in a chlorhexaderm or betadine solution daily. If there is a bacterial or fungal component to this disease process daily treatment may be able to help.
It is true that if they cut back to far into the "quick" they can disturb the germal tissue that gives rise to new nail tissue. But I generally do not cut back that far when removing a split or cracked nail. I remove enough nail so as to prevent further cracking or movement of the broken pieces and then bandage the foot for a day or two to prevent bleeding.
You can get chlorhexaderm and betadine at your local pharmacy. Give that a try, it will not hurt anything. Just rinse the pads of the feet after each treatment and dry them well. You should let them soak for at least five minutes.
If there is anything else I can answer please let me know.
I cannot think of something else causing this that you haven't already addressed. The best test right now with the least stress on your dog would be culturing a nail if she splits another one.
A fungal/mycobacterium infection could cause these ongoing symptoms and that is why I would recommend soaking in an antifungal such as the betadine and to a lesser extent chlorhexaderm.
I have been able to to do a punch biopsy of a nail bed and did not remove the whole claw and still get a definitive diagnosis for SLO on pathology. But sometimes it does require removing the whole claw. So maybe this is why your veterinarian is holding it out as a last resort as far as a diagnosis goes.
There is a spray called Malaseb that has an antifungal/antibacterial action that you could spray on the feet.
Do you think you could do the soaks?
Immune system related diseases such as SLO can show up at any time of life. Something triggers the immune system to inappropriately attack the bodies own cells. A medication, an allergen (insect bite), but many times the triggering event will never be discovered.
Ask your veterinarian about the malaseb spray. It will be easier to use than soaks or wet pads. And it does not sting.
Any other questions?