My Gordon Setter started to limp, was obviously in pain and on close examination, her hind claws were all split right down the underside.  Her foreclaws were ropey as well, some of these were thin on the top.  The quicks were showing and one was so bad that the vet had to cut it right back, and trim the rest right down, under unaesthetic.

After some research the vet diagnosed Symmetrical Lupoid Onychodystrophy - SLO.  None of the vets in the practice had seen it before.  It is an auto immune condition where the body's defenses attack a part of the body.  There is a human condition called SLE.

Recently I discovered another case in my village, a championship agility dog.  It had lost all its claws and having seemed to recover enough to train again, started to lose them again.  Her owner's vet had never seen the condition either.

Here are two links that describe this horrible condition.
 
http://www.bloodaxe.com/SLO.html

http://blackschutzhundshepherds.blogspot.com/2009/08/horrors-of-slo-symmetrical-lupoid.html

Well now for a bit of good news.  I have treated my girl by changing her evening kibble to a fish based product (salmon and salmon oil), high in Omega fatty acids.  She still has her morning tripe. The level of these compounds was boosted further with an evening primrose oil based product, followed more recently by starflower oil pills, cod liver oil and biotin pills, the oil twice a day.  I am considering Yumega Plus, reported elsewhere in this forum, as liquids are easier to give than capsules.  Biotin, by the way, is fed to horses to improve hoof growth and was in the evening primrose product.

After trimming the fur on her paws last week, I could see that the splits on her claws were confined to the outer half, the inner being healthy.  No claws have been lost.  Maybe we have discovered it early enough to prevent that.  Just maybe, it was not SLO.  Who knows.  A biopsy was impractical as her dew claws had been removed as a pup.  She has continued to train for agility and enjoy herself, and though wary of me handling her paws, has been pain free.

I will have to be careful not to overdose her, but she is very healthy and has no digestive problems.

I hope this doesn't apply to any of this forum's readers' dogs, but since it so rare and didn't seem to be mentioned here, I thought it might be useful information. 

My two dogs were diagnosed with this after they both broke nails. It was triggered by a v.nasty tummy bug and apparently thats one way that the auto immune illnesses are triggered, along with respiratory infections. At one point my boy had bandages on three feet when he broke a nail on each foot. They were both on tablets and oils for nearly two years and now are on oils and Biotin. Some of their nails look better as the regrowth hasn't always been great. I also use a Dremmel to trim their nails now as normal clippers make the nails crack and break up.