Lupus and/or polyarthritis

I have a 7 year old male GSD who, about 2 weeks ago, started having issues with his left hind leg. The hock was swollen and he stopped using it (thankfully he is very muscular and not having an issue with only using 3 legs). My first thought was cancer, but thankfully that has pretty much been ruled out. My vet has taken numerous x-rays; 2 joint taps and a full lab panel and everything is normal. The anti-imflammatory that he was given has not helped at all, but he is on Tramadol for pain, which seems to be helping. After consulting with a radiologist as well as an ortho surgeon, they have come to the conclusion that he has arthritis in multiple joints which is being caused by Lupus, even though the test was normal. Everyone seems to be stymed by him. Has anyone had any experience with Lupus in their dog? If so, any info you can provide me would be greatly appreciated.

Oh, dear ... how bizarre. No, I can thankfully say we've never had this problem. Sounds like your vet is trying hard to identify the issue, and I suppose you've done the usual Internet searches to see if anything turned up. Is there a vet school anywhere near? Sometimes they can be helpful with the hard-to-diagnose issues. We have one in Fort Collins, only about an hour away from the Denver area, and they have been found to be very helpful on occasion.

Just a thought ...