The vet does not think it is CUPS, after all, but a local manifestation of acute ulcerative stomatitis. Spencer did not present with the primary symptoms– bad breath, drooling, bleeding, inflamed gums-- and he did not have any fever or swelling at all. You could only see the lesions and the mess the tooth was in with a surgery light and scope when he was under anesthesia. I knew Basenjis were stoic little dogs, but it's incredible the amount of pain they can bear without telling us the extent.
That very back tooth appears to be the cause, which is why Spencer's throat was so affected. He did have a complete dental, and his other teeth weren't so bad. The infection was severe, however. Virulent liquid had seeped onto the back of his tongue where it lay against that back tooth and appears to have caused the lesions on his tongue and throat. The antibiotics are beginning to help. So the trick is to keep the dog hydrated and nourished via syringe for the week it may take to fight the infection and heal the lesions so he can eat.
I know this is way too much information, but I wanted to pass it along, in case it might help someone else-- perhaps in the future, through the search function. This forum is such a great resource, and I had never heard of a lot of the things that have been brought up. They helped me tremendously. Our dogs tell us so much, but we can learn even more from each other. So thank you!