Making a long story short, I thought Quentin looked much better before we reached the vet's office. The vet thought it was probably saddle thrombus, which meant that my two-year-old cat had a blocked blood supply to his hind legs. From what I have read about thromboembolism and, in this case, saddle thrombus, the prognosis is terrible. And I quote, it is something of a miracle if a cat recovers from saddle thrombus. This occurred on June 3, 2010, and I was prepared to have this be the summer of taking care of Quentin, the way last summer had been the summer of taking care of Ponzo. But believe it or not, Quentin was miraculously back to his old self three days later, before he even finished taking half of his medicine, which I was able to give him in canned catfood, something he has always refused to eat in the past. Actually, I had noticed a marked improvement in Quentin on day two, when I saw him watching Karma and obviously wishing he could chase her. On day three, he did just that. By day four, there was no sign that anything had been wrong with Quentin.
It has been exactly one month now since Quentin became suddenly and mysteriously ill. He is fine now, but since the condition can recur if it was indeed saddle thrombus, I watch Quentin, our happy, vocal, lively boy, for any signs of the condition.
I did ask the vet if it could have been a widow spider bite, which is usually fatal to small cats, and he did say it was a possibility. I guess I shall never know, but I do know Quentin did not have anything contagious because of the shared food, water, and litterboxes; no one else got sick. I am still inclined to blame a spider.