Poor little Dennis

My cats go outside a lot. When the weather is nice (i.e. when it's not raining) and we're home, we let them go out. There are lots of interesting cat play places around our house. A vegetable garden, chicken shed, and 2 big fields behind the house, one of which with horses. Then there's the neighbour's chicken shed (where they have caught many a sparrow and starling).
When they play outside, I usually leave the back door open so they can come in to eat, drink or use the littler box. And whenever I'm outside, they usually stay close to me. I like that, because I can see what they do.
Last Tuesday something went wrong, however.
As usual, Dennis and Cilla had been outside all afternoon. They came in when it got dark, but Dennis spent almost all night on the litter box. We were conspiring a birthday surprise for my brother in law, and from the litter box he could see what we were doing. We made fun of him: look, he found a new chair with a lovely view!
Little did we know that he had a problem...
Wednesday he went outside, but didn't come in. He stayed outside all night and all day Thursday. Nobody had seen him in the day time though, which is very unusual. At 21:30 he walked in and went straight to the litter box. He couldn't walk straight, his fur was pale and he was very skinny. I called the vet and he told me to come to the office straight away.
The vet examined him and found that his bladder was clogged and therefore he couldn't pee. It had been like that for a few days (apparently since Tuesday) and it had been infected. Because he couldn't get rid of his waste, it had gone back into his veins and had started to poison him, that's why he walked so funny. The vet flushed his bladder and called me at 23:00 that all was well and I could pick him up after work.
What happened? He was probably kicked by a horse, which made him bleed internally. The blood had clogged his bladder. The vet told me we were lucky he came in and we found him, because usually cats that feel that badly find a quiet spot to die..
I took him back there this morning and he got another shot of antibiotics and anti-infection. We'll go back again Monday to see how he's doing. In the mean time he has to eat a lot, sleep a lot and stay in. He does look a lot better now though, thank goodness!