Questions about sudden death of rabbit

Question
My rabbit was almost 8 months old before he died last night.
He was always extremely active and playful and got plenty of
exercise, rabbit pellets, and Timothy hay. He seemed to be
in good health and just as active and playful as ever before
he suddenly got diarrhea Friday night. Even then, he was
still lively and wanted to run around. A friend of mine who
has owned rabbits her whole life told me to feed him plain
oats. I gave him a bath, cleaned his cage, and I took out
all his food and gave him oats. The next day, I noticed he
hadn't touched his food or his water and he left a puddle of
diarrhea in his cage. He was just sitting there grumpy and
miserable, but was still responsive. I took him to the vet,
where they gave him LRS, Buprenorphine, Metacam, and Reglan
subcutaneously and prescribed him with Metacam, Cisapride,
Metronidazole, and Albon to take home. When I got him home,
he was way worse off and didn't seem to have the energy to
move. I gave him Albon, Metronidazole, and Cisapride as
instructed and he didn't resist, which is weird for him. I
gave him a large syringe full of water which he did drink,
and he just became weaker and weaker until he could barely
lift his head. After holding him for a few hours I laid him
down in his carrier with a warm water bottle to keep him
comfortable, because at this point I knew he wouldn't make
it. I went to check on him about 45 minutes later and he was
lying on his side breathing very heavily. I went to pet his
face and as soon as I did so he arched his back and opened
his mouth really wide as if he were trying to scream but no
sound came out. He did this about three times within 30
seconds while I was petting his face, laid his head down and
took one last big breath before he died. What I would really
like to know is what could have caused this seemingly sudden
illness and death, what I could have done to possibly
prevent it or could have done differently, and if he was in
any pain as he was seizing up right before he died. I would
like to never have to experience something like that for as
long as I live. It was truly awful and breaks my heart. He
was my baby and I took amazing care of him and want to know
where I went wrong.

Answer
Dear Amber,

I am sorry about the loss of your bunny.

Sadly, there is absolutely no way to confidently ascertain a cause of death without a post-mortem exam, preferably including histopathology of major organ tissues.  The signs you describe are a little vague, and could have been due to any of dozens of different things.

If you noticed signs of unusual behavior before he died, then please read:

www.bio.miami.edu/hare/sickbun.html

which might give some clues.  Since he had runny stool before he died, this might also be of some help:

www.bio.miami.edu/hare/poop.html

But unless you can provide more specific information, this is the best I can offer.  

If the body has been kept very cold (not frozen), and it has been less than 24 hours since death, a necropsy may still be possible, if you feel that would give you closure and peace of mind.  You can find an experienced rabbit vet here:

www.rabbit.org/vets

to call and find out what your options might be.

I am very sorry for your loss.

Sincerely,

Dana