Old age doesn't make a rabbit stop eating > Old age is not a disease but just a time of life when more problems are more likely to occur both because there is a decline in the immune system with age and also the organs and musculoskeletal system and entire body are just not as efficient as in a younger animal .
Rabbits do not die from "old age' butusyually a medical problem that occurred because of the aging process. it is the same with humans; there is always a medical condition behind the death ( I guess that I have been watching too much of Dr G Medical Examiner
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I think that Claire may be correct that there is a dental issue (often not able to be visualized in an office exam) that is causing him to be too uncomfortable to eat. He most likely needs both dental and head x-rays which may be able to pinpoint the problem or even to be anesthesized and examined by a rabbit knowledgeable vet.
Many vets ( not rabbit knowledeable) may take a scope and attempt to look into the rabbits mouth . Believe me they really cannot see much that way.
In the meantime this rabbit is going to deteriorate quickly eating only lettuce .
The owner should attempt to introduce other veggies (parsley, cilantro , kale, collard greens , dandelions etc) one veggies at a time daily in a small amount to see if the rabbit responds to other veggies.
Possibly this rabbit doesn't have access to haywhich is even more important than greens.
Possibly this rabbit has been fed nothing but pellets and junk ( terrible diet) and is in stasis...?
Stasis would create a situation where the slowdown of the gut would cause either no poops or small dried out ones.; gas builds up and creates a painful sitution in which the rabbit will be too uncomfortable to eat because he is in pain.
This will lead to death if the owner doesn't treat it and it CAN be treated.
Can youfind out more about this rabbit or ask the owner to join the forum and post.
Where is she located.? We could help her find a better vet.
If she is in Wi I know there are many rural areas with poor rabbit vet care but not so in the larger areas.
Maybe you can do some follow up on this rabbit (if the owner doesn't want to join) and give us more info.