Rabbits can get worms. We saw pinworm eggs in a young rabbit's fecal just recently and I know they can get other types of worms as well. I work at a veterinary clinic and we have the best local "bunny vet" on staff.

Please take your bunny to the vet - a rabbit losing weight is a very serious symptom. If it is worms, deworming medications can easily eradicate the problem.