I’m very sorry for your loss. 🙁
What region do you live in? I ask because there is a virus called RHDV2 that is spreading through north america (it’s in other parts of the world too) that can cause rapid death with very few symptoms. It is highly contagious so hopefully that’s not what it was.
Sadly rabbits hide their symptoms very well, and sometimes things can become serious very quickly. Often in those cases there is nothing that could be done.
Would you be able to bring Steve inside permanently? He would be safer and would likely appreciate the company since he lost his companion.
Again I’m so sorry for your loss. I’ve lost two buns where we had no idea of the cause and it all happened very quickly, it’s very traumatic. Sometimes these things happen but that doesn’t make it any easier.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.