I thought I replied to this earlier, but it must not have posted. I would suspect mites or possibly a fungal infection. Mites are common in rabbits and easily treated with Revolution (selamectin). It’s a prescription only med except in Australia. It’s a spot-on product that is applied at the nape of the neck. Another treatment option is ivermectin injections. You normally do 2 or, in severe cases, 3 rounds of treatment with approx. 14 days in between.

For a fungal infection, you’d most likely get an ointment, if severe you might get a systemic anti fungal med to be given by mouth.

A third possibility would be if the bun has been subject to poor housing, where it has had to sit in its own pee. That can cause urine scalding with compromised skin and hair loss as a result. You can use sudocream on the compromised skin and of course make sure the bun’s living conditions are improved.

When someone mentions external parasites, we always issue this warning: Never use any compound containing fipronil on a rabbit. Fipronil is sold under a variety of brand names, for example Frontline. It’s deadly toxic to rabbits.