A rabbit who gets a mite infestation usually has a depleted immune system, but it's often not possible to get to the root cause of what's wrong.
It's strange how quickly their coat can change from beautiful to scraggy, isn't it?
I think an injection is the way to go, though a lot of vets tend to use the spot on treatment these days. Just make sure all rabbits in contact are treated
The normal treatment for mites is ivermectin:
http://www.medirabbit.com/EN/Skin_di...e/fur_mite.htm
https://www.viovet.co.uk/Pharmaq_Ive..._Drops/c19097/
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