Apple tree damage

I have an apple tree about 2 years old and something has nibbled the bark off about 50% of the way round. I think its probably a rabbit, my question is do I need to treat the tree in any way? I have now put a cage around the base of the tree to protect it further.

I am not convinced by the use of fungicidal paints in this situation.  There is no evidence of fungal infection, and there is the risk of trapping any other infections under the paint.

The main risk is of ring-barking, which will kill the tree because the nutrients etc cannot move up and down the tree.  Hopefully the cage has sufficiently small holes to prevent rabbit or other predator continuing to nibble away.  You can get plastic tree guards which are specifically designed for that.