Itching also seems to be related to stress, common for eczema. It’s also possible that some rats infested with mites can keep them under control until their immune system is suppressed by stress. It’s also possible that some rats with mites eventually develop an allergy to them. If you have more than one rat with scabs, it’s probable that mites are the cause.
Regardless of the cause of the itching, when the rat scratches, her back toenails break the skin which can allow bacteria to enter. As the scratches heal, scabs form and can themselves itch which causes more scratching, resulting in a vicious cycle. The first step to breaking the itching cycle is to clip the rat’s back toenails. Trimming off the sharp tips reduces the amount of damage the nails do to the skin. You might need to clip the nails as often as once or twice a week. You can also put a brick or concrete block in your rat’s cage to help her wear down her toenails.
You might also apply a vitamin E cream or an antibiotic ointment to the scabs once or twice a day. Neosporin Plus contains a topical anesthetic which can be especially helpful. This treatment may be sufficient to clear up the scabs. If they recur, you need to look for another cause.