All animals, domestic or wild are intuitive to Reiki healing and can sense instinctively who can heal them, actively seeking them out. All animals take the energy they need to heal or relax and on no account does Reiki ever cause discomfort, pain or distress. The animal is the healer and the person is just the channel for the healing energy. 
Reiki can be used alongside veterinary treatment as it does not in any way interfere detrimentally with the treatment, but can help to speed up the healing process, de-stress a sick animal and ease any pain the animal is suffering.
Reiki can also be used when an animal is in transition, as it seems to calm them and prepare them for their journey to the spirit world. When Reiki is done for a dying animal the owner can benefit greatly seeing how calm and at peace, their beloved pet becomes and it can ease their grief and loss.