An abscess will frequently break open and drain on its own. In this case, clean the wound with hydrogen peroxide. You can also apply a topical antibiotic ointment such as Neosporin. If it does not break open and drain on its own and becomes hard and continues to grow, wait for a scab to appear on the surface and gently squeeze it, popping the scab allowing the old pus to be forced out. Clean the wound as described above. In all cases of abscesses, if it appears to be making your rat ill, you will need to get your rat to the vet to have it lanced. In all cases of abscesses, you may need to put your rat on oral antibiotics to fight the infection from within.