Sometimes when a cat gets an abscess, it occurs in a place where isn't enough pressure on the skin for it to burst or it grows extremely large and the cat becomes ill and these need to be lanced and drained. This is expensive because it is a surgical procedure: a cat needs to be anaesthetised so that the skin can be cut open and the pus flushed out. Thereafter, the vet might just leave drain holes open or they might stitch a drain to the skin so that the incision can continue to let any remaining pus come out. A drain is a piece of surgical tubing that acts as a foreign object and prevents the hole from closing.
A few years ago, my cat Ms P was unlucky enough to need abscess draining surgery twice. The first time, she was bitten badly on her shoulders when a tom cat tried to mate with her, even though she is spayed. She was in pain and wouldn't eat much so I took her to the vet who put her on antibiotics and she seemed to get better. A month later, I noticed that she had a hard lump in the area. Back to the vet, who used a needle to draw off some of the fluid in the lump. The fluid was not pussy but reddish. Under the microscope, the fluid showed many Neutrophils, which are white blood cells that fight off bacterial infections. Which goes to show that antibiotics can't always prevent abscesses, because they are formed in a pocket with poor blood supply. We gave the lump a few days to develop and then she was back to the vet for it to be drained.
Not happy: surgical tubing stitched across her shoulders
Because she is feral, she is not a good patient and about 2 days later, she ran away and hid. I was frantic with worry because the drain would prevent healing but to my immense relief she reappeared after 36 hours and I trapped her in the house and got her locked into the bathroom. By now, the drain was worrying her badly and I removed it. Fun fact: nail clippers work well for cutting sutures. The vet told me to syringe water into the holes to flush it clean and she was given an extra day of antibiotics and everything healed up nicely.
The next time she got into a fight, she was either bitten or clawed at the base of her tail as she ran away and again was in pain and wouldn't eat. In the beginning, I felt a small sausage-shaped lump at the base her tail but by the time I got her to the vet it was gone and the vet she didn't think that there was an abscess. I disagreed and asked her to anaesthetise and investigate. When she cut into the area, pus poured out, even though there was no external swelling. She had no drain this time, just an incision a short way up her tail and one on her hindquarters. She looked very odd with her shaved butt and tail and I took her home and locked her into the bathroom. She took advantage of a moment's carelessness on my part and escaped and ran away again. This time, she went deep inside a pile of timber and refused to come out. I gave up on trying to get her out and put food and water at the entrance to her hiding place. She eventually emerged about a week later but luckily it had healed up without complications.
I am very happy that she hasn't had any since I moved because she really is a terrible patient.