RedneckPete: The abcess does not look quite right to me, but it may or less just be the way it is photographed and tissue around it. I have seen some bizarre looking abcesses when I worked for the vet. I feel this question has drawn alot of good feedback, but going way out in left field on some matters. I honestly cannot make a suggestion to you as to what may of caused this, without having seen it myself. I can make assumptions but you may have a case of a simple rogue infection.
In my experience with abcesses in a pig we had a litter of piglets born let's see....last April I guess it was, one piglet had a puncture wound caused when momma broke the cord. He was very down when I got to him (they were born at night) and I pepped him up, flushed the wound, and ran him up to the vets office, as I did not feel like suturing a newborn piglet without some local anesthetic to keep him from wiggling away since I was the only one home to do so. Well, the vet flushed it again with saline, sutured, and was done with it. Never thought any more about it as he was fine. (In hind sight he did bleed alot but with a newborn piglet and the area I wasn't too concerned. The vet also used dissolving PDS II stiches which I also found odd but ignored it) Well about 6 weeks later he formed a knot near where the sutures had been. So I checked it and it was firm. Well I stuck a needle into the knot to see if I could pull out any fluid, I got a whitish fluid, very much resembling momma's milk.... finding this odd I took it to the vets office and proceeded to say I felt he had a hernia and to check the fluid. Well the vet (this is a new vet who I cannot stand, same who did the sutures) she proceeded to just stare at me and tell me it was impossible for it to be a hernia as she did the suturing and closed the wall, she couldn't of made a mistake). Anyways she took the syringe looked at it and told me that it did not come from a pig. I told her my thoughts and and left. So going on that it was a hernia I decided I would keep and eye on it as he was a butcher pig anyways and let it be. Well about 2 months later he starts forming knots, literarly overnight behind his ears, near the perineal and scrotal areas. I knew these were abcesses and put him on a round of antibiotics, he has a low grade fever and never was amiss in any other way. The next morning he was down, and had an anal prolapse, needless to say I put him down on the post. I checked him over and found he had more subq cysts on him. I performed the necropsy and discovered a prolific systemic series of abcesses. The cause was the kicker. I found that the when the vet sutured she caught the intestine, a hernia had indeed formed since it was attached to the outside wall and as he was growing it put more and more stress on the loop of intestine causing the intestine to begin to become necrotic the pig was nothing but a septic infection on four legs. However he never showed any signs of distress until the prolapse. He ate full feed, drank, ran, played, rooted, never slowed down. This amazes me as he had a large blockage that had to of been there for a while, but he never showed any distress. Well needless to say I took his carcass up to the office and set it on the table in front of her for her to see herself as this pig suffered needlessly.
Anyways...sometimes these things occur for various reasons. I have seen a seed cause an abcess when it was lodged under the skin from a laceration. In my opinion I doubt this was the case. It could of been a simple staph infection that went rampant. Who knows. In anycase I would keep and eye on the rest of the pigs you have, but I think this will probably be limited to him. I would also not touch the meat with a ten foot poll. Cooking may kill things but I'm a germ nut anyways. If this is a staph infection the last thing you need is to consume the meat. I posted in the goats forum to a lady who had a goat with an abcess. I told her that I saw a girl nearly loose one of her fingers from an infection caused by draining an abcess without gloves on. She had her nails done three days previous to that and the infection set in. Abcesses are nasty nasty things that the utmost hygeine and sterilization should be used in treating it, to include lancing and draining wounds, or removing them entirely. To chance this in the meat....well it's your health not mine.
Wish the best for your other ones, let us know if the present any signs or symptoms. I'll be curious to know. I feel this is an isolated incident however. Sorry I can't be of more help.