Mumsy, sorry to hear about the inflated bird. I want to link you to one of the possible causes but as it belongs to another chicken website, I'm not allowed to link it directly (I already got a warning for helping someone fix a broken link here).
I will quote these though: ""Air between the skin and flesh is caused by a germ known as Clostridium, it will have entered via a cut. The medication of choice is Bacitracin. Since you only have one infected bird, I'd advise that you fill some empty capsules with Bacitracin powder and give one morning and night until the air has gone. Its also a good idea to pierce the skin and let the air out, I'd use a large needle like the ones used for horses or cattle and just let the air escape.""
Another poster said: "True - air under the skin is either a result of trauma (causing sub-cutaneous emphysema) or from infection (fightingwith another bird, non-sterile needles being used for injections, injuries etc). You can deflate the air using a sterile needle but best to do as Sandman suggested and put the bird on some antibiotics, especially if the air pocket is returning. I've had birds get this before and the antibiotics (I've used baytril in the past for it) seem to clear it up pretty quick."
Some users are giving penicillin: "The penicillin should work, but 1 ml is a little too much, especially for an 8 week old bantam. If you dose the chick again, give 30 000 IU/kg. On the bottle of penicillin, it should say how many IU (international units) of penicillin per ml. Say there's 100 000 IU/ml. Your chick weighs around 300 grams, I would guess, which is about 1/3 of a kg. If you go to give your chick more penicillin (and the concentration is indeed 100 000 IU/ml) then give it no more than 10 units or 1/10 of a ml (0.1 ml). If the concentration is 300 000 IU/ml, then you'd give 3 units (0.03 ml)."
I know your chicken is already culled, but for future note, in case you see more with it.
