Tylan 50 injectable (also can give orally,) Gallimycin (erythromycin,) oxytetracycline, and LS50 can all be used to treat MS. The last 3 and procaine penicillin G injectable will also treat staph infections if that could be the cause--bumblefoot is a staph infection. Respiratory symptoms are swollen sinus or any facial or eye swelling, runny nose, eye drainage and crust, and any sneezing or throat rattles. I hope you can figure out the cause, but I would check into getting the chick tested for MG and MS. This can be done by a blood test drawn by the vet, or if the chick is sacrificed and sent to the state vet for a necropsy. You would really want to know if your flock is positive for either of those.