Do you have neighbors or farms with chickens in your area? Many backyard flocks are positive for MG, and wild birds can help to spread diseases. I would treat with Tylan 50 orally, at 1/4 ml per pound 2-3 times a day for 5 days. Give you chickens probiotics in feed or add some plain yogurt to their feed daily. If they have not been wormed lately, you may want to do that with Valbazen or Safeguard Liquid Goat Wormer. Close your flock to new birds, and see how it goes. Or you can separate the sick birds, and think about culling them. I would get a necropsy by your state vet to see what illness they have. Sometimes there can be a combination of MG and another disease. MG can reappear with stress such as cold westher or during a molt.
