Mycoplasma gallisepticum by using qPCR. Mycoplasma gallisepticum is the causative agent of chronic respiratory disease (CRD) in chickens and infectious sinusitis in turkeys, chickens, game birds, pigeons, and passerine birds of all ages. M. gallisepticum is transmitted through the eggs of carrier hens. Most commercial flocks are M. gallisepticum free, however, and the disease is most frequently introduced from carrier birds being brought into the flock. Stress is thought to lower the poults resistance to the disease, like in CRD.