Histology of air sac lesions induced in chickens by contact exposure to Mycoplasma synoviae.

Chickens were housed when 1 day old with chickens experimentally infected with Mycoplasma synoviae. Air sacs from 16 of the 2-week-old and eight of the 3-week-old contact-exposed chickens were given gross lesion scores and embedded in glycol methacrylate for slide preparation. Histologic lesions in air sacs with gross scores of 0-2 were mild edema resulting in a two to eightfold increase in air sac thickness, capillary proliferation, and exudate consisting largely of heterophils and necrotic debris. Histologic lesions in air sacs with gross scores of 3 and 4 were marked hyperplasia of epithelial cells and diffuse infiltration of the air sac connective tissue by mononuclear cells. Nine of 10 air sacs with gross scores of 0 had no histologic evidence of inflammation. The most severe histologic lesions were in those air sacs with gross scores of 4. The glycol methacrylate procedure resulted in 2-mum sections with excellent cellular detail.