Abstract
Adenomatous hyperplasia with or without cystic formation of allantoic epithelium was observed in 63/954 equine placentas examined from 1 February 1988 to 31 January 1990. In 61/63 placentas, the adenomatous hyperplasia was associated with other placental lesions: 49 with chronic or chronic-active placentitis, six with placental edema, three with fetal diarrhea, one with placentitis and fetal diarrhea, one with fetal diarrhea and placental edema, and one with hyperplasia of chorionic epithelium. When lesions were less severe, the hyperplastic lesions were not grossly visible, but when lesions were severe, nodular, tumorous, cystic masses were observed at or near the insertion site of umbilical blood vessels. Histologically, lesions classified as stage 1 were characterized by hypertrophy and hyperplasia of epithelial cells and formation of intraepithelial glands (lumina). Lesions classified as stage 2 were characterized by the presence of fibroadenomatous changes in the allantoic stroma. Lesions classified as stage 3 were characterized by the formation of the nodular masses, which were composed of glandular or cystic structures of various sizes lined by cuboidal or low to medium columnar epithelium. These glands or cysts were empty or contained amphophilic secretion, a mixture of neutrophils and secretion material, or neutrophils. The cause of adenomatous hyperplasia of the equine allantois is not certain; however, there is a close connection between chronic placental disorders and this hyperplastic lesion.